JP2964493B2 - Multicolor image forming device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multicolor image forming apparatus, and more specifically, to form an image using a plurality of types of toners using an electrostatic latent image. The present invention relates to a multi-color image forming apparatus such as a copying machine or a printer.

2. Description of the Related Art As a conventional multi-color image forming apparatus of this type, an image forming apparatus as shown in FIG. 29 is known.

In this image forming apparatus, a charger b for uniformly charging the latent image carrier a is disposed at a position facing the latent image carrier a, and a downstream side of the charger b, that is, the latent image carrier a The first latent image forming unit c and the first
The toner images formed on the latent image carrier a formed by the developing means d, the second latent image forming means e, the second developing means f, and the first and second developing means d and f have the same polarity. The pre-transfer charger g and the plurality of toner images on the latent image carrier a are transferred to the transfer medium i.
And a second developing means f.
Has a structure in which a magnet member is provided inside and a latent image carrier a in which a magnetic brush of a two-component developer composed of a toner and a magnetic carrier is formed between the latent image carrier a and the latent image carrier a.

In a conventional image forming apparatus of this type configured as described above, a color image is recorded through the following steps. That is, after the surface potential of the latent image carrier a is charged to one polarity (for example, minus) by the charger b (see FIG. 30 (a)), the first latent image forming means c supplies the first latent image. Forming an image (see FIG. 30 (b)), and
By applying a predetermined developing bias voltage to the first developing means d, a first toner image is formed by reversal development (see FIG. 30 (c)). Next, a second latent image is formed on the latent image carrier a by the second latent image forming means e (see FIG. 30 (d)), and a predetermined developing bias voltage is applied to the second developing means f. Is applied, a second toner image is formed by regular development (see FIG. 30 (e)), and the polarity of both toner images is set to the same polarity (for example, plus) by the pre-transfer charger g. After the alignment (see FIG. 30 (f)), both color toner images are collectively transferred onto the transfer medium i by the transfer means h, so that a color image is recorded. Here, the case where two color images are formed is described. However, when forming three or more color images, the number of latent image forming units and the number of developing units are increased according to the number of colors. Similarly, a multicolor image can be formed.

[Problems to be Solved by the Invention] In the conventional image forming apparatus configured as described above, in the second and subsequent developing units, the latent image on which the toner image is already carried by the first Since the image carrier a is again rubbed with the developer of the second and subsequent developing units, the toner image formed in the first developing step is The process has the disadvantage that it is significantly disturbed, and consequently the resulting color image is a highly disturbed color image. Therefore, in the image forming method using this kind of the overlapping developing method, how to develop the toner image formed by the first developing means d by the second and subsequent developing means without disturbing the toner image. Has become a very important issue.

As a means for solving the above-mentioned problems, it is conceivable to use a one-component developer for the second and subsequent developing units and perform the non-contact development. Since an AC bias is applied between the body and the developing means to form a toner image, it is difficult to increase the speed, and the first developed image is electrically affected in the second and subsequent development steps. The disadvantage is that the image is disturbed. Therefore, it is considered that a magnetic brush developing method using a two-component developer composed of a toner and a magnetic carrier is preferable for the second and subsequent developing means.

On the other hand, in the magnetic brush developing method, which is best known as a two-component developer, as described above, the developer in the developing area rubs the first-stage toner so as to rub the first-stage toner in the multi-color mode. There is a problem that the developed image is disturbed, and in order to solve this problem, a resin carrier (MT carrier) having a small particle diameter in which the developer of the second and subsequent developing means is in soft contact with the latent image carrier a Is used.

However, when the MT carrier is used, the charge becomes high in a region where the spatial frequency of the electrostatic latent image is high (for example, 4 to 10 lines / mm), so that the carrier is electrostatically attracted onto the latent image carrier a ( (Hereinafter, referred to as carrier adhesion) is likely to occur, and this carrier adhesion mainly occurs between the toner images in the second and subsequent development steps, resulting in a secondary adverse effect of deteriorating the image quality.

Therefore, it is an object to prevent carrier adhesion, which is a secondary adverse effect associated with the prevention of image disorder in the second and subsequent development steps.

Further, in this type of multi-color image forming apparatus, there is a problem that the developer of the first or previous developing unit is mixed with the developer of the second or subsequent stage in the developing process of the second or subsequent stage.

Therefore, in this type of multicolor image forming apparatus, firstly, it is necessary to prevent image disorder of the first-stage developed image in the second and subsequent developing steps, and secondly, to prevent image disorder. Third, it is important to prevent color mixing in the second and subsequent developing means to achieve high image quality and to increase the life of the apparatus. It has become a challenge.

