JP3090210B2 - Image forming device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention is used in many image forming apparatuses such as electrophotographic copying machines and printers, and performs processing necessary for a developed image formed on a latent image carrier. More particularly, the present invention relates to an image forming apparatus provided with a transfer process device for processing and simultaneously transferring a plurality of developed images formed on a latent image carrier. Things.

[Prior Art] Conventionally, there is an electrophotographic copying machine using the above-described transfer process apparatus as shown in FIG. That is, the electrophotographic copying machine reads an image of a document to be copied including color information by a document reading device (not shown),
Once converted to an electrical signal. Then, after performing predetermined image processing on the image information converted into the electric signal as necessary, the laser scanning unit 1000 (hereinafter referred to as RO
S (abbreviation of Raster Output Scaner)), and is converted into an optical signal again.
Scanning exposure is performed at the two exposure positions 1002 and 1003.

At this time, as shown in FIG. 70 (a), the surface of the photoreceptor drum 1001 is previously charged to a predetermined negative voltage by a primary charger 1004, and then the first and the second images are formed by the ROS 1000 as described above. Scanning exposure is performed at second exposure positions 1002 and 1003 to form an electrostatic latent image on the surface.

At the first exposure position 1002, exposure is performed by image writing for exposing only the image portion, and a first electrostatic latent image is formed as shown in FIG. 70 (b). Then, as shown in FIG. 70 (c), the first electrostatic latent image is subjected to reversal development by the first developing device 1005,
For example, develop to red. Thereafter, at a second exposure position 1003, exposure is performed by background writing for exposing a background portion, and as shown in FIG.
Is formed. Then, as shown in FIG. 70 (e), the second electrostatic latent image is developed into a normally black color by the second developing device 1006 by the regular developing method.

Next, the toner images developed by the first and second developing devices 1005 and 1006 are charged to one polarity by a pre-transfer charger 1007 as shown in FIG. The polarity of the toner images developed by the second developing devices 1005 and 1006 is made uniform.

Thereafter, the two toner images formed on the photosensitive drum 1001 are transferred onto a transfer paper 1008 of a predetermined size supplied from a paper feed cassette as shown in FIG. 70 (g).
The image is transferred from the photosensitive drum 1001 by the charging of 1009. Then, the transfer paper 1008 to which the toner image has been transferred is separated from the photosensitive drum 1001 by removing the charge of the separation charger 1010, and then conveyed to a fixing device (not shown) to transfer the two-color toner image to the transfer paper. Fixes simultaneously on 1008.

[Problems to be Solved by the Invention] However, the above-described conventional technology has the following problems. That is, in the above-mentioned electrophotographic copying machine, at the time of two-color simultaneous image copying, two-color toner images having different polarities are simultaneously formed on the photosensitive drum 1001, and the two-color toner images are charged by the transfer charger 1009. Since the image is transferred onto the transfer paper 1008 and the two-color image is copied simultaneously, the two-color toner formed on the surface of the photosensitive drum 1001 before the transfer of the toner image by the transfer charger 1009 is performed. The polarity of the image needs to be adjusted by charging the pre-transfer charger 1007. Therefore, the pre-transfer charger 1007 performs discharge of the same polarity as the polarity of one of the two color toner images formed on the photosensitive drum 1001, and inverts the polarity of the other toner image. The charge amount of one toner image is increased.

At that time, the toner image whose polarity is reversed is
Since the charge of the original polarity is canceled by the charge of 1007 and the charge of the opposite polarity is further applied, it is difficult to invert the polarity of this toner image efficiently and reliably. Therefore, when the toner image after the polarity inversion is transferred onto the transfer paper 1008 by charging the transfer charger 1009,
There is a problem that transfer unevenness, image omission, and the like occur in the toner image whose polarity has been inverted, and this is particularly remarkable in the case of a solid image.

In the case of a toner image in which the polarity is reversed, if the particle size of the toner is locally large, a gap is formed between the transfer paper 1008 and the photosensitive drum 1001 around the particle, and the transfer electric field does not act sufficiently. Of the toner image developed on the body drum 1001, the toner having a large particle diameter and the toner around the toner remain without being transferred onto the transfer paper 1008 and remain in a granular form, and a white granular image on the transfer paper 1008 in a solid image or the like. There is a problem that a dropout occurs.

On the other hand, the toner image whose polarity is not inverted is
07, the toner image is charged with the same polarity as the charged polarity of the toner image.
There is a problem that a so-called blur phenomenon occurs in which the charge amount of the toner is excessively increased due to the charging of the toner and the toner is disturbed.

The problem that arises when the polarities of these two-color toner images are made uniform is to directly lower the image quality of the copied image, and therefore becomes a serious problem in a copying machine or the like having a two-color simultaneous copying function. I have.

Further, in the above-described electrophotographic copying machine, when performing two-color simultaneous image copying, transfer characteristics differ depending on which polarity of the two-color toner image is inverted by charging of the pre-transfer charger 1007. There was a problem that it would. That is, the charging characteristics of the toner that is subjected to the pre-transfer charging by the pre-transfer charger 1007 are determined by the dielectric constant and particle diameter of the toner itself. And charging characteristics are also different. For this reason, there is a problem that the occurrence state of image disorder (blurred image) or transfer failure changes depending on which polarity of the toner image is inverted, that is, the polarity of the charging polarity of the pre-transfer charger 1007. .

Furthermore, the electrophotographic copying machine is provided with two developing devices 1005 and 1006 around the photosensitive drum 1001 for performing two-color simultaneous image copying and the like, and the pre-transfer charger 1007 includes:
Developing device 1006 around photoconductor drum 1001 and transfer charger 1009
The space required for installation is very small. Therefore, in the pre-transfer charger 1007, the distance between the discharge wire and the shield is inevitably reduced, so that abnormal discharge is easily generated between the discharge wire and the shield, and the discharge wire may be cut due to spark discharge. There was a problem that there is. In particular, when an opening is provided on the rear surface of the shield as necessary, spark discharge or the like is likely to occur between the edge of the opening and the discharge wire.

The pre-transfer charger 1007 includes a second developing device 1006
, The toner is liable to be contaminated by the toner scattered from the second developing device 1006, and the toner adheres to a discharge wire or the like to cause discharge unevenness, which causes a problem that unevenness occurs in a transferred image. In addition, there is a problem that it is difficult to maintain a predetermined charging characteristic of the pre-transfer charger 1007 for a long period of time.

Further, when a scorotron having a grid is used as the pre-transfer charger 1007, toner having a polarity opposite to the voltage applied to the grid tends to adhere to the grid, and the grid is quickly stained by the toner, and a predetermined discharge is caused. There was also a problem that the characteristics could not be maintained.

Further, in the above-described electrophotographic copying machine, when the transfer paper 1008 on which an image is once formed is transported again to the image forming section to perform double-sided copying or multiplex copying, the transfer paper 1008 is transported to the leading and trailing edges of the transfer paper 1008. Transfer paper is easy to be formed due to curving.
There was also a problem in that image missing or retransfer occurred at the leading and trailing edges of 1008.

Means for Solving the Problems Accordingly, the present invention has been made to solve the above-mentioned problems of the prior art. The first object of the present invention is to perform two-color simultaneous copying. It is an object of the present invention to provide an image forming apparatus capable of efficiently and reliably transferring a two-color developed image and transferring a high-quality image.

It is a second object of the present invention to provide an image forming apparatus which clarifies conditions for reversing the polarity of toner in consideration of the charging characteristics and the like of the toner.

Still another object of the present invention is to provide an image forming apparatus capable of preventing occurrence of abnormal discharge in a pre-transfer charger.

That is, the invention described in claim 1 of the present invention is:
A latent image carrier whose surface is uniformly charged to a predetermined polarity and potential by primary charging means; and a surface of the latent image carrier is exposed to a specific one-color image by an image portion exposure method, and is electrostatically charged. A first image exposing unit for forming a latent image, and an electrostatic latent image formed on the latent image carrier by the first image exposing unit, and a specific image having the same polarity as the charging polarity of the latent image carrier. A first developing unit for reversal developing with a color toner of a color, and a black image continuously exposed on a surface of the latent image carrier on which a specific one color toner image is formed by the first developing unit. Exposure method to form an electrostatic latent image
Image developing means, and second developing for normally developing the electrostatic latent image formed on the latent image carrier by the second image exposing means with black toner having a polarity opposite to the charging polarity of the latent image carrier. Means for transferring the two toner images having different polarities formed on the latent image carrier onto a transfer material before transferring the polarity of the specific one color toner image formed on the latent image carrier; To reverse the polarity of the two toner images,
And a pre-transfer charging unit for applying a charge having a polarity opposite to the charge polarity of the latent image carrier.

Also, in the invention described in claim 2 of the present invention, the first developing means may be an insulating magnetic brush (IM
The developing method using B) develops the electrostatic latent image formed on the latent image carrier with one specific color toner, and the second developing unit includes a resin carrier having a small particle diameter. The developing system using the (MT carrier) is configured to develop the electrostatic latent image formed on the latent image carrier with black toner.

Further, in the invention according to claim 3 of the present invention, the pre-transfer charging means, when the shield extends from the discharge wire, a length d0 of a perpendicular line extending from the discharge wire to the shield, Length d1 from to the end of the shield
The ratio dl / d0 is set to be 1.1 times or more.

Still further, the invention described in claim 4 of the present invention provides:
When the shield is retracted from the discharge wire, the pre-transfer charging means may have a length of a perpendicular 0 extending from the discharge wire to the shield and a length from the discharge wire to the end of the shield.
The ratio d1 / d0 to d1 is set to be 1.4 times or more.

[Operation] According to the first aspect of the present invention, a primary image is formed by the second developing unit using a specific one color toner for forming a subordinate image by the first developing unit. The black image forming a main image such as a character or a figure is not affected by the first developing means because of the use of black toner. , And a black image forming a main image such as a character or a figure can be formed with good image quality.
In the first developing means, a specific one color toner for forming a secondary image is used, and the specific one color toner image developed by the first developing means is almost all. In this case, an image such as a character or a figure is a secondary image that is used to emphasize or emphasize a main image such as a character or a figure by underlining or halftone dots or enclosing the image with a frame. Even if affected by the second developing means in the latter stage, it is not so noticeable.

