JPH09222807A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve a transfer efficiency at the time of transferring colors in an overlapping manner after passing the printing station by eliminating excessive electric charges applied to the toner of a recording medium passing the printing stations immediately before or after the transfer position of the printing stations positioned in the middle in a simple constitution. SOLUTION: In an image forming device, printing stations 21-24 for applying electric charges to the surface of a photoreceptor drum 3, forming an electrostatic latent image by exposure and developing the electrostatic latent image into a visible image are disposed on an endless belt-like medium transport belt, and the toner images developed into visible images by the printing stations are transferred in an overlapping manner on a recording paper sheet 18 transported by a paper transport belt. In this case, in the intermediate printing stations 22, 23 of the printing stations, a conductive film 24a for absorbing excessive electric charges on the transported recording paper sheet is disposed opposite to the paper transport belt in the upstream side part and the downstream side part of the transfer positions thereof.

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 printer which transfers images at a plurality of printing stations in an overlapping manner.

[0002]

2. Description of the Related Art As an image forming apparatus of this type, for example, in a color laser printer, yellow (Y), magenta (M), cyan ( It is configured by arranging four printing stations of C) and black (K) in succession. Each printing station includes an electrophotographic process mechanism including a photosensitive drum as an electrostatic latent image holding member, a charging unit, an exposure device including a laser diode, a developing device, and the like. Further, a transfer unit is provided at a position facing each printing station via a paper transport belt. By printing on the sheet conveyed by the sheet conveying belt by each of the printing stations, the respective colors are superimposed to form a color image.

Specifically, for example, in the case of the reversal developing method, the photosensitive drum is rotated in one direction, and the surface of the photosensitive drum is uniformly charged to about 680V by the charging section. Next, the exposure device irradiates the surface of the photosensitive drum with light based on the print data. As a result, the potential of the portion irradiated with light, that is, the portion of the printing portion that becomes characters and the like, is about 10
It drops to 0V. Subsequently, when the charged toner is supplied by the developing device, the toner adheres to the portion where the surface potential of the photosensitive drum is lowered to about 100V.

In this state, when the recording sheet is conveyed to a position where the photosensitive drum and the transfer portion face each other, a transfer electric field which is a potential difference between the transfer voltage of about -1000 V applied to the transfer portion and the surface potential of the photosensitive drum described above is generated. It is considered that, depending on the strength, electric discharge occurs between the surface of the photosensitive drum and the recording sheet on the upstream side of the transfer position. Due to the transfer electric field, the toner attached to the photosensitive drum is attracted and attached to the recording sheet. As a result, the transfer process is completed, and then the recording paper to which the toner is attached is sequentially conveyed to the next printing station, and the same printing process as described above is performed. In this way, the toner of each color is transferred onto the recording paper in an overlapping manner to form a color image.

By the way, in such an apparatus, depending on the color to be printed, it may not be necessary to perform transfer by the printing station located in the middle among the printing stations arranged in parallel. For example, in the case of printing green, only transfer by the yellow (Y) printing station located at the most upstream side and the cyan (C) printing station located at the third from the upstream side is required, and the transfer between these is performed. No transfer from the magenta (M) printing station located at. Therefore, in this case, the recording paper transferred at the yellow (Y) printing station remains attached to the photosensitive drum of the magenta (M) printing station while the yellow toner, which is positively charged with a constant charge amount, remains attached. It is conveyed to the cyan (C) printing station through the space between the transfer units.

At this time, since printing is not performed in the magenta (M) printing station, when the recording paper is inserted between the magenta (M) photosensitive drum and the transfer roller, the yellow toner of the recording paper is further added. Due to the discharge, the yellow toner is more strongly positively charged, and the amount of charge is increased. Therefore, at the time of transfer at the next cyan (C) printing station, since the yellow toner on the recording paper is positively overcharged, the transfer electric field for transferring the cyan toner from the photosensitive drum becomes weaker, and the cyan toner is transferred. There is a problem that transfer failure occurs.

In order to solve this problem, conventionally, for the purpose of preventing an increase in toner charge amount, a bias is applied by corona charging, brush charging, etc., or a charge eliminating needle or charge eliminating brush is installed and after transfer by each printing station. , Or immediately before the next transfer position.