The present invention has been made in view of the above circumstances, and in the developing means of the second and subsequent stages, while preventing the image disorder of the first-stage developed image, preventing the occurrence of carrier adhesion accompanying the image disorder prevention, It is an object of the present invention to provide a multicolor image forming apparatus characterized in that color mixing of a first-stage or previous-stage developer is prevented in a second-stage developing unit and higher-quality images can be achieved. .

[Means for Solving the Problems] In order to achieve the above object, a multicolor image forming apparatus according to the present invention includes a latent image carrier and a plurality of electrostatic images based on different image information on the latent image carrier. A plurality of latent image forming means for forming a latent image; and a developing carrier opposed to the latent image carrier, and each of the latent images on the latent image carrier is tonered with toner held on the developing carrier. A plurality of developing means for developing, and a transfer means for collectively transferring the plurality of toner images on the latent image carrier from the latent image carrier directly to a transfer medium, wherein at least the second and subsequent developing means include the developing means A multi-color image forming apparatus that encloses a magnet member in a carrier, forms a magnetic brush of a two-component developer including a toner and a magnetic carrier on the developing carrier, and develops a latent image on the latent image carrier is provided. Assuming that it is above the first stage developing means The image forming apparatus further comprises image disturbance preventing means for controlling the developed image to be developed on the latent image carrier to a toner image within a range not disturbed by the developing means of the second and subsequent stages.

In the present invention, the image disturbance preventing means is configured such that a first-stage developed image is rubbed by a magnetic brush in a second-stage or later developing process during a developing process in a second-stage or later developing device, or a carrier. In order to prevent the toner image from being disturbed by the adhesion or the like, and considering that there is a strong correlation between the density of the toner image and the image disorder level by the first-stage developing means, the first-stage developing means It is conceivable to take measures to prevent image disturbance, or to take measures to prevent image disturbance during the developing step by the developing means of the second and subsequent stages.

That is, in the former case, the density may be controlled to a level that does not cause image disorder.For example, the image disorder preventing means may be replaced with a reference pattern for forming a reference density pattern corresponding to the toner image density on the latent image carrier. An image forming unit, a reference pattern density detecting unit that detects the density of the reference pattern image, and a developing bias control unit that controls the developing bias of the first-stage developing unit based on a signal from the reference density detecting unit can do.

In this case, the reference pattern image forming section forms a reference density image corresponding to the first toner image density before forming the first latent image on the latent image carrier, and includes an arbitrary reference latent image pattern. And a developing unit for visualizing the reference latent image pattern. Preferably, the latent image forming unit is formed by the first-stage latent image forming unit. At the same time, it is better to form the developing means by the first developing means. In this case, it is necessary to accurately determine the relationship between the development density and the development applied bias potential under different environments of the developer to be used as the reference latent image pattern. For example, a mesh pattern of an arbitrary size having an area ratio of 10 to 30% is required. By doing so, the detection sensitivity can be improved.

Further, the reference pattern density detection unit may be any as long as it detects the density of the reference pattern image,
For example, it can be formed by optical density detecting means such as a photo sensor or a surface voltmeter. In this case, as long as the reference pattern density detection unit can detect the reference density image formed by the reference pattern image forming unit, the installation position may be an arbitrary position. Any location may be used as long as it is on the downstream side of the image forming unit, that is, on the post-processing side and upstream of the cleaning device, that is, on the pre-processing side.

The developing bias control unit controls the voltage applied to the developing bias applying unit in order to provide an image density in a region where the image of the developer does not occur in an environment in which the image forming apparatus is operated. A central processing unit (CP) that determines a predetermined bias voltage by comparing and calculating a signal detected by the density detection unit and a reference value measured in advance.
U).

Further, as an image disturbance preventing means, a toner is provided between the first-stage developing means and the second-stage developing means, and is developed on the latent image carrier by the first-stage developing means. A density control member for removing excess toner corresponding to the excess density from the image may be provided. In this case, a rotatable conductive roll provided with the density control member at a position close to the latent image carrier, a drive source for driving the conductive roll to rotate, and removing toner adhered on the conductive roll It may be formed by a cleaning member, or a conductive plate disposed at a position close to the latent image carrier, and a bias for selectively applying a bias of positive and reverse polarities to the conductive plate via switching means. It can be formed with an applied voltage source. In this case, the conductive roll and the conductive plate are:
For example, it can be formed of a conductive member made of aluminum, stainless steel, steel, or the like.