According to the first aspect of the present invention, the pre-transfer charging unit reverses the polarity of the specific one color toner image formed on the latent image carrier to change the polarity of the two toner images. Since the configuration is such that a charge is applied to align the toner, the polarity of the first color toner image formed on the latent image carrier is reversed, so that the charging polarity of the second color black toner image is: As it is, it is possible to surely prevent scattering of the black toner image forming the main image due to the pre-transfer charging.

Further, according to the second aspect of the present invention, the second developing means uses an electrostatic latent image formed on the latent image carrier by a developing method using a resin carrier (MT carrier) having a small particle diameter. Since the image is developed with the black toner, the disturbance of the toner image that occurs as a result of the kinetic energy of the developer of the second developing unit exerting on the color toner image of the first color is small by the second developing unit. Particle size resin carrier (MT carrier)
By using, the kinetic energy of the developer of the second developing unit is reduced, thereby making it possible to suppress the kinetic energy.

According to the third or fourth aspect of the present invention, as a result of various experiments in which the positional relationship between the discharge wire and the shield of the pre-transfer charging means is changed, when the shield is extended from the discharge wire, the shield from the discharge wire is used. Length of vertical line
The ratio d1 / d0 of d0 and the length d1 from the discharge wire to the end of the shield is set to be 1.1 times or more, or when the shield is retracted from the discharge wire, the above d1 / d0 is used. By setting the ratio to be 1.4 times or more, it becomes possible to increase the output of the electrostatic simulator, that is, to make it difficult to leak.

[Embodiments] The present invention will be described below based on the illustrated embodiments.

In this embodiment, first, a configuration of an electrophotographic copying machine as an image forming apparatus according to the present invention to which the transfer processing apparatus closely related to the configuration and operation of the image forming apparatus according to the present invention is applied, Then, the configuration of the transfer process apparatus according to the present invention and its evaluation will be described in detail. Prior to description, a table of contents for this embodiment is provided.

Table of Contents (1) Overview of electrophotographic copying machine (2) Type of developing device (3) Configuration of transfer process device (4) Type of charger before transfer (5) Selection of polarity reversal toner (5-1) Transferability ( 5-2) Reproducibility of fine lines (5-3) Background fog (5-4) Blur (5-5) Peelability (5-6) Toner charge (5-7) Measurement of drum potential (6) (7) Control of voltage applied to pre-transfer charger by image density (8) Dirt on pre-transfer charger (9) Prevention of abnormal discharge (9-1) Surface of shield Relationship between State and Abnormal Discharge (9-2) Relationship between State of Shield End and Abnormal Discharge This embodiment will be described below according to the table of contents.

(1) Schematic of electrophotographic copying machine FIG. 1 shows an electrophotographic copying machine to which a transfer processing apparatus according to the present invention can be applied. This copying machine is a copying machine having a composite function capable of performing copying such as simultaneous two-color copying, double-sided copying, and single-sided multiplex copying in addition to ordinary copying. Further, as shown in FIGS. 1 and 2, the paper feeding cassettes for supplying the paper are all housed inside the copying machine, so that the paper feeding cassette does not protrude outside the apparatus. The installation space is small and the external appearance is clean and good looking.

The structure of the copying machine will be described below together with the copying operation. That is, the image of the document to be copied is read by a document reading device (not shown) including the color information,
Once converted to electrical signals. The image information converted into the electric signal is subjected to predetermined image processing as needed, and then processed by a laser scanning unit 1 (hereinafter referred to as ROS [Raster Ou).
The signal is again converted into an optical signal, and is scanned and exposed on a photosensitive drum described later.

As shown in FIG. 3, this ROS1 is composed of two semiconductor lasers 2 and 3 for writing two types of images at different positions on the surface of the photosensitive drum, and a beam Bm from these semiconductor lasers 2 and 3 A polygon mirror 4 for guiding a beam Bm over a predetermined scanning range by reflecting on a reflecting surface during rotation, a polygon motor 5 for rotating the polygon mirror 4, and a beam Bm from the polygon mirror 4 Lens 6 for correcting the light beam to be guided onto the photosensitive drum at equal intervals, and mirrors 8, 9 and mirrors 10, 11 for reflecting the beam Bm onto the photosensitive drum 7.

The ROS 1 scans a laser beam Bm emitted from the semiconductor lasers 2 and 3 oscillating in accordance with an image signal along a photoconductor drum 7 axis direction by a polygon mirror 4, thereby forming a mirror on the photoconductor drum 7. 9 and mirror
First and second two exposure positions exposed by 11
An image corresponding to the image signal is scanned and exposed at L1 and L2.

As the photoconductor drum 7, for example, a photoconductor using an organic photoconductive substance (OPC) is used, but the photoconductor drum 7 is not limited to this. As shown in FIG. 1, the photosensitive drum 7 is rotatably arranged in a direction indicated by an arrow by driving means (not shown).

As shown in FIG. 4 (a), the photosensitive drum 7 is uniformly charged to a predetermined negative voltage by the primary charger 12 before the image is formed by the ROS1 in accordance with the copy mode as described above. First and second exposure positions L1, L2 shown in FIG.
Scanning exposure is performed on both or one of them, and an electrostatic latent image is formed on the surface.

At this time, when two-color simultaneous copying is performed, exposure is performed at the first exposure position L1 by image writing for exposing only the image portion, and as shown in FIG. An electrostatic latent image is formed. As shown in FIG. 4C, the first electrostatic latent image is developed by the first developing device 13 into, for example, red by a reversal developing method. Thereafter, at the second exposure position L2, exposure is performed by background lighting, which exposes the background portion, this time being superimposed on the first electrostatic latent image, and as shown in FIG. A second electrostatic latent image is formed so as to overlap the electrostatic latent image. As shown in FIG. 4E, the second electrostatic latent image is normally developed into black by the second developing device 14 by a regular developing method.

In the case of ordinary copying, double-sided copying, single-sided multiplex copying, etc., the first or second exposure position may be used as necessary.
The image exposure and development steps in L1 and L2 are performed once or plural times.

The first and second developing devices 13 and 14 can be operated at the same time or selectively. The first and second developing devices 13 and 14 operate simultaneously when performing two-color simultaneous copying. One operates selectively. In the first developing device 13, a developing device using a toner of a color such as blue is used by replacing the developing device in addition to red.
Mainly, when performing simultaneous two-color copying, etc., an image of a predetermined color is developed. In the second developing device 14, black toner is usually used, and when performing normal copying or simultaneous two-color copying, etc. Then, a black image is developed.

As shown in FIG. 4 (f), the toner images developed by the first and second developing devices 13 and 14 are charged by the pre-transfer charger 15 as shown in FIG. Device 13,
The polarity of the two toner images developed by 14 is made uniform.

Thereafter, the two toner images formed on the photosensitive drum 7 are transferred to a plurality of paper feed cassettes 16 arranged in the copying machine main body.
As shown in FIG. 4 (g), the transfer charger 21 is placed on the transfer paper 20 of a predetermined size supplied from any one of 17, 18, and 19.
Is transferred from the photosensitive drum 7 by the charging of the photosensitive drum 7. The transfer paper 20 to which the toner image has been transferred is separated from the photosensitive drum 7 by charging of the separation charger 22, and then is conveyed to the fixing device 23 where the toner image is fixed on the transfer paper 20.

The transfer paper 20 on which the toner image has been fixed remains in the discharge tray in the normal copy mode or the two-color simultaneous copy mode.
Discharged on 24. The surface of the photosensitive drum 7 is cleaned by the cleaner C to remove residual toner, paper dust, and the like, and prepares for the next image forming process.

However, in the above-described modes such as double-sided copying and single-sided multiplex copying, the transfer paper 20 on which the toner image has been fixed is not discharged onto the discharge tray 24 as it is, or is transferred through the transport path 25 and the intermediate tray 26 as it is or on the front and back. Is turned over and transported again to the image forming section, where a predetermined toner image is copied. After the transfer and fixing of the predetermined image are repeated, the transfer paper 20 is discharged onto the discharge tray 24 for the first time.

Further, in the electrophotographic copying machine, when performing two-color simultaneous copying, after performing the first image exposure at the first exposure position L1 as described above, the first developing device 13 performs this static exposure. The electrostatic latent image is developed, and a second image exposure is further performed on the first toner image developed on the photosensitive drum 7 to form an electrostatic latent image. 2 developing device
Develop with 14. Therefore, the first and second developing devices 13 and 14 can perform clear development even when the potential contrast is small.
Disturb the toner image developed by the developing device 13 of
It is designed so that it can be developed without causing color mixture of toner.

(2) Kinds of Developing Apparatus The present invention relates to a transfer processing apparatus for processing and simultaneously transferring a plurality of developed images formed on a latent image carrier, and a transfer processing apparatus according to the present invention. In, the developed image to be subjected to the transfer process is a developed image having what characteristics, that is, the developed image to be subjected to the transfer process is a developed image developed by a developing device employing any development method. Is a big problem.

Therefore, next, prior to the description of the configuration of the transfer process device, the configurations of the first and second developing devices will be described.

FIG. 6 shows the first developing device. That is, the first developing device 13 is detachably provided to the copying machine main body via a holding portion (not shown), and can be replaced with a developing device of another color as needed. I have.

As shown in FIGS. 6 to 8, the first developing device 13 includes a developing roller 29 disposed at an opening 28 of a housing 27 so as to face the photosensitive drum 7;
A first screw auger 31 and a second screw auger 32 provided side by side on the rear side of 29 via a partition 30;
The main part is constituted by a third screw auger 34 provided above the augers 31 and 32 via a receiving plate 33. Also, on the back side of the housing 27,
A toner box 37 having a toner stirring coiled auger 35 and a toner stirring screw auger 36 is provided.

Inside the developing device 13, a two-component developer composed of a color toner T of a predetermined color and a carrier C as magnetic particles is provided.
D1 is stored.

As the toner T, it is preferable to use a toner obtained by cutting a toner having a particle size of 60 μm or more. By doing so, it is possible to prevent toner having a large particle size from being developed on the photosensitive drum 7. Therefore, since the toner image developed on the photosensitive drum 7 does not include a toner having a large particle diameter, the unevenness of the toner image on the surface of the photosensitive drum 7 is reduced.
It is possible to prevent a white granular image from being omitted.

The developing roller 29 has a cylindrical sleeve 38 that rotates in the direction of the arrow, and a magnet roller fixedly disposed inside the sleeve 38.
The surplus developer D1 is scraped off by a trimmer bar 40 disposed above the developing roller 29, and a fixed amount of magnetic brush is formed on the outer peripheral surface thereof. .

