JP2717544B2 - Image forming device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (1) Object of the Invention (Industrial Application Field) The present invention relates to an image forming apparatus utilizing an electrostatic transfer process, such as an electrostatic copying machine and the same printer.

(Problems to be Solved with the Related Art) A transfer roller as a transfer member is pressed against an endlessly running image carrier, usually having a generally cylindrical configuration, and a sheet-like material mainly comprising paper is provided between the two. 2. Description of the Related Art An image forming apparatus configured to transfer a transferable toner image formed on the surface of an image bearing member to a transfer material by the action of a transfer bias applied to a transfer roller side is well known in the art. However, in such an apparatus, when the original image is larger than the size of the transfer material, and the toner on the image carrier is directly transferred to the transfer roller, or when a jam occurs, the transfer roller is remarkably moved. There is a problem in that the transfer material is contaminated and the subsequent transfer material is soiled, or a transfer bias is substantially insufficient to cause transfer failure.

As means for avoiding such a problem, a bias having the same polarity as that of the toner is applied to the transfer roller at the time of non-transfer when the transfer material is not passing between the image carrier and the transfer roller. Means for transferring the toner to the carrier and cleaning the transfer roller have been proposed.

Recently, with the spread of computers as well as copiers, printers based on electrophotography using lasers or LEDs have become widely used. In order to reduce the time as much as possible to extend the life, image exposure is generally used, and the reversal development method is often used for visualizing the image.

In the case of a cleaning unit that applies a bias having the same polarity as the toner to the transfer roller when the transfer material does not pass as described above, in the case of an image forming apparatus using a reversal development method, the charge polarity of the image carrier and that of the toner are different. Because of the same polarity, sufficient cleaning could not be performed when a bias was applied in the reverse direction during transfer during non-transfer.

SUMMARY OF THE INVENTION The present invention has been made in view of the above situation, and has as its object to form an electric field for transferring toner from a transfer member to an image bearing member so that the transfer member can be cleaned well. It is still another object of the present invention to provide an image forming apparatus capable of stably obtaining a high-quality image by cleaning the transfer rotary member with toner.

(2) Configuration of the Invention (Technical Means for Solving the Problems, Action of the Invention) In order to achieve the above object, the present invention provides a movable image carrier, and an image forming device for forming a toner image on the image carrier. An image forming apparatus comprising: charging means for charging the image carrier to a first potential; and developing means for developing an electrostatic image on the image carrier with toner having the same polarity as the charging polarity of the charging means. Means, and a transfer member that forms a nip portion with the image carrier, and transfers the toner image from the image carrier to a transfer material at the nip portion, while the nip portion has no transfer material. An image forming apparatus comprising: a period in which a voltage having the same polarity as the toner image is applied to the transfer member to form an electric field for transferring a toner having the same polarity as the charge polarity of the toner image from the transfer member to the image carrier. Before the nip in the period Assuming that the potential of the portion of the image carrier is a second potential, the absolute value of the second potential is made smaller than the absolute value of the first potential, and the absolute value of the voltage applied to the transfer member during the period. In the image forming apparatus (1) or (1), the second potential is the same polarity as the charging polarity of the charging unit or 0 V. In the image forming apparatus (2) or (1) or (2), the transfer member is provided with the toner image while the toner image is transferred from the image carrier to a transfer material at the nip portion. The image forming apparatus (3) or (3), wherein a voltage having a polarity opposite to the charging polarity is applied to the transfer member while the transfer material is not present in the nip portion. Polarity opposite to the charging polarity of the toner image The voltage of the image forming apparatus characterized by switching is performed between the same polarity as the charging polarity of the voltage of the toner image (4), or in that of any of the above (1) to (4),
In the image forming apparatus (5) or (5), the second potential is set before passing the developing position of the developing unit.
In the image forming apparatus (6) or (5) or (6), the potential is set in advance by the charging unit, and the potential of the image carrier in the nip portion in the period is set. The image forming apparatus (7), wherein when the portion passes through the developing position in advance, a voltage is applied to the developing unit to prevent toner from adhering to the image carrier.

(Description of Embodiment) FIG. 1 is a schematic side view of a main part of an image forming apparatus suitable for applying the present invention, and is a cylinder that extends in a direction perpendicular to the paper and rotates in the direction of arrow A. A primary charger 2 for uniformly charging the photosensitive layer on the surface of the image carrier 1 in the shape of a circle, and irradiating the charged surface with optical information such as a reflected light image of an original image and an image-modulated laser beam. An exposure unit 3 for forming an electrostatic latent image and developing device 4 for supplying toner to the latent image to visualize the latent image, and being in pressure contact with the image carrier, and transferred to this pressure contact nip portion A transfer roller 5 serving as a transfer rotator for feeding a material (not shown) to transfer a toner visible image onto the transfer material; a power supply 7 for applying a developing bias to a developing portion of the developing device; Is provided.

Since the general image forming process of such an image forming apparatus is well known, it is not necessary to describe the details thereof, but in order to facilitate understanding of the present invention, The operation will be schematically described with specific numerical values including potential fluctuation during operation of the member.