[0008]

However, in the image forming apparatus for applying the bias such as the corona charging and the brush charging as described above, since a new power supply for supplying electric power to these is required, the parts are not required. There is a problem that it leads to an increase in the number of points and cost, and there is also a problem that the size of the entire device cannot be reduced because it is necessary to secure a space for installing these parts.

Further, in the case where the static elimination needle or the static elimination brush is installed, it is not necessary to provide a power source for supplying electric power, but since the tips of the static elimination needle and the static elimination brush are sharp, these static elimination needles are formed. There was a problem that intensive transfer of charges to the static elimination brush occurred. Further, since there is no limitation on the amount of mobile charge, there is a problem in that the adhesion between the recording paper and the toner is weakened and the toner image is disturbed.

Therefore, according to the present invention, it is possible with a simple structure to eliminate the excess electric charge charged on the toner of the recording medium passing immediately before or immediately after the transfer position of the intermediate printing station, and after the passing, the colors are transferred in a superimposed manner. It is an object of the present invention to provide an image forming apparatus capable of improving the transfer efficiency when performing.

[0011]

According to the present invention of claim 1, the surface of an electrostatic latent image holding member is charged, an electrostatic latent image is formed by exposure, and the electrostatic latent image is visualized. In an image forming apparatus, at least three printing stations are arranged on an endless belt-shaped medium conveying belt, and a toner image visualized by the printing station is transferred onto a recording medium conveyed by the medium conveying belt in an overlapping manner. Of all the intermediate printing stations, at least one site on the upstream side and the downstream side of the transfer position of the transfer position,
In this configuration, a plate-shaped conductive member that absorbs excess electric charge on the conveyed recording medium is disposed so as to face the medium conveying belt.

According to a second aspect of the present invention, there is provided charge detecting means for detecting extra charge on the recording medium absorbed by the conductive member,
A charge absorption amount changing means for changing the charge absorption amount of the conductive member is provided, and the charge absorption amount changing means is controlled according to the charge absorption amount by the charge detecting means to change the charge absorption amount of the conductive member to thereby change the recording medium. The upper charge amount is kept constant.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention applied to a color laser printer will be described below with reference to the drawings. FIG. 1 is a cross-sectional view showing the structure of a color laser printer according to the present embodiment. Numerals 21 to 24 are arranged continuously with a sheet conveying belt 1 as an endless belt-shaped medium conveying belt stretched inside the apparatus main body. There are four printing stations.

Each of the printing stations 21 to 24 is provided with a charger 4, an exposure device 5, a developing device 6, around a photosensitive drum 3 as an electrostatic latent image holding member composed of a single-layer photoconductor for positive charging.
The drum cleaner 7 and the static elimination lamp 8 are arranged to face each other, and the transfer roller 9 is also arranged to face the photosensitive drum 3 via the paper transport belt 1.

The charge eliminating lamp 8 referred to here is for eliminating the charge once to improve the charging effect before the surface of the photosensitive drum 3 whose toner has been scraped off by the drum cleaner 7 is uniformly charged again by the charger 4. It is a thing. Therefore, its purpose is different from that of the static eliminator 24 described later.

The developing device 6 of each of the printing stations 21 to 24 stores yellow (Y), magenta (M), cyan (C), and black (K) toners, and the developing roller 10 is rotatably provided at the bottom. The toner is supplied to the photosensitive drum 3 by the contact charging. The transfer roller 9 is made of conductive urethane rubber.

A belt charging device 11 is arranged at a conveyance start position of the sheet conveying belt 1, and a tension roller 12 for giving a predetermined tension to the sheet conveying belt 1 is arranged. The sheet conveying belt 1 is made of a conductive material, and its surface is covered with an insulating layer. For example, it is made of semi-conductive polyimide. Then, the conductive material is stretched around the pair of guide rollers 14 and 15 while maintaining a predetermined tension so as to come into contact with each other.

Below the paper conveying belt 1, a paper feeding device 19 in which recording papers 18 are stacked and accommodated is detachably provided. A paper feed roller 23 for picking up the recording paper 18 and a conveying roller for guiding the recording paper 18 to the conveying path 13 are provided above the paper feeding device 19. A suction roller is provided on the upstream side of the paper transport belt 1 to electrostatically attract the recording paper 18 guided to the paper transport belt 1 so as not to shift it.

In the vicinity of one guide roller 14, a conveying path 13 for guiding the recording paper 18 from the paper feeding device 19 to the end of the paper conveying belt 1 is provided. Further, the fixing device 20 is arranged close to the outside of the other guide roller 15.