On the other hand, as shown in FIG. 8, the screw augers 31, 32, and 34 have a rotating shaft 42 that is rotationally driven at low speed by a driving motor (not shown) via a driving gear 41, and these rotating shafts.
The first screw auger 31 is made of a non-magnetic stainless steel material (non-magnetic SUS304), and the first screw auger 31 is made of a non-magnetic stainless steel material (non-magnetic SUS304). Thus, the magnetic roller 39 is not affected by the magnetic force.

As shown in FIG. 8, an opening 44 for supplying the toner conveyed by the second screw auger 32 to the first screw auger 31 is opened at one end of the partition 30. , At both ends of the receiving plate 33,
An opening 45 for supplying the toner conveyed by the screw auger 32 to the side wall of the housing 27 and supplying the toner to the third screw auger 34, and supplying the toner conveyed by the third screw auger 34 to the first screw auger 31 The toner supply from the toner box 37 is conveyed to the second screw auger 32 and the third screw auger 34, and supplied from the first screw auger 31 to the developing roller 29. It has become so.

Then, the first electrostatic latent image formed on the photosensitive drum 7 is developed by the magnetic brush of the developer D1 formed around the developing roller 29.

The first developing device 13 can be moved by a retract mechanism 47 between a developing position close to the photosensitive drum 7 and a non-developing position separated from the photosensitive drum 7 as necessary. .

As shown in FIGS. 9 to 11, the retract mechanism 47 includes a crankshaft 48 rotated by a drive source (not shown).
, A yoke cam 50 that follows the crank 49, the crank 49 and the yoke cam
While energizing the elastic force in the direction that always contacts 50,
Return spring 51 for separating developing device 13 and developing device
The pressure contact spring 52 constantly biases the elastic force in a direction in which the photosensitive drum 13 approaches the photosensitive drum 7.

In this case, the yoke cam 50 is formed with a guide slot 56 for guiding a guide pin 55 slidably fitted to a bifurcated arm 54 provided on the holding portion 53 of the developing device 13. (See FIGS. 10 and 11).

The crank part 49 of the crankshaft 48 formed as described above
The yoke cam 50 and the yoke cam 50 are engaged as shown in FIGS. 10 and 11, and when the developing device 13 is separated, when the crankshaft 48 rotates counterclockwise from the state shown in FIG. The yoke cam 50 is moved in the separating direction against the elastic force of 52, and the developing device 13 is reliably separated from the photosensitive drum 7. In order to bring the developing device 13 close to the photosensitive drum 7, when the crankshaft 48 is rotated counterclockwise from the state shown by the imaginary line in FIG. 10, the yoke cam 50 comes close to the elastic force of the return spring 51. Direction (left side in Fig. 10)
The developing device 13 is moved closer to the photosensitive drum 7 side and is set in a developing state. In this case, the pressing spring 52 projects from the holding portion 53 holding the housing 27 of the developing device 13 toward the rear side (the right side in FIG. 9) of the housing 27 and slides on the U-shaped movable body 57. It is formed of a coil-shaped compression spring fitted to a pair of guide shafts 58 movably fitted and contracted between the holding portion 53 and the movable body 57. Also, the above return spring
51 is formed by a coil-shaped tension spring having one end attached to the fixed side of the copying machine main body and the other end attached to the movable body 57. The guide pins 55 to be inserted into the bifurcated arms 54 of the holding part 53 and the guide long holes 56 of the yoke cam 50 are provided at both ends of the U-shaped movable body 57, respectively.

In FIG. 9, reference numeral 59 denotes a positioning roll provided on both sides of the first and second developing devices 13 and 14, and 60
Is a guide groove of the guide frame 61 provided in the copying machine main body.
It is a guide pin slidably supported in 62.

FIG. 12 and FIG. 13 show the second developing device.

That is, as shown in FIG. 12, the second developing device 14 includes a developing roller 65 disposed in the opening 64 of the housing 63 so as to face the photosensitive drum 7,
First and second screw augers 66 and 67 are provided on the rear side of the auger 65, and a third screw auger 69 provided via a receiving plate 68 above the augers 66 and 67.
And a coil-shaped agitator 70 provided on the back side of the augers 66, 67, and 69 for supplying and stirring the toner, the main part of which is formed. A toner box (not shown) having a toner agitating coil auger and a toner conveying screw auger is provided.

Inside the developing device 14, a two-component developer D2 including a black toner T and a carrier C as magnetic particles is stored.

First, the developing roller 65 is composed of a cylindrical sleeve 71 rotating in the direction of the arrow and a magnet roller 72 fixed and arranged inside the sleeve. A trimmer bar 73 disposed above the developing sleeve 71 is provided. As a result, the excess developer D2 is scraped off, and a fixed amount of magnetic brush is formed on the outer peripheral surface.

On the other hand, as shown in FIG. 12, the screw augers 66, 67, and 69 have a rotating shaft 74 that is rotationally driven at a low speed by a driving motor via a driving gear (not shown), and these rotating shafts.
And a transport blade 75 spirally formed in the length direction of 74. The first screw auger 66 has a transfer blade 75 made of a non-magnetic stainless steel material (non-magnetic SUS304).
Is not affected by the magnetic force.

Then, the toner T supplied into the developing device 14 by the agitator 70 is transferred to the carrier C by the agitator 70.
And then the first through the second screw auger 67
The toner is conveyed to the screw auger 66, is exchanged between the first screw auger 66 and the third screw auger 69, is sufficiently stirred, and is then supplied to the developing roller 65.

Then, the second electrostatic latent image formed on the photosensitive drum 7 is developed by the magnetic brush of the developer D2 formed around the developing roller 65.

At this time, the magnetic brush formed on the surface of the developing roller 65 has a low density, and rubs the first toner image already formed on the photosensitive drum 7 with the magnetic brush.
The first toner image is not disturbed or the toner is not mixed.

As shown in FIG. 12, a catch-up roll 76 is disposed downstream of the developing roller 65 at the opening 64 of the housing 63 of the second developing device 14 so as to face the photosensitive drum 7. The catch-up roll 76 is rotatable in the direction of the arrow by driving means (not shown). Inside the catch-up roll 76, a catch-up magnet roll 77 is provided in a fixed state. Further, a scraper 78 for removing the carrier C captured on the catch-up roll 76 is in contact with the outer peripheral surface of the catch-up roll 76.

By doing so, the carrier C adhered to the photosensitive drum 7 during development is captured by the catch-up roll 76.

The second developing device 14 has a retracting mechanism 47 configured in the same manner as the first developing device 13, and has a developing position close to the photosensitive drum 7 and a non-developing position separated from the photosensitive drum 7. In addition, it can be moved as needed.

By the way, in the above-mentioned electrophotographic copying machine, it is necessary to form two color toner images of red and black on the photosensitive drum 7 at the time of simultaneous two-color image copying.
Developing devices 13 and 14 are provided.

For this reason, the development method in which the first and second developing devices 13 and 14 perform development has a large direct influence on the transfer characteristics of the transfer process device according to the present invention.

That is, the transfer process device according to the present invention performs predetermined electrical processing on the two-color toner images having different polarities formed on the photosensitive drum 7, and then transfers these toner images onto transfer paper. It is a device for. Therefore, the first
In the above embodiment, as the second developing devices 13 and 14, the first developing device 13 employs an IMB developing method using a two-component developer composed of a red non-magnetic toner and a carrier. The developing device 14 employs an MT developing method using a two-component developer composed of a black nonmagnetic toner and a carrier having a small particle diameter.

(3) Configuration of Transfer Processing Apparatus FIGS. 14 to 18 show a basic configuration of an embodiment of the transfer processing apparatus according to the present invention. This transfer process device is for performing necessary processing on the toner image formed on the photosensitive drum 7 and transferring the toner image onto the transfer paper 20. The pre-transfer charger 15 and the transfer charger 21 Its main part is configured.

As shown in FIG. 15, the pre-transfer charger 15 includes a shield 80 made of a metal such as stainless steel and formed in an elongated box shape having an irregular cross section. This shield 80
As shown in FIG. 16, a long side wall 82 is provided at one end in the width direction of the central wall 81.
Is formed so as to form an angle larger than 90 degrees, and a short side wall 83 is formed so as to be orthogonal to the other end in the width direction of the central wall 81, so that one side surface facing the central wall 81 is large. The shield 80 is provided so as to be open, and the opening 84 is disposed so as to face the photosensitive drum 7 as shown in FIG.

As shown in FIG. 15, insulating blocks 85 and 86 are fixed to both ends of the shield 80. Between these two insulation blocks 85 and 86, a discharge wire 87 to which a high voltage is applied is provided.
A ring 88 for connection is fixed to both ends of the discharge wire 87 by means such as caulking or spot welding. As the discharge wire 87, for example, a tungsten wire having a diameter of 60 μm, which is electropolished, is used. However, the present invention is not limited to this, and the surface of a wire made of an alloy such as tungsten (W) and rhenium (Re) may be used. And those plated with gold are used. Since the wire made of an alloy of tungsten (W) and rhenium (Re) has a breaking strength about 30% higher than that of a normal wire, it is effective in an apparatus for automatically cleaning the discharge wire 87 and the like.

The one ring 88 is engaged with a projection 90 provided on an insulating block 86 via a spring 89, and the other ring 88 is engaged with a projection 92 of a power terminal 91 provided on the insulating block 85. Have been combined. Discharge wire 87 above
Is connected to a high-voltage power supply (not shown) via a power supply terminal 91, a high voltage is applied to the discharge wire 87 by the high-voltage power supply (not shown), and a corona is provided between the discharge wire 87 and the shield 80. Discharge can be generated. The polarity of the corona discharge is determined by the polarity of the high voltage applied to the discharge wire 87.

The pre-transfer charger 15 charges the first and second toner images formed on the photoconductor drum 7 with ions generated by corona discharge, and charges the first and second toner images. It is configured such that the polarities are aligned to one polarity.

In addition, as shown in FIG. 17, an opening 84 is formed along the longitudinal direction of the central wall 81 of the shield 80 in order to prevent stagnation of discharge products, reduce discharge unevenness, reduce power supply load, and the like. It is provided over substantially the entire length.

That is, when the opening 84 is not provided in the center wall 81 of the shield 80, O ³ or NO _X generated by corona discharge
Discharge product is liable to stay in the shield 80 and the like, O ³ or the like cause a problem such as image blurring at high humidity environment adhering to the surface of the photosensitive drum 7.