The photosensitive layer on the surface of the image bearing member is an organic photosensitive member that is charged to a negative polarity.
It is charged to -700 V as a potential.

The charging means is not limited to a corona charger, and it is a matter of course that a contact charging device configured to apply a voltage to a roller, a brush, a blade, or the like to contact the image carrier can be used.

Next, the irradiation area is attenuated to -100 V by the light information irradiation in the exposure section 3, and this portion forms an electrostatic latent image.

A developing bias of -500 V is applied to the developing device 4 by the power supply 7, and the negatively charged toner adheres to the potential attenuated portion by the reversal developing method to form a toner image.

On the other hand, a transfer material (not shown) supplied by the transport path 6
The image carrier 1 is synchronized with the timing of the toner image.
When the transfer material passes through the transfer material, a +500 V transfer bias is applied to the transfer roller by the power supply 8 to transfer the toner image to the transfer material.

In this case, in this embodiment, the charger 2 is stopped when the image is not imaged (when the transfer material is not passed), that is, when the optical information is not irradiated on the image carrier, and the surface potential is set as the second potential.
Set to 0V.

At this time, when the non-charged area having the surface potential of zero is in the developing area facing the developing device, the developing bias is turned off or the developing bias is switched to the positive polarity to prevent the toner from adhering to the image carrier. It shall be.

In a printer using a laser beam, the amount of laser output changes depending on the temperature, humidity, and the like. It is common to continuously irradiate the laser during rotation, detect the light amount, and control so as to maintain a constant light amount. At this time, if the image carrier is charged, the potential of the laser irradiation area becomes Attenuated, two types of latent images corresponding to the portions initially charged at -700 V and -100 V are formed. Applying a developing bias that does not develop both of these is due to problems such as reversal fog and carrier adhesion. Although it is difficult, as described above, by setting the potential of the image carrier to zero, it is possible to select a bias that does not cause toner adhesion over a wide range.

For example, in the case of a magnetic brush developing method using a two-component developer to which a DC bias is applied, from the viewpoint of the adhesion of the developer to the image bearing member, the potential of the image bearing member is in the range of several hundred V near 0 V, A developing bias value at the time of an image can be selected.

In addition, using a one-component insulating magnetic developer, the developer thin layer is opposed to the image carrier with a gap at the developing site, and an alternating electric field is formed at the developing site by superimposing a direct current on an alternating current. In the case of jumping development, the developing bias at the time of non-image, various means such as weakening or turning off only the peak-to-peak voltage of the AC component, changing the DC component or turning off both, A developing bias that can prevent toner from adhering to the image carrier can be selected. Then, contamination of the transfer roller in contact with the non-image area of the image carrier can be prevented.

When the non-charged area of the image carrier is in a transfer area where the image carrier and the transfer roller are in pressure contact with each other, that is, when the transfer material is not passing, the image carrier is in direct contact with the transfer roller. To transfer the toner of the transfer roller to the image carrier, a bias of the same polarity as that of the toner is applied to the roller at -500V. That is, the absolute value of the surface potential of the image carrier when the transfer material does not pass is made smaller than the absolute value of the bias voltage applied to the transfer roller.

Since the surface potential of the image carrier is 0 V, the toner is transferred from the transfer roller to the image carrier even when the voltage applied to the transfer roller is lower than when the surface potential of the image carrier is kept at -700 V. And a sufficient electric field is formed, and the transfer roller is well cleaned.

That is, the second potential (0 V) is the first potential (-) with the polarity (minus) of the charge of the toner having the same charge polarity as the toner image.
700V) in the opposite direction, the transfer roller becomes easier to clean the toner.

FIG. 2 is a timing chart showing the operation of the embodiment of the present invention, wherein A represents the surface potential of the image carrier;
The unevenness in the image portion indicates a potential pattern due to light information irradiation.

B indicates switching of the developing bias. Since the non-charged portion is visualized at the timing of bias application during normal development, in the present invention, the transfer material does not pass between the image carrier and the transfer roller. In other words, it is necessary to switch the bias so as to overlap the non-image non-charged area.

C indicates the bias application to the transfer roller and the switching timing thereof. In the image area (when the transfer material passes), the polarity is positive.
In the non-image area (when the transfer material does not pass), a negative bias is applied.

As described above, the developing bias is switched so as to overlap the non-charged portion from the viewpoint of toner adhesion to the image bearing member. The bias becomes 0V,
Normal polarity, in this case, regardless of the negatively charged toner, but a strong reverse electric field is formed for a short time, so it is included in the one-component developer and charged to the positive polarity. There is a possibility that the so-called reversal toner or the positively charged carrier in the two-component developer may not adhere. If a bias having the same polarity as the toner is applied to the transfer roller in this portion, Since the transfer roller may be attracted by attracting particles of opposite polarity, the transfer roller may not be stained.Therefore, the bias to the transfer roller is the same as that of the toner in the non-charged area where the opposite polarity particles do not adhere to the image carrier. It is desirable to switch to polarity.

