JPH05107951A - Image forming device - Google Patents

Abstract

PURPOSE:To simplify a mechanism for carrying a sheet from a sheet feeding cassette to a transfer area and to facilitate the separation of the sheet from a photosensitive drum by making the diameter of a transfer roller greater than that of the photosensitive drum and applying a voltage to the transfer roller. CONSTITUTION:The sheet sent from the sheet feeding cassette 42 is electrostatically attracted around the periphery of the roller 71 on and on by the application of a voltage to the transfer roller 71, thereby carrying the sheet to the transfer area C. Therefore, the mechanism for carrying the sheet from a sheet feeding part 4 to the transfer area C can be simplified. Additionally, since the outside diameter D of the transfer roller 71 is greater than the outside diameter (d) of the photosensitive drum 31 by the prescribed ratio, the sheet which has passed through the transfer area C is carried along the transfer roller of the greater outside diameter and the separation of the sheet from the photosensitive drum 31 can be stably carried out without a destaticizing means. A voltage application control means 73 starts the application of a voltage when a detection switch 70 detects the front end of the sheet and stops it after the lapse of a prescribed length of time following the passing of the rear end of the sheet through the transfer area.

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 a copying machine or a laser beam printer, and more particularly to an image forming apparatus using a transfer roller.

[0002]

2. Description of the Related Art Conventionally, in an image forming apparatus such as a copying machine, an electric charge having a first polarity is applied to a photosensitive drum and an optical image as image information is projected onto the photosensitive drum to form an electrostatic latent image. After being formed, a toner image is formed with toner having a second polarity (first polarity when reverse development is performed) different from the first polarity, and a toner image is formed between the photoconductor drum and the transfer roller by the sheet conveying means. There is provided a so-called transfer roller system in which a toner image is transferred onto a sheet introduced into the transfer area. The transfer roller has an outer diameter much smaller than that of the photosensitive drum, and a voltage (including ground) having a polarity different from that of the toner is usually applied to the transfer roller so that the transfer roller contacts the paper with the photosensitive drum. A so-called back surface charge is applied to the surface that does not.

In this transfer roller system, it is not necessary to apply an excessive back surface charge to the paper as in the case of transfer by corona discharge, so that the toner on the photosensitive drum is transferred onto the paper in the pre-process and the post-process of the transfer. There is an advantage that it is possible to suppress the generation of an image scattered around the characters due to being attracted to. Further, since the paper is introduced between the photoconductor drum and the transfer roller and comes into contact with the photoconductor drum side with a predetermined pressing force, the paper is reliably conveyed and the transfer misalignment due to the misalignment between the photoconductor drum and the paper is caused. There is an advantage that does not occur.

However, when an image having a relatively low printing rate (a large amount of white paper is formed) is formed mainly when reversal development is performed, after the paper passes through the transfer area, the back surface charge of the paper and the surface charge of the photosensitive drum. There is a problem that the sheet is attracted to the photoconductor drum side by the electrostatic attraction force of, and the separation of the sheet from the photoconductor drum becomes unstable. Therefore, in order to stably separate the sheet, an image forming apparatus is provided in which a charge eliminating needle for eliminating charge on the back surface of the sheet is provided at a position near the downstream side of the transfer roller in the sheet conveying direction (for example, See Kaihei 1-26969).

[0005]

However, since the static elimination needle is made of a conductive rigid body such as SUS, it must be separated from the photoconductor drum by a predetermined distance so as not to damage the photoconductor drum. Therefore, the static elimination needle cannot contact the back surface of the paper reliably, and the static elimination of the paper becomes insufficient,
The paper was sometimes wrapped around the photosensitive drum. Therefore, there remains a problem that the separation of the paper is unstable.

In addition to the static elimination needles, a power source means for applying a bias to the static elimination needles, a large number of pairs of conveying rollers, a conveying guide, etc. for guiding a sheet sent from a sheet feeding cassette or the like to a transfer area. Therefore, there is a problem that the manufacturing cost is high. 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 capable of stably separating a sheet from a photosensitive drum and achieving cost reduction.

[0007]

As an image forming apparatus of the present invention for achieving the above object, an electrostatic latent image formed on a photosensitive drum is visualized as a toner image by a developing device, and then the toner image is formed. In an image forming apparatus that transfers a toner image onto a sheet by a transfer roller, the outer diameter of the transfer roller is set to be the outer diameter of the transfer roller so that the sheet that has passed through a transfer area between the photosensitive drum and the transfer roller is along the transfer roller side. The outer diameter of the transfer roller is larger than the outer diameter of the transfer roller, and a paper feeding unit for feeding the paper and following the predetermined outer circumference of the transfer roller is arranged close to the transfer roller. In order to electrostatically adsorb the sheet of paper to the transfer roller, voltage application to the transfer roller is started at a predetermined timing, and after the trailing edge of the sheet has passed the transfer area between the photosensitive drum and the transfer roller. Time after and is characterized in that a voltage application control means for releasing the voltage application to the transfer roller.

[0008]

In general, a sheet that is conveyed by being sandwiched between two curved surfaces that rotate synchronously, after being passed between the curved surfaces,
It tries to follow the curved surface side with a small curvature. Therefore, in the image forming apparatus having the above-described configuration, the sheet that has passed through the transfer area between the photoconductor drum and the transfer roller is guided to the transfer roller side that is larger than the outer diameter of the photoconductor drum (that is, has a smaller curvature). Can be made.

Further, the voltage application control means starts applying a voltage to the transfer roller to electrostatically attract the paper along the predetermined portion of the outer periphery of the transfer roller to the transfer roller,
It can be transported to the transfer area.