Further, when the opening 84 is not provided in the center wall 81 of the shield 80, a discharge from the discharge wire 87 toward the center wall 81 of the shield 80 occurs, but this discharge includes dust or the like attached to the discharge wire 87. This causes unevenness. Therefore, when the discharge from the discharge wire 87 to the central wall 81 of the shield 80 becomes uneven, the discharge from the discharge wire 87 to the photosensitive drum 7 opposite to the center wall 81 of the shield 80 also becomes uneven.

Further, when the opening 84 is not provided in the central wall 81 of the shield 80, the central wire 81 of the shield 80 is connected to the discharge wire 87.
Therefore, in order to cause the photosensitive drum 7 to perform predetermined charging, the voltage applied to the discharge wire 87 must be increased accordingly, and the load on the power supply increases.

Therefore, the opening 84 is formed in the central wall 81 of the shield 80 as described above.
By providing the above, it becomes possible to prevent stagnation of discharge products, reduce uneven discharge, reduce power supply load, and the like.

In addition, as the insulating blocks 85 and 86 of the pre-transfer charger 15, in particular, water droplets that have condensed on the surfaces of the insulating blocks 85 and 86 do not leak and ignite on the surfaces of the insulating blocks 85 and 86. It is desirable that 86 be made of flame-retardant PP resin (for example, nitrogen 2654 manufactured by Chisso Corporation), ABS resin, PBT resin, Virograss (trade name, manufactured by Toyobo Co., Ltd.), BMC resin, or the like.

On the other hand, as shown in FIG. 17 (b), the shape of the insulating blocks 85 and 86 of the pre-transfer charger 15 is such that the contact portions below the shield 80 and the insulating blocks 85 and 86 do not collect water drops. In order to leave a gap G and to increase the creepage distance for the contact portion between the insulating blocks 85 and 86 and the discharge wire 87,
The positioning member A of the discharge wire 87 is formed in a column shape on the bottom walls of the insulating blocks 85 and 86.

Further, in order to suppress arc discharge between the shield 80 and the discharge wire 87, a constant voltage element B is connected to the shield 80 as shown in FIG. Is biased to the shield 80.

As shown in FIG. 18, the transfer charger 21 is disposed around the photosensitive drum 7 and downstream of the pre-transfer charger 15 via a supply unit 94 for the transfer paper 20. As shown in FIG. 18, the transfer charger 21 is composed of a corotron having a shield 95 and one discharge wire 96. The transfer charger 21 has a shield 97 and two discharge wires 98. A separate charger 22 made of corotron is integrally formed. In the figure, reference numeral 99 denotes a paper guide provided at the opening of the separation charger 22.

Then, the transfer charger 21 and the separation charger 22 are formed on the photosensitive drum 7 by corona discharge ions generated by applying a high voltage to the discharge wires 96 and 98, and the charge of the pre-transfer charger 15 is performed. The first and second toner images, whose polarities have been made uniform by the charging, are transferred onto the transfer paper 20, and the transfer paper 20 onto which these toner images have been transferred is separated from the photosensitive drum 7.

The transfer charger 21 and the separation charger 22 are shielded.
As shown in FIG. 18, the central walls of 95 and 97 are provided with openings 100 and 100 over substantially the entire length along the longitudinal direction, similarly to the pre-transfer charger 15, and to prevent leakage. It has the same material and shape as the pre-transfer charger 15.

(4) Kinds of Pre-Transfer Charger In the above-described embodiment, the case where a corotron including a shield and a discharge wire is used as the pre-transfer charger 15 has been described, but the present invention is not limited to this. That is,
As the pre-transfer charger, a so-called scorotron in which a grid is provided at the opening of the shield may be used in addition to the shield and the discharge wire. Further, as the pre-transfer charger, a corotron or a combination of a scorotron and a lamp may be used.

Next, the present inventors conducted an experiment to examine how transfer characteristics are basically different between a case where a corotron is used as a pre-transfer charger and a case where a scorotron is used.

First, an experiment for confirming the toner charge amount and transferability was performed. Transferability when the first image, that is, only the toner image developed by the first developing device 13 is inverted by the pre-transfer charger 15 composed of corotron or scorotron, the toner charge amount on the photosensitive drum 7 and the photosensitive member An experiment for measuring the surface potential of the drum 7 was performed.

The conditions of the experiment were based on the copy machine FX5030 manufactured by Fuji Xerox Co., Ltd.
A modified one was used as needed, and the process speed was 130 mm / s.

FIG. 19 shows the result of a measurement for examining the relationship between the toner charge amount on the photosensitive drum and the transferability.

This measurement was performed by changing the DC voltage applied to the discharge wire of the pre-transfer charger and evaluating the transferability of the test chart. At that time, if a scorotron was used as the pre-transfer charger 15, AC4KV (RMS), 1K
The measurement was performed by applying a voltage of 751 V to the grid at Hz.

As a result of the above measurement, after the pre-transfer charging is performed by the pre-transfer charger 15, the charge amount of the toner on the photosensitive drum 7 is reduced to 15
When it was at least μc / g, it was found that the transferability was good. The charge amount of the toner on the photosensitive drum 7 is 15 μc /
When the value was between g and 5 μc / g, transfer failure occurred at the leading and trailing ends of the ladder portion of the test chart. Further, when the charge amount of the toner on the photosensitive drum 7 was 5 μc / g or less, transfer failure occurred on the entire ladder portion of the test chart.

Next, a test was conducted to see the difference between Corotron and Scorotron.

20 and 21 show the relationship between the charge amount of the toner on the photosensitive drum 7 and the surface potential of the photosensitive drum, and the charge amount of the toner on the photosensitive drum 7 and the discharge wire of the pre-transfer charger. 4 shows a relationship with an applied DC voltage. In FIG. 20, A shows the case where a corotron is used as the pre-transfer charger 15, and B shows the case where a scorotron is used as the pre-transfer charger 15. Also,
In FIG. 21, A shows a case where a corotron is used as the pre-transfer charger 15, and B to D show a case where a scorotron is used as the pre-transfer charger 15.

FIG. 22 shows the relationship between the surface potential of the photosensitive drum 7 and the DC voltage applied to the discharge wire of the pre-transfer charger. In the figure, A is a case where a corotron is used as the pre-transfer charger 15, B is a case where a scorotron is used as the pre-transfer charger 15, and C is a combination of a scorotron and a lamp as the pre-transfer charger 15. Each case is shown.

Summarizing the above measurement results, the amount of toner charge required to transfer a toner image with reversed polarity in good condition
To obtain 15μc / g or more, pre-transfer charging of a scorotron, which has a wide latitude with respect to fluctuations in the DC voltage applied to the discharge wire of the pre-transfer charger 15 and easily controls the amount of toner charge by adjusting the grid voltage It is desirable to use it as a vessel 15. When a scorotron is used as the pre-transfer charger 15, as is apparent from FIG. 22, the surface potential (VH, VL) of the photosensitive drum after reversing the polarity of the first toner image is kept low. Accordingly, overcharging of the second toner image can be reduced. It should be noted that not only a scorotron but also a combination of a scorotron and a lamp can be used as the pre-transfer charger 15 to obtain the same or better effects as described above.

Next, the present inventors performed measurements to confirm the image quality of the second toner image when a scorotron was used as the pre-transfer charger 15.

FIGS. 23 and 24 show the relationship between the charge amount of the toner on the photosensitive drum 7 and the surface potential of the photosensitive drum, and the charge amount of the toner on the photosensitive drum 7 and the discharge wire of the pre-transfer charger. 4 shows a relationship with an applied DC voltage. In the figure, A indicates the first toner image, and B indicates the second toner image.

As a result of this measurement, it was found that the image quality of the second toner image was degraded in the blur level.

Therefore, as a type of the pre-transfer charger, it was found that the scorotron is preferable in terms of the amount of toner charge of the first toner image and the like. However, in consideration of the image quality of the second toner image, the pre-transfer charger is not necessarily used. As a result, it was found that scorotron is not optimal, and it is necessary to consider other factors.

(5) Selection of Polarity-Reversed Toner Next, the present inventors determine which of the two types of toner images formed on the photosensitive drum 7 should be reversed in polarity, namely, transfer. An experiment was conducted to determine whether the discharge polarity of the pre-charger should be determined to be positive or negative.

That is, the first developing device 13 is provided on the photosensitive drum 7.
A negative color toner image developed by
And a positive black toner image developed by the developing device 14. Therefore, of the two types of toner images formed on the photosensitive drum 7, the polarity of the negative color toner image is inverted and the polarity of the positive black toner image is inverted. It was examined how transferability, image quality, and the like were different.

As test items, in addition to transferability and image quality, peelability, toner charge amount, and photoconductor drum potential were performed.

Here, as for the transferability, a test for confirming the transferability of a uniformly-filled area, that is, a so-called solid portion, was performed by changing the voltage applied to the pre-transfer charger 15 and the transfer charger 21. As for the image quality, blur, background fog, and reproducibility of fine lines (line width and density) were evaluated. The peelability was evaluated by observing the posture of the transfer paper at the time of peeling. Regarding the toner charge amount, the charge amount of the toner on the photosensitive drum after passing through the pre-transfer charger was measured. Regarding the photoconductor drum potential, the surface potential of the photoconductor drum after passing through the pre-transfer charger, the transfer charger, and the separation charger was measured.

The main body of the copying machine used for the measurement is a copy machine FX5030 manufactured by Fuji Xerox Co., Ltd.
s. The measurement was performed in a laboratory environment.

Further, the diameter of the beam Bm for writing an image on the photoconductor drum 7 is 89.0 μm in the main scanning direction (axial direction of the photoconductor drum 7) and the sub-scanning direction (the axis of the photoconductor drum 7) for the first image exposure. In the second image exposure, the main scanning direction was 84.0 μm and the sub-scanning direction was 128.0 μm. Further, the potential of the photosensitive drum 7 is such that the dark portion potential VH = -630 V, the bright portion potential VL = -100 V, the developing bias VB = -450 V for the first image, and the dark portion potential VH = -630 V for the second image. , Bright part potential VL = −200 V, developing bias V
B = -300V was set.

