JPH05333720A - Image forming device - Google Patents

Abstract

PURPOSE:To prevent toner from adhering to a transferring roller, whether the polarity of the toner is positive or negative, by making an electric circuit from the surface of a photo conductive layer to a base substance a nearly insulating state other than a time when a transferring bias is impressed on the transferring roller. CONSTITUTION:The toner 8 discharged form a developing device 6 is electrostatically charged, it is considered to be equivalent to charge, and a current flows when the toner moves. The toner can not move when the circuit of the current at the time is broken. Since the time when the image carrier 1 is brought into contact with a certain surface of the transferring roller 9 is normally about 0.1 seconds, the integral value of the current flowing in limited time becomes small so that the movement of the toner or the amount of that may be limited by making the flow of the current difficult. Normally, the transfer roller 9 is connected with a bias power source grounded, and the conductive base substance 2 of the image carrier 1 is also grounded. The toner is prevented from adhering to the roller by insulating the transferring roller 9 against a ground plane other than the time when transferring is performed, or performing grounding with a value which is equal to or above a specified electric resistance value (for example, 10<5>OMEGA) at such a time.

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 for forming an image based on the electrophotographic principle, and more particularly to an image forming apparatus using a contact type transfer device.

[0002]

2. Description of the Related Art In a conventional contact type transfer device, as disclosed in JP-A-51-9840, a transfer material is passed between a moving latent image carrier and a transfer roller opposed thereto. When a transfer material is present between the image carrier and the transfer roller, a transfer bias voltage having a polarity opposite to that of the developing toner is applied to the transfer roller to transfer the developed image on the latent image carrier to the transfer material. When the transfer material is not present, a cleaning bias having the same polarity as that of the toner is applied to prevent the toner from adhering to the transfer roller, or the toner is more positively returned to the image carrier.

Further, as one method of development, there is contact development or pressure development in which toner is constantly in contact with the image carrier. In particular, the one-component pressure contact development method aiming at simplification of the developing device, stabilization of the development characteristics, and high resolution of the image has been practically applied in recent years.

[0004]

When the toner adheres to the surface of the transfer roller, the transfer material becomes insufficient due to the back stain on the transfer material, the stain in the fixing process caused by the stain, and the increase in the transfer roller surface resistance, resulting in insufficient transfer electric field. The image quality such as a defect has deteriorated.

FIG. 2 is a charge amount distribution chart of a typical negatively chargeable toner in one-component development. 2A shows an example in which the distribution peak has a negative polarity, but the distribution also spreads to the positive polarity side, and FIG. 2B shows an example in which both the negative polarity and the positive polarity have peaks. ing. These are negatively charged as a whole, but when attention is paid to individual toners, there are positively charged toners. In pressure contact development, the toner is always pressed against the image carrier, and both the positive polarity toner and the negative polarity toner adhere to the surface of the image carrier, although the amount is very small.

FIG. 3 is a diagram showing the relationship between the bias voltage value and the toner development amount when a bias voltage is applied between the image carrier and the developing device using a one-component pressure contact developing device. From this, it can be seen that the toner is developed if there is a potential difference between the image carrier and the developing device. Depending on the potential difference, the amount of development can be made extremely small, but in many cases the surface potential of the image carrier is not uniform during the non-transfer period, and a small amount of toner is developed on the image carrier, which may cause Is mainly developed with positively charged toner.

In the above-mentioned prior art, the polarity of the cleaning bias applied to the transfer roller is described. For example, in the case of performing reversal development in which negatively charged toner is developed on a negatively chargeable image carrier, negatively charged toner is negatively charged by applying the same polarity to the transfer roller, that is, a negative bias. It was possible to prevent the toner from adhering to the surface of the transfer roller. However, in contact development or pressure contact development, especially in the one-component pressure contact development method,
As described above, the positively charged toner is often present as the background fog toner on the image carrier even when the image is not formed.By applying the negative cleaning bias, the positively charged toner is attracted to the transfer roller surface. There was a problem that caused it.

Further, in order to apply the cleaning bias, a power source having a polarity opposite to that of the transfer bias applying power source is required, which causes an increase in size of the apparatus and an increase in cost.