Further, the image disturbance preventing means may be constituted by an AC and DC superimposed bias applying unit for applying a developing bias to the first stage developing means. In this case, as long as the developing efficiency can be increased and the electric coupling force of the developed image on the latent image carrier can be strengthened to prevent image disturbance, an AC and DC superimposed bias voltage to the developing unit is used. May be any voltage, but it is preferable to set the gap between the latent image carrier and the developing roll of the developing means as narrow as possible (for example, 0.4 mm).

As described above, the reason why the first-stage developed image is disturbed in the second and subsequent development steps is that the mechanical rubbing force of the developer is caused by the first-stage latent image. Because it is caused by exceeding the electrostatic adhesion between the toner and the toner.
Second, the first-stage excess toner is far from the latent image, and the charge amount of the toner is also low.

Therefore, it is meaningful to take measures for preventing image disturbance in advance in the first-stage developing unit. Further, as another countermeasure at the first stage, it is preferable to set the contrast potential of the latent image to be high, to increase the charge amount of the toner, not to over-develop, and to use a small diameter toner. There is.

On the other hand, in order to achieve both the securing of the developing ability and the prevention of image disorder in the latter, that is, the developing means of the second and subsequent stages, the optimal developing agent for the latent image carrier is used as the developing means of the second and subsequent stages. It is necessary to secure the sliding force.

That is, when the latent image is rubbed by the chain of the developed image as in the case of the normal development with the magnetic brush in the developing means of the second and subsequent stages, the toner image of the first stage is mainly mechanically disturbed. . Therefore, in order to prevent the toner image from being disturbed, for example, when the flow rate of the developer or the density of the developer in the gap between the latent image carriers and the Cannot be supplied, and a desired toner density cannot be obtained. Therefore,
As means for achieving both of these, for example, the above-mentioned image disturbance preventing means is provided by a pair of oppositely opposed polarities of the latent image carrier to constitute a magnet member of the development carrier in the second and subsequent developing units. And the peripheral speed of the developing carrier with respect to the latent image carrier is 0.6 to 1.4.
In this case, it is preferable that the repulsion difference between the two repulsion magnetic poles is 350 to 100 Gauss. In this case, the configuration of the repulsion magnetic pole may be changed as appropriate by, for example, providing a pair of magnetic poles in the repulsion magnetic pole portion of the magnet member by a magnetizing method, or arranging magnets of the same polarity adjacent to each other. Although it does not matter, it is preferable to form a concave groove along the axial direction of the developer carrier on the surface of the main pole, and appropriately set the magnetic force and repulsion difference of the repulsive magnetic pole according to the depth and width of the concave groove. It is better to do it. Further, the magnetic force densities of both repulsive magnetic poles may be the same, but preferably the magnetic force of the repulsive magnetic pole located on the upstream side in the rotation direction of the latent image carrier is A, and the magnetic force is located on the downstream side in the rotational direction of the latent image carrier. When the magnetic force of the repulsive magnetic pole is B, BA ≧ 0.
It is better to have a 1B relationship.

Further, the peripheral speed of the developing carrier with respect to the latent image carrier may be set to be constant, but it is preferable that the developing carrier is switchable between the single color mode and the multicolor mode in order to achieve high image quality in the single color mode. Better.

Further, in the developing means of the second and subsequent stages, the developer is magnetically separated from the development carrier in the development area, and the developer is dispersed individually to reduce the mechanical energy. In order to reduce the mechanical energy of the developer, it is necessary to reduce the size and density of the carrier. Therefore, a magnetic carrier of, for example, a range of 20 to 60 μm can be used as the magnetic carrier of the two-component developer in the developing means of the second and subsequent stages. However, in order to prevent image quality deterioration due to carrier adhesion, the center of the carrier is preferably used. When the particle size is 45 μm, it is preferable to make the magnetic carrier having 50% or less of the median particle size of the carrier 10% by weight or less of the whole magnetic carrier.

By taking the above measures, it is possible to sufficiently prevent the image disorder of the first-stage developed image, but if it is not possible to prevent carrier adhesion during the developing process of the second-stage and later developing means,
If the carrier recovery means for collecting the magnetic carrier adhering to the latent image carrier is arranged downstream of the latent image carrier in the rotation direction of the developing means of the second and subsequent stages, more thorough under any condition As a result, the speed can be increased, and a high-quality image can be obtained. At this time, the bias applied to the carrier collecting means may be set to a constant value. However, in general, in a superposition developing system in which normal development is performed after reversal development, the developed image toner (first Since the polarity of the developed image toner) and the polarity of the carrier of the developer of the second stage developing means (the carrier of the second developer) are the same, the bias of the carrier collecting means is opposite to that of the carrier of the second developer. Polarity cannot be set and carrier recovery means
A bias having the same polarity as that of the carrier of the developer is applied. However, this cannot achieve high image quality by preventing the carrier from adhering at the time of single-color copying of the second-stage developing means occupying about 80% of the whole. There is an inconvenience. Therefore,
In the multicolor mode, a bias having the same polarity as that of the carriers of the first developing toner and the second developer is applied to the carrier collecting unit in order to prevent the toner of the first developed image from being attracted to the carrier collecting unit in the multicolor mode. In the monochrome mode, it is preferable that the bias can be switched so that a bias having a polarity opposite to the polarity of the carrier of the second developer is applied to the carrier collecting means.