Further, in the first developing device 13, the developers D1 and D2 used in the first and second developing devices 13 and 14 are a developer (red), a toner (negative polarity), a styrene / acrylic copolymer, a red dye ( Red toner carrier with a mean particle size of 11.5 (Positive polarity) made by kneading lithol scarlet) A carrier with a mean particle size of about 80 μm coated with a ferrite core with a methyl terpolymer is used. ) Toner (Positive polarity) Styrene / acrylic copolymer, carbon black positive charge control agent melt-kneaded / crushed / classified, average particle size 11.5
μm black toner carrier (negative polarity) A polymer-based carrier core having an average particle size of about 45 μm obtained by melting and kneading a styrene / acrylic copolymer and magnetite was used.

(5-1) Transferability In conducting a transferability test, the inventors of the present invention set the DC voltage applied to the discharge wire 77 of the pre-transfer charger 15 to +0.5 to +1. In the case of negative discharge in the range of 5 KV, it was changed in the range of -1.0 to -2.0 KV. An AC voltage having an effective value of 3.5 KV and a frequency of 1 KHz was applied to the discharge wire 77 of the pre-transfer charger 15 in a manner superimposed on the DC voltage.

Further, the DC voltage applied to the discharge wire of the transfer charger 21 is in the range of −0.5 to −3.0 KV when charged positively by the pre-transfer charger 15, and is negative when discharged by the pre-transfer charger 15. Was changed in the range of -1.5 to +2.0 KV. In addition, the discharge wire of the transfer charger 21 includes:
An AC voltage having an effective value of 3.3 KV and a frequency of 1 KHz was applied while being superimposed on the DC voltage.

As shown in FIG. 25, the evaluation of the transferability was performed by a solid image portion 101a having a square planar shape and a line image portion 101 having sequentially different line thicknesses.
b, using a test chart 101 including a line image portion 101c in which a plurality of lines are densely arranged with each other, a character portion 101d in which characters such as alphabets and kanji are described, and the like.
The measurement was performed by measuring the density of the solid portion 101a located at the center of 01. The concentration was measured using a Macbeth densitometer RD914.

FIG. 26 shows the measurement results of the transferability. In the figure, A, B, and C show the case where the toner image is charged to the plus side by the pre-transfer charger 15, and A is +0.5 KV.
In addition, B shows the case where it is charged to +1.0 KV, and C shows the case where it is charged to +1.5 KV. Further, in the same figure, D, E, and F show the case where the toner image is charged to the negative side by the pre-transfer charger 15; D is -1.0 KV, E is -1.5 KV, and F is- The case where it is charged to 2.0 KV is shown. Further, in the same figure, a solid line indicates a black toner image, and a broken line indicates a red toner image.

As is apparent from this graph, when the DC voltage applied to the transfer charger 21 changes, the transfer density is greatly reduced. However, if the DC voltage is equal to or higher than a predetermined value, the pre-transfer charger 15 discharges positively. Although a practically sufficient density was obtained between the case where the toner was charged and the case where the toner was negatively charged, it was found that stable transfer was possible when the amount of the DC component on the minus side was large. That is, as is apparent from FIG. 26, by charging the toner image to the negative side by the pre-transfer charger 15, a predetermined image density can be obtained at a relatively low DC applied voltage of the transfer charger 21. I understood. that time,
-1.5K as DC voltage to discharge wire 87 of pre-transfer charger 15
It is desirable to apply a voltage that is equal to or higher than V and has an effective value of 3.5 KV and a frequency of 1 KHz superimposed as an AC voltage.

(5-2) Reproducibility of Fine Lines Next, the present inventors write 1-bit and 3-bit fine line images on the photosensitive drum 7 by ROS1 in the vertical and horizontal directions, and how much these fine line images are reproduced. Was tested.

At this time, when the DC voltage applied to the discharge wire 77 of the pre-transfer charger 15 is positively charged, the DC voltage is +1.0 and +1.5.
-1.5 and -2.0 KV for negative discharge to KV
Was changed to. The discharge wire of the pre-transfer charger 15
To 77, an AC voltage having an effective value of 3.5 KV and a frequency of 1 KHz was applied so as to be superimposed on the DC voltage as in the transferability test.

Further, the DC voltage applied to the discharge wire of the transfer charger 21 is -2.0 and -2.5 KV when charged positively by the pre-transfer charger 15 and negative when the pre-transfer charger 15 discharges negatively. It was changed to +1.0 and +1.5 KV. The discharge wire of the transfer charger 15 has an effective value
An AC voltage of 3.3 KV and a frequency of 1 KHz was applied while being superimposed on the DC voltage.

The reproducibility of the thin line was evaluated based on the line width and density of the 1-bit and 3-bit lines and the Raggedness of the 3-bit line. Here, the Raggedness indicates the sharpness of the edge of the image as shown in FIG.

FIG. 28 to FIG. 30 show the results of the above measurement.

In FIGS. 28 to 30, A represents one bit line,
B indicates a 3-bit line. Also, the 28th
30 to 30, the broken line indicates the case where the pre-transfer charger 15 is turned off, the solid line indicates the case where the discharge polarity of the pre-transfer charger 15 is negative, and the dashed line indicates the discharge polarity of the pre-transfer charger 15. Indicates the case of positive polarity, respectively. 28 to 30, (a) and (b) show red toner images, (a) shows a horizontal straight line, (b) shows a vertical straight line, and (c) , (D) show a black toner image and (c)
Indicates a horizontal straight line, and (d) indicates a vertical straight line.

As is clear from these graphs, it was found that the reproducibility of the fine line was hardly different between the case where the pre-transfer charger 15 charged positively and the case where the thin line was charged negatively.

(5-3) Background fog The present inventors conducted an experiment to examine the state of occurrence of background fog.

That is, as the first and second developing devices 13 and 14, a new one and a 50,000-copyed one (using an old developer) are used, and a solid portion (image portion) of a predetermined density or more is used. Was obtained, and the range of toner density satisfying the condition that the fog of the background (background portion) was equal to or less than a predetermined density was obtained.

As for the density of the solid part, the red image is 1.0
As described above, the black image was set to 1.4 or more, and the background fog was set to 0.014 or less for red and 0.02 or less for black.

Here, the image density was evaluated by the value of Log (R white reference) / (R solid portion), where R is the reflectance of the image.

FIG. 31 to FIG. 33 show the results of the above measurements, and Table 1 below summarizes the results of the above measurements. 31 to 33, the broken line indicates the case where the pre-transfer charger 15 is turned off, the solid line indicates the case where the discharge polarity of the pre-transfer charger 15 is negative, and the dashed line indicates the pre-transfer charger. Fifteen
Respectively show the case where the discharge polarity is positive.

In this table, the red toner used in the developing device was negatively charged, and the black toner was positively charged. The used developing device used was 50,000 copies.

As is apparent from Table 1, it is found that the allowable range of the toner density becomes narrower and worse for background fog, regardless of whether the polarity of red or black is inverted. It was also found that the allowable width was reduced to about half or less in the developing device after 50,000 copies.

As is clear from the results of Table 1 and FIGS. 31 to 33, when the polarity of the red toner is reversed, the toner density is 2.
It is sufficient if the toner density is in the range of 6.2 to 13.5% when the second developing device for developing black does not have latitude of 6 to 4.6% and the toner density does not have a latitude in a used state.
In the case where the polarity of the black toner is reversed, the first developing device for developing the red color has a toner density of 2.7 in a new state.
~ 4.5%, toner concentration 2.2 ~ 2.4 when used
%, When the second developing device for developing black is new, the toner concentration may be in the range of 7.5 to 13.5%. Therefore, when the polarity of the black toner is inverted, the latitude of the toner density is slightly larger than when the polarity of the red toner is inverted.

(5-4) Blur Next, the present inventors use a pre-transfer charger.
An experiment was conducted to examine how the state of occurrence of blur changes between the case where the toner image is positively charged and the case where the toner image is negatively charged.

At this time, when the DC voltage applied to the discharge wire 77 of the pre-transfer charger 15 is positively charged, the DC voltage is +1.0 and +1.5.
-1.5 and -2.0K for negative discharge to 6KV
V, and compared with the case where the pre-transfer charger 15 was turned off. The discharge wire 77 of the pre-transfer charger 15 includes:
As in the transferability test, an AC voltage having an effective value of 3.5 KV and a frequency of 1 KHz was applied while being superimposed on the DC voltage.

At that time, the toner density of the developing devices 13 and 14 was also changed, and the state of occurrence of blur was examined.

The evaluation of the degree of blur was performed according to the evaluation of the grade of the blur scale. Here, as shown in FIG. 34, the grade evaluation of the blur scale is performed by classifying the bad blur to prepare a limit sample and classifying an actual copy with reference to the limit sample.

FIG. 35 and FIG. 36 show the results of the above test. FIG. 35 shows the case of a red toner image, and FIG. 36 shows the case of a black toner image. 35 to 36, the broken line indicates the case where the pre-transfer charger 15 is turned off, the solid line indicates the case where the discharge polarity of the pre-transfer charger 15 is negative, and the one-dot chain line indicates the pre-transfer charger. 15 shows the case where the discharge polarity is positive. Further, in FIG. 35, A shows the case where 1.5 KV was applied, B shows the case where 1.0 KV was applied, C shows the case where 1.5 KV was applied, and D shows the case where 2.0 KV was applied. In FIG. 36, A is
B applies when 1.0KV is applied, and B applies when 1.5KV is applied.
C shows the case where 2.0 KV was applied, and D shows the case where 1.5 KV was applied.

As is clear from these graphs, in the red image, when the pre-transfer charging is performed, the blur level is about G0.5 to 1.5 as compared with the case where the pre-transfer charger is turned off in the blur scale. It turned out to be bad.
Moreover, when the toner is positively charged by the pre-transfer charger 15, the toner charge amount after the polarity inversion becomes too high, and when the toner is negatively charged by the pre-transfer charger 15, the toner charge amount becomes low. It turns out that the change is small and good.

When the pre-transfer charging is performed on a black image, the pre-transfer charger is turned off in the blur scale.
It was found that the blur level became worse by about G0.5 to 2.5 as compared with the case where the blurring was performed. In addition, when the toner is positively charged by the pre-transfer charger 15, the change in the toner charge amount is large, which is bad against blurring. When the toner is negatively charged by the pre-transfer charger 15, the toner after the polarity reversal is used. It turned out that it becomes worse if the charge amount is too high.

From the above results, it was found that the occurrence of blurring by the pre-transfer charger 15 is better when the pre-transfer charger 15 is charged negatively.