The present invention solves such a problem, and an object of the present invention is to provide a smaller and cheaper image forming apparatus capable of obtaining stable high image quality for a long period of time.

[0010]

An image forming apparatus of the present invention has an image carrier having a photosensitive layer formed on a conductive substrate and a contact type transfer means having a conductive layer formed on the conductive substrate. ,
An image forming apparatus that transfers and fixes a developed toner image on an image carrier onto a transfer material, wherein the surface of the conductive layer of the contact-type transfer means passes through a conductive substrate outside the period of transferring the developed toner image to the transfer material. It is characterized in that the electric circuit of the image carrier to the conductive substrate has a period of being in a substantially insulating state. Further, in the above-mentioned substantially insulating state, the resistance value of the electric circuit from the surface of the conductive layer of the contact type transfer means to the conductive base of the image carrier through the conductive base is 10 ⁵ Ω or more.

Further, the above-mentioned period of being in the substantially insulating state is characterized in that it is all outside the period of transferring the developed toner image onto the transfer material.

The present inventor pays attention to the fact that a current flows between the image carrier and the transfer roller during the transfer, and makes it difficult for the current to flow between the transfer roller and the image carrier during the non-transfer, so that It has been found that toner can be prevented from adhering to the transfer roller.

The toner that comes out of the developing device is charged and can be regarded as equivalent to the electric charge. Therefore, when the toner moves, a current flows. If the circuit through which the current flows at this time is cut off, the toner cannot move. In addition, the time during which the image carrier and any surface with the transfer roller are in contact is usually 0.1.
Since it is a finite time of about a second or less, by making it difficult for the current to flow, the integrated value of the current flowing within the finite time also becomes small, and the movement of toner or the amount of moved toner can be limited. Usually, the transfer roller is connected to a bias power source which is grounded, and the conductive substrate of the image carrier is also grounded. Therefore, the above-described action is exhibited by insulating the transfer roller from the ground surface except when transferring or by grounding the transfer roller at a predetermined electric resistance value or more. As a result, it is possible to prevent both positive polarity toner and negative polarity toner from adhering to the transfer roller, and a power supply for a cleaning bias is not required, which facilitates downsizing of the apparatus and cost reduction.

[0014]

【Example】

(Embodiment 1) FIG. 1 is a sectional view of a main part of an image forming apparatus according to an embodiment of the present invention. The image carrier 1 has a photosensitive layer 3 made of an organic photoconductor formed on a conductive substrate 2 made of an aluminum drum. The image carrier 1 is initially charged by a charger 4 composed of a corona discharger. Exposure device 5
The toner 8 is developed on the electrostatic latent image formed by the developing device 6 of the one-component pressure contact developing system, and the toner image is transferred to the recording paper 14 conveyed between the image carrier 1 and the transfer roller 9. The toner 8 is conveyed by the toner carrier 7 made of a conductive elastic roller, and the toner carrier 7 is in pressure contact with the image carrier 1. The transfer roller 9 is a metallic shaft 10.
The conductive layer 11 was formed on the conductive elastic body to form a roller. As the material of the conductive layer 11, for example, conductive rubber can be used, but foamed silicone, foamed urethane, etc. are preferable. In this embodiment, conductive urethane foam is used for the conductive layer 11, and the electric resistance value between the shaft 10 and the surface of the conductive layer 11 is set to 10 ³ Ω. The transfer roller 9 is brought into pressure contact with the image carrier 1 by a pressure device 12 formed of a coil spring. Further, the transfer roller 9 has a shaft 10 connected to a grounded bias control device 13 including a bias power source, and a transfer bias can be applied. After the transfer, the surface of the image carrier 1 is cleaned by the cleaner 15 and discharged by the charge eliminator 16. The static eliminator 16 may be omitted.

The charge amount distribution of the toner 8 of this embodiment is shown in FIG.
It is the same as (a), and the developing characteristics by the developing device 6 are the characteristics shown in FIG. Further, the bias control device 1
A power source for disconnecting an electric circuit with the ground plane was used as the element 3 except when the transfer bias was applied. Therefore, there is no circuit through which current flows between the transfer roller 9 and the image carrier 1 except during transfer.