In addition, in order to collect the carrier collected by the carrier collection unit, it is preferable to form a collection box with the developing unit. In this case, the collecting box may be formed integrally with the developing means, but by forming the collecting box detachably with respect to the developing means, the carrier can be easily taken out after collecting.

[Operation] The above technical means works as follows.

First, when the mode is set to the multi-color mode, the latent image carrier is uniformly charged by the charger, and the surface potential of the latent image carrier is charged to one polarity. In this state, the first
A first latent image is formed by the latent image forming means, and a predetermined developing bias voltage is applied to the first developing means, whereby the first toner image is formed by inversion or regular development. Next, a latent image is formed on the latent image carrier by the second and subsequent latent image forming means, and a predetermined developing bias voltage is applied to each of the second and subsequent developing means to form the toner image. Is formed by regular or reversal development, and
After the polarity of both toner images is aligned by the pre-transfer charger,
A color image is recorded by transferring both toner images onto the transfer medium at once by the transfer means. In the course of such a multi-color image forming process, the image disturbance preventing means causes the latent image carrier on the latent image carrier during the developing step of the first-stage developing means or the developing means of the second-stage developing means or later. The image disturbance of the toner image formed on the surface of the image is prevented, and the color mixture or the carrier adhesion caused by the prevention of the image disturbance can be prevented. When the mode is set to the single color mode, the peripheral speed of the developing carrier is switched, so that the image quality is maintained at a higher image quality at the time of single color copying than at the time of the multicolor mode.

Embodiment An embodiment of the present invention will be described below in detail with reference to the drawings.

<< Table of Contents >> I. Outline of the equipment II. Circuit configuration of the equipment III. Specific configuration of the equipment IV. Characteristics of the equipment 1) Automatic density control (ADC) in the first stage developing means
2) First development density control by density control member 3) Prevention of image disturbance by improving development efficiency in first stage 4) Image disturbance by regulation of magnetic flux density of magnet member in second and subsequent stages Prevention 5) Prevention of carrier adhesion by regulating the magnetic flux density of the magnet member in the second and subsequent stages 6) Improvement of the image quality of monochromatic images by switching the peripheral speed ratio in the developing carrier of the developing means in the second and subsequent stages 7) Prevention of Carrier Adhesion by Specifying Particle Size Distribution of Magnetic Carriers in Second and Subsequent Developing Means 8) Prevention of Carrier Adhesion by Switching Applied Bias of Carrier Recovery Means after Second and Subsequent Development 9) Second Stage Prevention of Color Mixing Following Carrier Recovery After Development V. Summary I. Overview of Apparatus FIG. 1 is an external perspective view of a digital color copying machine having a multicolor image forming apparatus of the present invention, and FIG. Its schematic cross-sectional view is shown.

The color copier includes a platen 12 provided on an upper surface side of a copier body 10 for mounting a document (not shown), and image information for inputting image information from a document provided below the platen 12. A reading unit 14, an image forming apparatus 20 that receives a signal from the image information reading unit 14 and forms a predetermined image;
A sheet as a transfer medium for transferring the image formed on the image forming apparatus 20
The paper transfer unit 30 includes a transfer unit 30 that transfers the paper 16, and a paper supply unit 40 and a paper transport unit 50 that supply and transport the paper 16.

The paper supply unit includes three paper trays 41 to 43 each accommodating paper 16 according to the size, a large-capacity tray 44, a two-sided tray 45, and a manual feed tray 46. The reversing chute 51, the first paper transport path 52 for transporting the paper sent from each of the trays 41 to 46 to the image forming apparatus 20, and the copied paper again in the image forming apparatus 20.
And a second paper transport path 53 for transporting the paper to the second paper transport path.

The sheet on which the toner image has been transferred by the image forming apparatus 20 is sent to the fixing device 60 through the discharge side conveyance path 54, and the fixing device 60
After the toner image is fixed by the pressure roll 61 and the heating roll 62, the toner image is conveyed to the discharge tray 47.