(5-5) Peelability Next, the present inventors discuss how the peelability of the transfer paper 20 differs between the case where the pre-transfer charger 15 is positively charged and the case where the transfer paper 20 is negatively charged. An experiment was conducted.

In conducting the peeling test, the present inventors set the DC voltage applied to the discharge wire 77 of the pre-transfer charger 15 to +1.5 KV when positively charged, and −2.0 KV to the transfer charger 21. KV, and a voltage obtained by superimposing a DC voltage of +780 V on an AC voltage of 4.1 KV as an effective value was applied to the separation charger 22. Further, the DC voltage applied to the discharge wire 77 of the pre-transfer charger 15 is set to −2.0 KV when negatively charged,
+2.0 KV was applied to the transfer charger 21, and a voltage obtained by superposing a DC voltage of −780 V on an AC voltage of 4.1 KV as an effective value was applied to the separation charger 22.

FIGS. 37 to 39 schematically show observation results of the posture of the transfer sheet in the transfer / separation unit.

As is clear from these figures, it was found that when the pre-transfer charger 15 was negatively charged, the peeling characteristics of the transfer paper 20 were reduced.

It should be noted that no finger mark or the like of a peeling claw for reliably peeling the transfer paper 20 was generated on the transfer paper 20.

(5-6) Toner Charge Next, the present inventors conducted an experiment to measure the charge of the toner image on the photosensitive drum 7 after passing through the pre-transfer charger 15. The measurement of the charge amount of the toner image was performed by a suction method. Here, the suction method is, as shown in FIG.
The toner of the solid image developed into a predetermined pattern on the photosensitive drum 7 is sucked into the measuring device 103 by the operation of the suction device 102, and the weight and the charge amount of the toner of the sucked toner image are measured by the measuring device 103 and the measuring device 103. The measurement was performed by a micrometer 104 connected to the measuring instrument 103.

As shown in FIG. 41, the measuring device 103 has an elongated suction hole 105 at one end side, and sucks air by a suction device 102 from a discharge port opened at the other end side. The toner image developed on the photosensitive drum 7 is sucked from the hole 105. Then, the toner sucked into the measuring device 103 is attached to the filter 106, and the weight and the charge amount of the captured toner are measured.

At this time, a masking 107 as shown in FIG. 42 is applied onto the photosensitive drum 7 so as to suck only the toner in a predetermined area and not to suck the other toner.

FIG. 43 shows the results of the above measurement. In the figure, the solid line indicates a black toner image, the broken line indicates a red toner image, and A and C indicate the pre-transfer charger 15.
B and D are pre-transfer chargers when
15 shows the case where the battery was discharged to the negative polarity.

With respect to the applied voltage of the pre-transfer charger 15 and the accompanying variation of the drum potential, the variation of the toner charge amount is smaller than that of the red toner.
It was found that the black toner was larger. This variation in the amount of charge of the black toner is considered to be one factor in the occurrence of blur.

(5-7) Measurement of Drum Potential Next, the present inventors set the pre-transfer charger 15 and the transfer charger
21, the surface potential of the photosensitive drum 7 after passing through the separation charger 22 was measured and evaluated.

The measurement of the surface potential of the photosensitive drum 7 is performed by the pre-transfer charger 15.
Was measured at the position of the separation charger 22 under the condition that the discharge of the transfer charger 21 was stopped.

FIG. 44 shows the DC voltage of the pre-transfer charger 15 and the photosensitive drum 7
3 shows the relationship with the surface potential. In the drawing, a solid line indicates VH of the photosensitive drum 7 and a broken line indicates VL of the photosensitive drum 7.

FIG. 45 shows a change in the surface potential of the photosensitive drum 7 after passing through the transfer charger 21 and the separation charger 22.

When the pre-transfer charger 15 is discharged to a negative potential, the potential on the photosensitive drum 7 increases, and the peeling property deteriorates.
Further, the potential after passing through the transfer charger 21 and the separation charger 22 is also high, which may adversely affect the cleaning performance.

(6) Improving the transferability of the leading and trailing edges of the transfer sheet In a copying machine as shown in FIG. 1, the transfer sheet 20 on which an image has been formed is transported through the transport path 25 at the time of single-sided multiplex copying or duplex copying. When the transfer paper 20 is conveyed again to the image forming unit, the transfer paper 20 has a peculiarity due to the curvature at the time of conveyance at the front and rear ends. For this reason, at the time of transfer and separation of the transfer paper 20, an image is missing or re-transfer to the photosensitive drum 7, and the image quality is reduced.

Therefore, in this embodiment, at the time of single-sided multiplex copying or double-sided copying, the voltage applied to the separation charger 22 is changed according to the mode.

That is, during one-sided copying, as shown in FIG.
When the voltage applied to the separation charger 22 using discharge is set to, for example, a DC component of -200 V, and multiple copies are performed again on one side,
As shown in FIG. 46, +200 V DC component is applied to the separation charger. At the time of backside copying during duplex copying, as shown in FIG. 47, a DC voltage of +200 V is applied to the separation charger 22 only at the leading and trailing ends of the transfer sheet 20, and at the center of the transfer sheet 20, a + 700V voltage is applied. Is applied.

By doing so, it is possible to solve problems such as transfer failure during single-sided multiple copying and retransfer during double-sided copying.

(7) Control of Voltage Applied to Pre-Transfer Charger Based on Image Density Next, an example in which the voltage applied to the pre-transfer charger and the like according to the image density is controlled will be described.

FIG. 48 shows an electrophotographic copying machine according to this embodiment. This electrophotographic copying machine has basically the same configuration as the apparatus shown in FIG.

In the figure, 110 indicates an image reading device,
The image is read by the image reading device 110 and the image processing device 111 is read.
Is detected by the detecting means 113, the ratio of each color is calculated from the image signal by the calculating means 114, and the calculated value calculated by the calculating means 114 is a central processing unit which is an arithmetic processing means. (CPU) 115 performs a comparison operation with reference data measured in advance. When the image information is of two colors, a signal from the CPU 115 is transmitted to the first control means 117 and the second control means 118 by the switching means 116 such as a changeover switch, and the first and second control means 117,
From 118, a predetermined voltage is applied to the pre-transfer charger 15 and the transfer charger 21, respectively. In this case, the first and second control means 117 and 118 respectively control the control device 11
9 and a predetermined applied bias voltage based on the signal converted by the control device 119.
And a power supply 120 applied to the power supply 21. When the image information is a single color of red or black, the switching means 116
The signal from the PU 115 is transmitted to the second control means 118,
The control unit 118 applies a predetermined voltage to the transfer charger 21. The pre-transfer charger 15 and the transfer charger 21 have a structure in which a grid electrode is provided in an opening of a shield having a U-shaped cross section in which a wire electrode is provided, and an AC high voltage is applied to the wire electrode. A DC control voltage is applied to the grid electrodes.

Next, regarding the operation mode of the image forming apparatus of the present invention,
This will be described with reference to the flowchart shown in FIG.

First, an image signal read from a document (not shown) by the image reading device 110 and arranged by image processing is detected by the detection unit 113 (step A), and the detection signal is transmitted to the calculation unit 114 to be transmitted to the red ratio or the black ratio. Is calculated (step B), the signal from the calculating means 114 is transmitted to the CPU 115, and the CPU 115 determines whether or not the image is a red and black image (step C), and compares the image with the reference data measured in advance. When the image signal is a red or black image, the signal is transmitted to the first control means 117 by the switching of the switching means 116, and the first control voltage is applied to the pre-transfer charger (step D). The pre-transfer charger 15 charges the photosensitive drum 7 with a predetermined bias voltage corresponding to a red ratio / black ratio, which will be described later, so that the red toner and the black toner on the photosensitive drum 7 are aligned on the same electrode. (Step E), a second control voltage is applied to the transfer charger 21 by the second control means 118 (Step F), and two colors of two colors are applied to the recording sheet by the opposite polarity charge by the transfer charger 21. The toner image is transferred (Step G).

On the other hand, if the image signal is not a red or black image,
In step S15, it is determined whether or not only the red ratio is present (step H), and the red ratio or the black ratio is compared with previously measured data. If only the red rate is present, the switching means 11
The signal is transmitted to the second control means by the switching of step 6, and the second
Is applied to the transfer charger 21 (step I),
The red toner image on the photoreceptor drum is transferred onto the transfer paper by charging the transfer charger 21 with a predetermined bias voltage corresponding to the red rate described later (step J). When the image signal is a black image, the switching means is the same as in the case of only the red ratio.
By switching the switch 116, a signal is transmitted to the second control means 118, and the second control voltage is applied to the transfer charger 21 (step K). The black toner image on the photosensitive drum 7 is transferred onto the transfer paper by charging the voltage (step L).

The first control voltage is, as shown in FIG.
A DC control voltage in a range of +1.0 to 2.0 KV for a black ratio of 10/90 to 90/10 is set to be applied to the grid electrode of the pre-transfer charger 15. When the image signal has only the red ratio in the monochromatic mode, as shown in FIG. 51, the DC ratio is in the range of +1.5 to 2.5 KV for the red ratio (red image density) of 5 to 25%. When a voltage is applied to the grid electrode of the transfer charger 21 and the image signal is only the black ratio, as shown in FIG. 52, the black ratio (black image density) is -1.5 to + 25%.
The DC control voltage in the range of 2.5 KV is set to be applied to the grid electrode of the transfer charger 21.

Therefore, in order to form a two-color image mode and a red or black single-color mode image using the image forming apparatus of the present invention, the control voltage is applied to the pre-transfer charger 15 and / or the transfer as shown in Table 2 below. This is performed by applying the voltage to the charger 21. That is, in the color mode, when the toner polarity is adjusted by the pre-transfer charger, the DC applied voltage of the grid electrode of the pre-transfer charger 15 is increased by +1.0 to 2.0 KV according to the red ratio / black ratio.
In addition, in the monochromatic mode, the operation of the pre-transfer charger 15 is turned off, and the polarity of the transfer charger 21 is switched according to the polarity of the toner, and according to the red rate or the black rate. The DC voltage applied to the grid electrode of the transfer charger 21 is variably adjusted in the range of +1.5 to 2.5 KV or -1.0 to 2.5 KV.

(8) Dirt on the pre-transfer charger Next, the present inventors conducted the following experiment to confirm the state of the pre-transfer charger. In this case, the test was performed using a negative polarity toner. As the pre-transfer charger 15, a scorotron having a grid G was used.