[0010]

Embodiments will be described in detail below with reference to the accompanying drawings showing embodiments. FIG. 1 is a schematic configuration diagram of a copying machine as an image forming apparatus showing an embodiment of the present invention. This copy machine A
An optical system 2 for illuminating and scanning an original placed on the contact glass 11 in the main body 1 of the apparatus to guide the reflected light from the original P to the photosensitive drum 31, and an electrostatic system formed on the photosensitive drum 31. An image forming unit 3 that visualizes the latent image into a toner image by the developing device 33 and then transfers the latent image to a sheet by a transfer roller 71.
A sheet feeding unit 4 for feeding the sheet from the sheet feeding cassette 42 to the image forming unit 3 and a fixing unit 51 for fixing the toner image transferred to the sheet are arranged in the middle, and the transferred sheet is ejected from the sheet ejection unit 54. The paper transport unit 5 is provided.

As the image forming unit 3, the photosensitive drum 31 is used.
A charging charger 32, a developing device 33, a transfer device B, and a cleaner 34 are arranged around the periphery of the above in this order along the rotation direction of the photosensitive drum 31. The transfer device B is
A transfer roller 71 that introduces a sheet into a transfer area C between the photoconductor drum 31, a cleaning device (not shown) that scrapes off the toner adhering to the transfer roller 71, and a transfer voltage is applied to the transfer roller 71. And a voltage application controller 73 for controlling voltage application to the transfer roller 71.

The outer diameter D of the transfer roller 71 is set to be larger than the outer diameter d of the photoconductor drum 31 by a predetermined ratio, whereby the transfer area between the photoconductor drum 31 and the transfer roller 71 is set. The sheet passing through C is arranged along the transfer roller 71 side having a large outer diameter. The ratio between the outer diameter d of the photosensitive drum 31 and the outer diameter D of the transfer roller 71 is appropriately selected according to the material of the transfer roller 71. For example, the transfer whose surface is made of a non-conductive material such as urethane. The roller 71 is set to D / d = 1.5 or more, and the transfer roller 71 made of a conductive material such as silicone rubber is set to D / d = 1.1 or more.

The voltage application control means 73 starts voltage application to the transfer roller 71 when the detection switch 70 provided facing the guide plate 43, which will be described later, detects the leading edge of the sheet and the trailing edge of the sheet. Is configured to release the voltage application after a predetermined time (about several seconds) after passing through the transfer area C. The paper feeding unit 4 is arranged close to the outer circumference of the transfer roller 71, and the paper feeding cassette 4
A sheet feeding roller 41 for feeding out the sheets inside the sheet 2 one by one, and a guide plate 43 for aligning the leading end of the fed sheet with a portion of the outer periphery of the transfer roller 71 which is substantially opposite to the transfer area C are provided.

With the above-mentioned structure, the sheets fed from the sheet feeding cassette 42 are sequentially electrostatically attracted to the outer periphery of the transfer roller 71 by applying a voltage to the transfer roller 71,
It can be transported to the transfer area C. Therefore, the mechanism for transporting the sheet from the sheet feeding unit 4 to the transfer area C can be simplified. Further, since the outer diameter D of the transfer roller 71 is set to be larger than the outer diameter d of the photoconductor drum 31 by a predetermined ratio, the sheet passing through the transfer area C is conveyed along the transfer roller 71 side having the larger outer diameter. As a result, it is possible to stably separate the paper from the photoconductor drum 31 without requiring a charge eliminating unit.

The paper feed section 4 may be one that feeds paper from a manual paper feed tray instead of the paper feed cassette 42, and the paper may be fed inside the apparatus for double-sided copying. Alternatively, the sheet may be sent out from a so-called intermediate tray that temporarily waits.

[0016]

【The invention's effect】 . As described above, according to the image forming apparatus of the present invention, the outer diameter of the transfer roller is set to be larger than the outer diameter of the photosensitive drum, and the sheet passing between the photosensitive drum and the transfer roller is transferred to the transfer roller. Since it is arranged along the side, it is possible to stably separate the sheet from the photosensitive drum with a simple configuration at low cost.

Further, since the paper delivered from the paper feeding unit can be electrostatically attracted to the outer periphery of the transfer roller and can be conveyed to the transfer region, a mechanism for conveying the paper from the paper feeding unit to the transfer region. As a result, the manufacturing cost can be further reduced.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of a copying machine according to an embodiment of the present invention.

[Explanation of symbols]

 4 Paper Feeding Section 31 Photoreceptor Drum 33 Developing Device 71 Transfer Roller 73 Voltage Application Control Unit C Transfer Area

Claims (1)

[Claims] 1. An image forming apparatus for developing an electrostatic latent image formed on a photosensitive drum into a toner image by a developing device, and then transferring the toner image onto a sheet by a transfer roller. In order to pass the paper that has passed through the transfer area between the rollers and the transfer roller side, the outer diameter of the transfer roller is made larger than the outer diameter of the photosensitive drum, and the paper is fed close to the transfer roller. And a paper feed section along a predetermined outer circumference of the transfer roller is arranged, and a voltage applied to the transfer roller at a predetermined timing so that the paper along the predetermined outer circumference of the transfer roller is electrostatically attracted to the transfer roller. Provided is a voltage application control means for starting the application and releasing the voltage application to the transfer roller a predetermined time after the trailing edge of the paper passes through the transfer area between the photosensitive drum and the transfer roller. Image forming apparatus characterized by.