The image carrier 1 is charged by the charger 4 during image formation.
The initial charging potential was set to -600V, the developing bias applied to the developing device 6 was set to -250V, and the transfer bias applied to the transfer roller 9 was set to + 500V, and 5,000 sheets of images were formed. As a result, toner adhesion was not recognized on the surface of the transfer roller 9, back stain of the recording paper 14 did not occur, and good printing without transfer failure was obtained.

Further, since the cleaning bias is not applied, the power source for the cleaning bias, which has been required in the past, is unnecessary, and the apparatus can be downsized and the cost can be reduced. (Embodiment 2) In the same image forming apparatus as in Embodiment 1, the transfer roller 9 has an electric resistance value of 10 ⁵ Ω and a transfer bias of +.
It was set to 900V. The bias control device 13 used is one whose output terminal is not insulated from the ground plane except when the transfer bias is applied. Of course, it may be insulated. Therefore, the resistance value of the electric circuit from the surface of the transfer roller 9 to the shaft 10, the bias control device 13, the ground plane, and the conductive substrate 2 is always 10 ⁵ Ω or more.

The initial charging potential of the image carrier 1 by the charger 4 is -600 V, and the developing bias applied to the developing device 6 is-.
An image was formed on 5000 sheets by setting the voltage to 250V. As a result, toner adhesion was not recognized on the surface of the transfer roller 9, back stain of the recording paper 14 did not occur, and good printing without transfer failure was obtained.

Comparative Example 1 In the same image forming apparatus as in Example 1, the transfer roller 9 had an electric resistance value of 10 ⁴ Ω and a transfer bias of + 700V. Further, the bias control device 1
For No. 3, the output terminal of which was not insulated from the ground plane except when the transfer bias was applied was used. From the surface of the transfer roller 9 to the shaft 10, the bias control device 13,
The resistance value of the electric circuit reaching the ground plane and the conductive substrate 2 was approximately 10 ⁴ Ω.

The initial charging potential of the image carrier 1 by the charger 4 is -600 V, and the developing bias applied to the developing device 6 is-.
An image was formed on 5000 sheets by setting the voltage to 250V. As a result, toner adhesion was recognized on the surface of the transfer roller 9, and the image density was slightly lowered due to transfer failure. When the transfer bias was set to +800 V, the initial image density was obtained.

[0021]

As described above, according to the present invention, toner adhesion to the transfer roller can be prevented without applying a cleaning bias. In particular, it is suitable for an image forming apparatus that performs contact or pressure contact type development. Of course, the same effect can be obtained with a non-contact developing type image forming apparatus.

Further, a power supply for applying a cleaning bias is not required, the image forming process sequence is simplified, and the size and cost can be easily reduced.

[Brief description of drawings]

FIG. 1 is a sectional view of a main part of an image forming apparatus according to an embodiment of the present invention.

FIG. 2 is a charge amount distribution chart of toner for one-component development.

FIG. 3 is a relationship diagram of a developing bias and a developing amount of a one-component developing device.

[Explanation of symbols]

 1 Image Carrier 2 Conductive Substrate 3 Photosensitive Layer 6 Developing Device 7 Toner Carrier 8 Toner 9 Transfer Roller 10 Shaft 11 Conductive Layer 12 Pressurizer 13 Bias Control Device 14 Recording Paper

Claims (3)

[Claims] 1. An image carrier having a photosensitive layer formed on a conductive substrate, and a contact-type transfer means having a conductive layer formed on the conductive substrate, wherein a developed toner image on the image carrier is transferred. In an image forming apparatus for transferring to a material, outside the period of transferring the developed toner image to the transfer material, from the surface of the conductive layer of the contact type transfer means to the conductive base to the conductive base of the image carrier. An image forming apparatus having a period in which an electric circuit leading to is substantially insulated. 2. The resistance value of an electric circuit from the surface of the conductive layer of the contact type transfer means to the conductive base of the image carrier in the substantially insulating state through the conductive base,
The image forming apparatus according to claim 1, wherein the image forming apparatus has a resistance of 10 ⁵ Ω or more. 3. The image forming apparatus according to claim 1, wherein the period of being in the substantially insulating state is the entire period other than the period of transferring the developing toner image to the transfer material.