At that time, the discharge wire 77 of the pre-transfer charger 15 has an effective value
A voltage obtained by superimposing a DC voltage of -0.5 KV on an AC voltage of 3.5 KV was applied, and the voltage applied to the grid G was changed.

Further, as shown in FIG. 53, the copy image used was a stripe image in which black solid portions and white portions were alternately arranged along the axial direction of the photosensitive drum 7.

 FIG. 54 shows the degree of contamination of the grid.

As is apparent from this figure, a curve in the form of a "bathtub" was obtained with respect to the contamination of the grid. That is, it was found that when the voltage of the grid was −100 V to −500 V, almost no contamination occurred. On the other hand, in the positive region including the grid voltage of 0 V, the toner is negative, and therefore the adhesion is severe. When the grid voltage is 0V, the pre-transfer charger
Regardless of whether the 15 was on or off, it was found that the soil in the center of the grid was more intense than on the plus side.

Therefore, by applying a voltage to the grid G of the pre-transfer charger 15 where almost no dirt is generated, that is, a voltage of -100 V to -500 V, the dirt is prevented from being generated on the grid G of the pre-transfer charger 15. be able to. In the above-described embodiment, dirt on the grid G of the pre-transfer charger 15 has been described. However, when a corotron is used as the pre-transfer charger 15, by applying the same voltage to the shield as described above, It is possible to prevent the shield of the pre-transfer charger 15 from being stained.

(9) Prevention of Abnormal Discharge Next, the present inventors conducted the following experiment in order to clarify conditions for preventing the occurrence of abnormal discharge in the pre-transfer charger.

(9-1) Relationship Between Shield Surface State and Abnormal Discharge Next, the present inventors conducted an experiment as the pre-transfer charger 15 to examine how the leak rate changes depending on the shield surface state. Was.

 FIG. 55 shows the results of the above experiment.

In the figure, A shows the case where the inner surface of the shield was deeply scratched with a cutter, B shows the case where the inner surface of the shield was roughened 20 times with a No. 1000 sandpaper, and C shows the inner surface of the shield with a No. 60 sandpaper. D shows the case where the surface was roughened 20 times, and D shows the case where the shield with mirror finish was used.

As is clear from this figure, when there is a flaw on the inner surface of the shield, it is easy to leak, but it is found that there is almost no difference in the ease of leakage depending on the type of flaw on the inner surface of the shield.

(9-2) Relationship Between Shield End State and Abnormal Discharge Next, the present inventors conducted an experiment to examine how the leak occurrence rate changes depending on the shield end state of the pre-transfer charger 15. Was performed.

FIG. 56 (a) shows the shield 70 as shown in FIG. 56 (b).
With the end of the shield 70 simply cut off without any treatment on the end of the
The relationship between the moving distance of the end of the shield 70 from the position of the perpendicular line lowered to the end of the shield 70 and the voltage at the time of occurrence of leakage was measured. In FIG. 56 (a), A moves the end of the shield 70 to the right in FIG. 56 (b), and B moves the end of the shield 70 to the left in FIG. 56 (b). Each case is shown.

In FIG. 56 (a), the higher the output of the electrostatic simulator, the less the leakage is likely. As is clear from FIG.
When the end of the shield 70 is extended to the right side in FIG. 56 (b), that is, extended as shown in FIG.
The ratio d1 / d0 of the length d0 of the perpendicular line dropped from 77 to the shield 70 and the length d1 from the discharge wire 77 to the end of the shield 70 is
When the end of the shield 70 is retracted to the left in FIG. 56 (b), that is, retracted, as shown at B in FIG. 56 (a) so that it becomes 1.1 times or more, d1 / d0 becomes 1.4 times or more. So, shield
It has been found that by extending or retracting the end of 70, the output of the electrostatic simulator can be increased, ie, less leaky. Note that d0 is set to 6 mm.

The numerical values such as 1.1 times and 1.4 times are obtained by connecting the discharge wire 77 to the shield 70 and d0 = as shown in FIG. 56 (b).
When the shield voltage is 8.75 KV or more when the shield 70 is set to an infinite flat surface with a gap of 6 mm, it is judged that a safe range where there is no possibility of a seek, and as shown in FIG. 56 (b),
When the moving distance of the edge of the shield 70 when each measured value is 8.75 KV is l, And d0, that is, d1 / d0. At that time, the second decimal place was rounded up.

FIG. 57 shows a state in which the end of the shield 70 is bent to the back side as shown in FIG.
The relationship between the distance traveled from the position of the perpendicular drawn down to the end of the shield 70 to the end of the shield 70 and the voltage at the time of occurrence of leakage was measured.

In the figure, A and B are the same data of A and B in FIG. 56 (b), C is the case where the end of the shield 70 is moved to the right side in FIG. 58, and D is the end of the shield 70. FIG. 58 shows a case where the section is moved to the left side in FIG. 58.

In FIG. 57, the higher the output of the electrostatic simulator, the less the leakage is likely. As is apparent from FIG.
When the end of the shield 70 is extended to the right side in FIG. 58, that is, as shown in FIG. 58, the length d0 of the perpendicular extending from the discharge wire 77 to the shield 70 and the end of the shield 70 from the discharge wire 77 So that the ratio d1 / d0 to the length d1 up to 1.1 times or more,
When the end of the shield 70 is retracted to the left in FIG. 58, that is, when the end of the shield 70 is retracted as shown in FIG. 57B, the end of the shield 70 must be extended or retracted so that d1 / d0 becomes 1.5 times or more. Thus, it was found that the output of the electrostatic simulator can be made high, that is, it is difficult to leak. D0 is 6mm
Is set to

FIG. 59 shows a state in which a mylar sheet is stuck to the back surface of the shield 70 as shown in FIG.
The relationship between the distance to the end and the voltage at the time of leakage was measured.

In the figure, A and B are the same data as A and B in FIG. 56 (b), C is the case where the end of the shield 70 is moved to the right side in FIG. 60, and D is the end of the shield 70. 60 shows a case where the section is moved to the left in FIG.

As is clear from FIG. 59, if Mylar was attached to the back of the shield 70, the output of the electrostatic simulator would be low. Therefore, it was found that it was preferable not to attach Mylar to the back of the shield 70. When a mylar is stuck on the back of the shield 70, as shown in FIG.
If the end of the right side in FIG. 60 is extended, that is, the length d0 of a perpendicular line lowered from the discharge wire 77 to the shield 70,
Ratio from the length d1 from the discharge wire 77 to the end of the shield 70
When the end of the shield 70 is retracted to the left in FIG. 58, that is, when the end of the shield 70 is retracted as shown in D of FIG. 59 so that d1 / d0 is 1.5 times or more as an estimated value, d1 / d0 is 1.6 times or more. It has been found that by extending or retracting the end of the shield 70, the output of the electrostatic simulator can be increased, ie, less leaky. Note that d0 is set to 6 mm.

FIG. 61 shows the distance between the discharge wire 77 and the end of the mylar sheet from the perpendicular to the shield 70 when the mylar sheet is stuck to the inner surface of the shield 70 as shown in FIG. It measures the relationship with the voltage at the time.

In the figure, A and B represent the same data as A and B in FIG. 56 (b), C represents the case where the end of the mylar was moved to the right side in FIG. 62, and D represents the end of the mylar. FIG. 62 shows the case where it is moved to the left side in FIG.

As is clear from FIG. 61, when a mylar sheet is stuck on the inner surface of the shield 70, the output of the electrostatic simulator is significantly reduced. Therefore, it was found that it is preferable not to stick a mylar on the inner surface of the shield 70. In addition, when a mylar is stuck on the back surface of the shield 70, when the end of the shield 70 is extended rightward in FIG. 62, that is, as shown in FIG. 61C, the shield 70 is lowered from the discharge wire 77 to the shield 70. The length d0 of the perpendicular and the length d1 from the discharge wire 77 to the end of the mylar
When the end of the mylar is retracted to the left side in FIG. 62, that is, as shown in D of FIG. 61, so that the ratio d1 / d0 becomes 1.6 times or more, d1 / d0 becomes 1.7 times or more. Thus, it has been found that by extending or retracting the end of the mylar, the output of the electrostatic simulator can be increased, that is, made less likely to leak. Note that d0 is set to 6 mm.

FIG. 63 shows a state in which a Mylar sheet is stuck to the surface of the shield 70 and the end of the Mylar sheet is discharged by 2 mm from the end of the shield 70 as shown in FIG. The relationship between the distance from the position of the perpendicular line to the end of the shield 70 and the voltage at the time of occurrence of leakage was measured.

In the figure, A and B represent the same data as A and B in FIG. 56 (b), C represents the case where the end of the mylar was moved to the right side in FIG. 64, and D represents the end of the mylar. FIG. 64 shows the case where it is moved to the left side.

As is clear from FIG. 63, a mylar sheet is stuck on the inner surface of the shield 70, and the end of the mylar sheet is
It was found that, in a state where 2 mm was ejected from the end portion, the output of the electrostatic simulator was high, so that leakage was difficult.

FIG. 65 shows a state where the Mylar sheet is stuck to the surface of the shield 70 and the end of the Mylar sheet is discharged 5 mm from the end of the shield 70 as shown in FIG. The relationship between the distance from the position of the lowered perpendicular line to the end of the shield and the voltage at the time of leakage was measured.

In the figure, A and B represent the same data as A and B in FIG. 56 (b), C represents the case where the end of the mylar was moved to the right in FIG. 66, and D represents the end of the mylar. FIG. 66 shows the case where it is moved to the left side.

As is clear from FIG. 65, a mylar sheet is attached to the inner surface of the shield 70, and the end of the mylar sheet is
It was found that in a state in which 5 mm was ejected from the end of, the output of the electrostatic simulator became considerably high, so that it was difficult to leak.

FIG. 67 shows a state in which the Mylar sheet is stuck to the surface of the shield 70 and the end of the Mylar sheet is discharged 10 mm from the end of the shield 70 as shown in FIG. The relationship between the distance from the position of the lowered perpendicular line to the end of the shield and the voltage at the time of leakage was measured.

In the figure, A and B represent the same data as A and B in FIG. 56 (b), C represents the case where the end of the mylar was moved to the right side in FIG. 68, and D represents the end of the mylar. FIG. 68 shows the case where it is moved to the left side in FIG.

As is clear from FIG. 67, a mylar sheet is attached to the inner surface of the shield 70, and the end of the mylar sheet is
It was found that, in a state where it protruded by 10 mm from the end of, the output of the electrostatic simulator was greatly increased, and thus almost no leak problem occurred.