In this apparatus, after the recording paper 18 fed from the paper feeding device 19 is adsorbed on the paper conveying belt 1 by the adsorbing roller, the yellow (Y) image is transferred at the printing station 21 and the magenta image is transferred at the printing station 22. Image transfer is performed, and the cyan image is transferred at the printing station 23 to perform color printing by superimposing colors.
The fixing device 20 thermally fixes and discharges the sheet to the sheet discharge unit 21 or 22.

Of the printing stations 21 to 24,
All the intermediate printing stations 22 and 23 except the printing stations 21 and 24 located at both ends are provided with a neutralization device 24 as shown in FIG. Note that, in FIG. 2, the exposure device 5, the developing device 6, and the like are not shown for convenience of description.

The static eliminator 24 is connected to the conductive film 24a as a conductive member provided on the upstream side and the downstream side of the position (transfer position) where the photosensitive drum 3 and the transfer roller are opposed to each other. Electrometer 24b as a charge detecting means for measuring the amount of charge on the recording paper 18, and the amount of movement of the charge from the recording paper 18 to the conductive film 24a is changed according to the measurement result from the electrometer 24b. A resistance circuit 24c is provided as a means for varying the amount of absorbed charge.

The conductive film 24a has a predetermined area facing the upper surface of the sheet conveying belt 1 along the longitudinal direction of the photosensitive drum 3 and is formed in a plate shape. The conductive film 24a is for absorbing the excess electric charge charged on the recording paper 18 passing through the printing stations 22 and 23.

FIG. 3 is a block diagram showing the circuit configuration of the color laser printer shown in FIG. 1. 25 is a CPU (central processing unit) that constitutes the main body of the control unit, and 26 is this CPU 25.
Is a ROM (read only memory) storing program data for controlling each part, 27 is a RAM (random access memory) provided with a memory used for data processing, and 28 is an I / O memory. An O (input / output) port, and 29 is an I / F (interface). CP above
U25, ROM26, RAM27, I / O port 28,
The I / F 29 is electrically connected by the bus line 31.

The I / O port 28 is connected to the printing stations 21 to 24, the fixing device 20, the electrometer 24b and the resistance circuit 24c of the static eliminator 24, and a jam of the recording paper 18 (not shown) is connected. Various sensors such as a sensor for detecting and an operation panel for an operator to perform various operations are connected. A host computer 34 is connected to the I / F 29 via a communication line.

When the printing process is performed, the CPU 25 appropriately performs the static elimination process shown in FIG. That is, the CPU 25
Is a magenta (M) printing station 2 during the printing process.
2 and the cyan (C) printing station 23, the amount of current flowing from the recording paper 18 through the conductive film 24c of the static eliminator 24 is measured by the electrometer 24b.
The measurement value is stored in the RAM 27. Then, this measured value is compared with a predetermined value of the preset current amount.

At this time, when it is determined that the measured value of the amount of current flowing from the recording paper 18 exceeds the predetermined value, the resistance circuit 24c of the static eliminator 24 is controlled to control the conductive film 24.
The amount of current flowing into a is suppressed. As a result, the toner on the recording paper 18 always has an appropriate amount of charge.

In the embodiment of the present invention having such a structure, when printing from the intermediate printing stations 22 and 23 is unnecessary, for example, when printing green, the recording carried by the paper carrying belt 1 is performed. The paper 18 is first transferred from the yellow (Y) printing station 21. Then, the yellow toner on the recording paper 18 is charged with a constant charge amount. Thereafter, the recording paper 18 to which the yellow toner has been transferred is further conveyed and passes through the magenta (M) printing station 22.

When passing the position (transfer position) facing the magenta (M) transfer roller 9, the recording paper 18 is first placed under the conductive film 24a of the static eliminator 24 provided on the upstream side of the paper transport belt 1. Pass through. Then, the electric charge charged on the yellow toner on the recording paper 18 is removed by the static eliminator 24.
Then, the excess charges are removed by gradually flowing into the conductive film 24a. Further, when the recording paper 18 passes through the magenta (M) transfer position, discharge is generated, and the amount of charge of the yellow toner on the recording paper 18 increases according to the strength of the transfer electric field. However, the conductive film 2 of the static eliminator 24, which is further transported and provided on the downstream side of the sheet transport belt 1.
After passing under 4a, the charges increased by the discharge on the recording paper 18 flow into the conductive film 24a and the excess charges are removed.