In the above embodiment, the charger is stopped to form the second potential in the non-image area of the image carrier (when the transfer material does not pass), but without stopping, the charging voltage of the charger is higher than the image area. Or when the corona charger has a grid, the bias of the grid may be controlled to lower the potential of the non-image area of the image carrier.

Further, in order to form the second potential, the non-image area may be charged in the same manner as the image area, and the non-image area of the image carrier may be discharged later.

For example, when the image bearing member is a photosensitive member, the potential can be reduced by irradiating light to a non-image area which has been withstand in advance. At this time, the second non-image area of the image carrier
The potential is not limited to 0 V as in the above embodiment.

In short, when cleaning the transfer roller, the absolute value of the surface potential of the image carrier when the transfer material does not pass is made smaller than the absolute value of the first potential and the absolute value of the bias voltage applied to the transfer roller. Is also to make it smaller.

In order to do this, it is conceivable to lower the surface potential of the image carrier when passing through the transfer material and to increase the bias voltage applied to the transfer roller. For example, if the surface potential of the image carrier is maintained at -700 V in the above-described embodiment, a bias voltage having the same polarity as that of the toner is applied to the transfer roller to transfer the toner from the transfer roller to the image carrier. In order to form the electric field, it is necessary to set the bias to the transfer roller to -1200V.

As described above, it is necessary to apply a high bias to the transfer roller in order to obtain a potential contrast that can sufficiently clean the transfer roller, which causes a problem of a large power supply device and a high-pressure leak.

Therefore, it is preferable to lower the potential of the image carrier when the transfer material does not pass, as in the above-described embodiment.

Optimally, the potential is set to 0V. By doing so, the voltage applied to the transfer roller can be reduced, and the dielectric breakdown of the image carrier does not occur, and the leakage can be prevented.

For example, it is effective for a material having a very low withstand voltage such as amorphous silicon and OPC.

Of course, it is of course possible to use a transfer belt instead of the transfer roller.

(3) Effects of the Invention As described above, according to the present invention, the electric potential of the portion of the image carrier in the nip portion is changed to the second electric potential during the period of forming the electric field for transferring the toner from the transfer member to the image carrier. In this case, the absolute value of the second potential is made smaller than the absolute value of the first potential by the charging unit and smaller than the absolute value of the voltage applied to the transfer member in the period. In addition, the toner adhered to the transfer member is easily transferred to the image carrier, so that the back contamination of the transfer material can be prevented.

[Brief description of the drawings]

FIG. 1 is a schematic side view of a main part of an image forming apparatus suitable for applying the present invention, and FIG. 2 is a diagram showing an image carrier potential, a developing bias, and the like according to the present invention.
6 is a timing chart illustrating a relationship of a transfer bias. 1 image carrier, 2 primary charger, 3 exposure unit, 4
... Developing device, 5... Transfer roller.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Junji Araya 3-30-2 Shimomaruko, Ota-ku, Tokyo Inside Canon Inc. (72) Inventor Yoji Tomoyuki 3-30-2 Shimomaruko, Ota-ku, Tokyo Within Canon Inc. (72) Inventor Hiroki Kisu 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (56) References JP-A-54-114248 (JP, A) JP-A-54-1004836 (JP, A) JP-A-51-9840 (JP, A) JP-A-54-63838 (JP, A)

Claims (7)

(57) [Claims] 1. A movable image carrier, an image forming unit for forming a toner image on the image carrier, a charging unit for charging the image carrier to a first potential, and a charging unit for charging the image carrier. Developing means for developing an electrostatic image of the image carrier with toner having the same polarity as the image forming means; forming a nip portion with the image carrier, and transferring the nip portion from the image carrier at the nip portion A transfer member for transferring the toner image to a material, and applying a voltage having the same polarity as that of the toner image to the transfer member while the transfer material is not present in the nip portion, so as to have the same charge polarity as the toner image. An image forming apparatus including a period for forming an electric field for transferring a polar toner from the transfer member to the image carrier, wherein a potential of a portion of the image carrier in the nip portion is a second potential in the period. The absolute value of the second potential is the first potential And an absolute value of a voltage applied to the transfer member during the period. 2. The image forming apparatus according to claim 1, wherein the second potential has the same polarity as the charging polarity of the charging unit or 0V. 3. A transfer member to which a voltage having a polarity opposite to a charge polarity of the toner image is applied while the toner image is transferred from the image carrier to a transfer material in the nip portion. The image forming apparatus according to claim 1 or 2. 4. The transfer member switches between a voltage having a polarity opposite to a charging polarity of the toner image and a voltage having the same polarity as the charging polarity of the toner image while the transfer material is not present in the nip portion. The image forming apparatus according to claim 3, wherein 5. The image forming apparatus according to claim 1, wherein said second potential is set before passing through a developing position of said developing means. 6. An image forming apparatus according to claim 5, wherein said second potential is set in advance by said charging means. 7. A voltage for preventing toner from adhering to the image carrier when the portion of the image carrier in the nip portion passes through the developing position in advance during the period. The image forming apparatus according to claim 5, wherein