[Effects of the Invention] The present invention has the above configuration and operation, and pre-transfer charging for imparting charge to the toner image before transferring the toner image formed on the latent image carrier onto the transfer material. Since the means is provided, it is possible to transfer the toner images having different polarities formed on the latent image carrier with good polarity and to transfer a good image.

That is, according to the first aspect of the present invention, a primary image is formed by the second developing unit by using a specific one color toner for forming a secondary image by the first developing unit. Since the black toner is used, the black image forming the main image such as a character or a figure is not affected by the first developing means, so that no image disturbance occurs, and the image density and sharpness etc. are reduced. It is possible to form a black image forming a main image such as a character or a figure with good image quality without deterioration. In the first developing means, a specific one color toner for forming a secondary image is used, and the specific one color toner image developed by the first developing means is almost all. In this case, an image such as a character or a figure is a secondary image that is used to emphasize or emphasize a main image such as a character or a figure by underlining or halftone dots or enclosing the image with a frame. Even if affected by the second developing means in the latter stage, it is not so noticeable.

According to the first aspect of the present invention, the pre-transfer charging unit reverses the polarity of the specific one color toner image formed on the latent image carrier to change the polarity of the two toner images. Since the configuration is such that a charge is applied to align the toner, the polarity of the first color toner image formed on the latent image carrier is reversed, so that the charging polarity of the second color black toner image is: As it is, the black toner image forming the main image can be reliably prevented from scattering due to pre-transfer charging.

Further, according to the second aspect of the present invention, the second developing means uses an electrostatic latent image formed on the latent image carrier by a developing method using a resin carrier (MT carrier) having a small particle diameter. Since the image is developed with the black toner, the disturbance of the toner image that occurs as a result of the kinetic energy of the developer of the second developing unit exerting on the color toner image of the first color is small by the second developing unit. Particle size resin carrier (MT carrier)
By using, the kinetic energy of the developer of the second developing unit is reduced, thereby making it possible to suppress the kinetic energy.

According to the third or fourth aspect of the present invention, as a result of various experiments in which the positional relationship between the discharge wire and the shield of the pre-transfer charging means is changed, when the shield is extended from the discharge wire, the shield from the discharge wire is used. Length of vertical line
The ratio d1 / d0 of d0 and the length d1 from the discharge wire to the end of the shield is set to be 1.1 times or more, or when the shield is retracted from the discharge wire, the above d1 / d0 is used. By setting the ratio to be 1.4 times or more, it becomes possible to increase the output of the electrostatic simulator, that is, to make it difficult to leak.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an electrophotographic copying machine as one embodiment of an image forming apparatus according to the present invention, FIG. 2 is a perspective view showing the appearance of the electrophotographic copying machine, and FIG. 4 (a) to 4 (g) are process diagrams each showing an image forming process, FIG. 5 is a configuration diagram showing a main part of FIG. 1, and FIG. 6 is a first developing device. FIG. 7 is a perspective explanatory view of the developing device, FIG. 8 is an explanatory view of a partially broken portion of the developing device, FIG. 9 is a plan view showing a retracting mechanism, FIG.
FIGS. 11 and 11 are side views of the essential parts showing the operation of the retracting mechanism, respectively. FIGS. 12 and 13 are sectional views of the essential parts and the whole sectional view showing the second developing device, and FIG. FIG. 15 is a perspective view showing a pre-transfer charger, FIG. 16 is a cross-sectional view of the pre-transfer charger, and FIG. 17 (a) is a pre-transfer charger. FIG. 17 (b) is a perspective view showing a main part of the pre-transfer charger, FIG. 17 (c) is a circuit diagram showing an electric system of the pre-transfer charger, and FIG. FIG. 19 is a plan view showing a charger and a separate charger, FIG. 19 is a graph showing a relationship between an applied voltage of a pre-transfer charger and a toner charge amount, and FIG. 20 is a relationship between a surface potential of a photosensitive drum and a toner charge amount. FIG. 21 is a graph showing the relationship between the applied voltage of the pre-transfer charger and the toner charge amount, and FIG. 22 is a graph showing the applied voltage of the pre-transfer charger and the photosensitive drum. 23 is a graph showing the relationship between the surface potential of the photosensitive drum and the toner charge amount, and FIG. 24 is a graph showing the relationship between the applied voltage of the pre-transfer charger and the toner charge amount. Graph showing the 25th
FIG. 26 is an explanatory view showing a test pattern, FIG. 26 is a graph showing a relationship between an applied voltage of a transfer charger and an image density, FIG. 27 is an explanatory view showing a state of radiance, and FIGS.
(D) is a graph showing the relationship between the toner density and the line width, respectively. FIGS. 29 (a) to (d) are graphs showing the relationship between the toner density and the image density, respectively, and FIGS. 30 (a) to (d). ) Is a graph showing the relationship between the toner density and the rigidity, respectively, FIGS. 31 to 33 are graphs showing the relationship between the toner density and the image density, respectively, FIG. 34 is a schematic diagram showing the degree of blur, FIG. FIG. 36 is a graph showing the relationship between the toner density and the degree of blur, respectively. FIGS. 37 to 39 are schematic diagrams showing the peeling state of the transfer paper, respectively. FIG. 40 is an apparatus for measuring the density of the toner image. FIG. 41 is a sectional view showing a main part of FIG. 40, FIG. 42 is a plan view showing a mask member,
FIG. 43 is a graph showing the relationship between the potential of the photosensitive drum and the amount of toner charge, FIG. 44 is a graph showing the relationship between the applied voltage of the pre-transfer charger and the surface potential of the photosensitive drum, and FIG. FIGS. 46 and 47 are schematic diagrams respectively showing changes in the surface potential of the photosensitive drum, FIGS. 46 and 47 are graphs showing the applied voltage of the transfer charger, and FIG. 48 is another embodiment of the present invention. FIG. 49 is a flowchart showing the operation of the apparatus shown in FIG. 48, and FIGS. 50 to 52 show graphs, charts, and FIG. 53 showing copied images showing the operation of the apparatus shown in FIG. 48, respectively. FIG. 54 is a graph showing the relationship between the grid voltage of the pre-transfer charger and the contamination of the grid, FIG. 55 is a graph showing the relationship between the output of the electrostatic simulator and the leak occurrence rate, and FIG. ) And (b) show the movement distance of the edge of the shield and the electrostatic simulator at the time of leakage. And FIG. 57 and FIG. 58 are graphs showing the relationship between the output of the shield wire and the positional relationship between the discharge wire and the edge of the shield. FIGS. 59 and 60 are graphs showing the relationship and the positional relationship between the discharge wire and the edge of the shield, and FIGS. 59 and 60 are graphs showing the relationship between the moving distance of the edge of the shield and the output of the electrostatic simulator when a leak occurs. And an explanatory diagram showing the positional relationship between the discharge wire and the edge of the shield, FIG. 61 and FIG. FIG. 63 and FIG. 64 are explanatory diagrams showing the positional relationship with the edge of the mylar, and FIGS. 63 and 64 are graphs showing the relationship between the moving distance of the edge of the shield and the output of the electrostatic simulator at the time of leakage. FIGS. 65 and 66 are graphs showing the relationship between the movement distance of the shield edge and the output of the electrostatic simulator when a leak occurs, and FIG. FIG. 67 and FIG. 68 are graphs showing the relationship between the moving distance of the edge of the shield and the output of the electrostatic simulator when a leak occurs, and the discharge wire and the edge of the shield. FIG. 69 is a configuration diagram showing a main part of an electrophotographic copying machine to which a conventional transfer process device is applied, and FIGS. 70 (a) to 70 (g) are conventional electrophotographic copying machines. FIG. 4 is a process chart showing the image forming process of FIG. [Explanation of Symbols] 7: Photoconductor drum (latent image carrier) 13: First developing device 14: Second developing device 15: Pre-transfer charger (pre-transfer charging unit) 21: Transfer Charger 80 Shield 87 Discharge wire

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Takayuki Tsutsui 2274 Hongo, Ebina-shi, Kanagawa Fuji Xerox Co., Ltd. Ebina Works (56) References JP-A-1-259390 (JP, A) JP-A-62- 94863 (JP, A) Actually open sho 64-30554 (JP, U)

Claims (4)

(57) [Claims] 1. A latent image carrier whose surface is uniformly charged to a predetermined polarity and potential by a primary charging means, and a specific one color image on the surface of the latent image carrier by an image portion exposure system. A first image exposure means for exposing to form an electrostatic latent image; and an electrostatic latent image formed on the latent image carrier by the first image exposure means having the same polarity as the charge polarity of the latent image carrier. A first developing means for reversal developing with a specific color toner of a specific polarity; and a black image continuously formed on the surface of the latent image carrier on which the specific one color toner image is formed by the first developing means. A second image exposure means for exposing the latent image by a background exposure method to form an electrostatic latent image, and an electrostatic latent image formed on the latent image carrier by the second image exposure means. Second development in which regular development is performed with black toner having a polarity opposite to the charged polarity of the body Means for transferring the two toner images having different polarities formed on the latent image carrier onto a transfer material before transferring the polarity of the specific one color toner image formed on the latent image carrier; An image forming apparatus comprising: a pre-transfer charging unit for applying a charge having a polarity opposite to the charge polarity of the latent image carrier in order to make the two toner images have the same polarity by reversing the polarity. 2. The image forming apparatus according to claim 1, wherein the first developing means converts the electrostatic latent image formed on the latent image carrier to a specific color toner by a developing method using an insulating magnetic brush (IMB). And developing the electrostatic latent image formed on the latent image carrier with black toner by a developing method using a resin carrier (MT carrier) having a small particle diameter. The image forming apparatus according to claim 1, wherein: 3. The pre-transfer charging means, when the shield extends from the discharge wire, includes a length d0 of a perpendicular line extending from the discharge wire to the shield, and a length d1 from the discharge wire to an end of the shield. 3. The image forming apparatus according to claim 1, wherein the ratio d1 / d0 is set to be 1.1 times or more. 4. The pre-transfer charging means, when the shield is retracted from the discharge wire, has a length d0 of a perpendicular extending from the discharge wire to the shield, and a length d1 from the discharge wire to an end of the shield. 3. The image forming apparatus according to claim 1, wherein the ratio d1 / d0 is set to be 1.4 times or more.