As a result, even if discharge occurs in the magenta (M) printing station 22, the recording paper 18 can pass through the magenta (M) printing station 22 without increasing the amount of yellow toner charge. . For this reason, the transfer efficiency at the next cyan printing station 23 can be improved, and the image disturbance can be prevented.

The amount of charges flowing into the conductive film 24a is measured by the electrometer 24b and stored in the RAM 27. When the value measured by the electrometer 24b exceeds a predetermined value, the resistance circuit 2
4c is controlled, and the amount of charges flowing into the conductive film 24a is limited. As a result, the charge amount of the recording paper 18 can always be maintained at a certain value or more. Therefore, the yellow toner on the recording paper 18 can be prevented from peeling off.

Further, since the static eliminator 24 is composed of the plate-shaped conductive film 24a having a constant area, the charges on the recording paper 18 gradually flow into the conductive film 24a. Therefore, it is possible to prevent the concentrated movement of electric charges that would occur when a conventional charge elimination needle or charge elimination brush is provided.

Further, since the above effect can be achieved only by providing the static eliminator 24 having a simple structure provided with the conductive film 24a, a new addition which is required when a bias such as corona charging or brush charging is applied as in the conventional case. The power supply can be eliminated.

It should be noted that when the amount of charge movement measured by the electrometer 24a is small, the toner charge amount on the recording paper 18 is such that it does not adversely affect the transfer of colors in a superimposed manner. Does not mean

[0035]

As described in detail above, according to the present invention, a plate-shaped conductive member is provided at at least one of the upstream side and the downstream side of the transfer position of the transfer position in the printing station located at the intermediate portion. With a simple configuration that can be provided, it is possible to eliminate the excess electric charge charged to the toner of the recording medium passing immediately before or immediately after the transfer position, and to improve the transfer efficiency when transferring the color after that. You can
As a result, image distortion can be suppressed. Furthermore, since the conductive member is formed in a plate shape, it is possible to prevent the concentrated movement of toner charges.

Further, the charge absorbed by the conductive member is detected, and the charge absorption amount of the conductive member is changed according to the detected amount, so that the charge on the recording medium passing through the printing station located at the intermediate portion is charged. The amount can always be kept above a certain level. As a result, it is possible to prevent an excessive amount of electric charge from being absorbed and toner that has already adhered to be separated from the recording medium.

In the above embodiments, the case where the present invention is applied to the color laser printer has been described, but the present invention is not necessarily limited to this and may be applied to a color copying machine or the like.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing a main configuration of a color laser printer according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating a configuration of a printing station located in the middle shown in FIG.

FIG. 3 is a block diagram showing a configuration of a control unit of the color laser printer shown in FIG.

FIG. 4 is a flowchart showing static elimination processing performed by the CPU shown in FIG.

[Explanation of symbols]

 1 ... Paper transport belt (medium transport belt) 21 to 24 ... Printing station 3 ... Photosensitive drum (electrostatic latent image carrier) 9 ... Transfer roller 18 ... Recording paper (recording medium) 24 ... Static erasing device 24a ... Conductive film ( Conductive member) 24b ... Electrometer (charge detection means) 24c ... Resistor circuit (charge absorption amount varying means)

Claims (2)

[Claims] 1. A printing station for charging a surface of an electrostatic latent image carrier, forming an electrostatic latent image by exposure, and visualizing the electrostatic latent image at least on an endless belt-shaped medium carrying belt. In an image forming apparatus in which three or more are arranged and the toner images visualized by the printing station are superposed and transferred onto a recording medium conveyed by the medium conveying belt,
A plate-shaped conductive material that absorbs excess charges on the recording medium that has been conveyed to at least one of upstream and downstream sides of the transfer position of the transfer position in all of the intermediate printing stations among the printing stations. An image forming apparatus, wherein a member is disposed so as to face the medium carrying belt. 2. A charge detection means for detecting a charge absorbed by the conductive member and a charge absorption amount changing means for changing a charge absorption amount of the conductive member are provided, and a charge absorption amount by the charge detection means is set. 2. The image forming apparatus according to claim 1, wherein the charge absorption amount varying means is controlled accordingly to change the charge absorption amount of the conductive member to maintain the charge amount on the recording medium at a constant amount.