II. Circuit Configuration of Apparatus FIG. 3 is a block diagram showing an outline of a circuit configuration of a color copying machine which is an image forming apparatus of the present invention.

As shown in FIG. 3, a distributed CPU architecture using serial communication centered on the main CPU 121 is adopted. This is to enable optimal arrangement of the controller and to provide optimal cost performance. Furthermore,
From the viewpoint of product development of an image forming apparatus, it is expected that software development by module design can be shortened, made more efficient, reduced in cost, and easier to troubleshoot.

In addition, since the processing efficiency is improved by decentralizing the processing by a plurality of CPUs, for example, a program that requires complicated and high-speed processing by using an inexpensive 8-bit CPU instead of using an expensive 16-bit CPU Can be performed.

Further, such processing decentralization facilitates model development. That is, even when a new input / output device or the like is developed, it may not be necessary to modify the program on the main unit (main CPU side), and even when a change is necessary, the program can be minimized.

Further, regarding the printed circuit board on the main body side, there is no need to store unnecessary I / O boats and programs due to the distribution of the CPU. Therefore, the cost of the printed circuit board can be reduced, and the degree of freedom of the arrangement space is improved.

This color copier is the main CPU 1 in the copier body 10.
The main body 10 is controlled by 21 and the CPU 122 for the inter-image lamp. CPU1 for this inter-image lamp
Reference numeral 22 denotes a CPU that specializes in controlling an inter-image lamp. The inter-image lamp irradiates a latent image carrier 70 (hereinafter, referred to as a photosensitive drum) after exposure with an electrostatic latent image before development. It is a lamp used for erasing a part or the like. Further, since the copier of this embodiment has a function of editing an image and the like, an inter-image lamp is used to limit the formation of an electrostatic latent image to a predetermined area. For this purpose, an independent CPU is used in addition to the main CPU 121.

In the copying machine of this embodiment, the following CPUs are used, and the CPUs are connected by communication lamps 123 and 124.

Document feed CPU 125; this is a CPU for controlling an automatic paper feeder and the like.

Sorter CPU 126; this is a CPU arranged in a sorter (not shown).

Display CPU 127: This is a CPU used to display various information in Chinese characters on the liquid crystal display unit 11a attached to the console panel 11, and to display an area for editing.

Tray control CPU 128; this is a CPU for controlling trays added to the copying machine body 10. This CPU is arranged on the rear side of the tray cabinet that accommodates each tray, and performs these controls according to the trays to be arranged. Intermediate tray 45 among these trays
The (double-sided tray) independently has a motor for transporting copy paper, and complicated control is performed such that the storage position of the copy paper stored in this tray varies depending on its size.

Card CPU 129; This is a part for reading and writing when an IC card is used for designating the coordinates of a document.

FIG. 4 more specifically shows a circuit configuration centered on the main CPU 121.

As described above, the main CPU 121 is connected to the DADF 13 (double-sided automatic document feeder), the sorter 15, the liquid crystal display 11a, the edit pad 132, and the serial communication lines 123 and 124 as described above.
It is connected to each unit of the inter-image lamp controller 139 and the tray control unit 133.

The main CPU 121 has an A / D converter built therein and is connected to the following units via an analog data line 134. Such CPUs include, for example, μPD7810CW and μPD7811CW manufactured by NEC Corporation and MB
An 8-bit one-chip CPU such as 89713X can be used.

Light amount sensor 135; this is a sensor used when detecting the amount of light from an exposure lamp (not shown) and controlling the same.

Temperature sensor group 136; these are sensors such as a soft touch sensor for controlling the fixing temperature.

Paper size sensor group 137; this is a sensor for detecting the size of the paper stored in the paper supply unit 50 or the like. According to the system configuration of the copying machine of this embodiment, copy sheets can be sent from up to four types of trays.

Further, the main CPU 121 is reset by a reset circuit 138 at runaway or initial rise, and the bus line 121A
Are connected to the following units via

Keyboard / Display LSI (Large Scale Integrated Circuit) 121
B; This is a circuit for mediating data with the console panel 11.

Timer / counter LSI 121C; this is a circuit for controlling driving of the main motor 140.

ROM 121D; this is a read-only memory having a capacity of 56 Kbytes and storing various control programs.

RAM 121E; This is a random access memory for storage of data having a capacity of 6 Kbytes. A non-volatile memory (NVM) 121F is connected to the RAM 121E so that necessary data can be stored even when the power of the copying machine is turned off.

First I / O controller 121G; this is a filter circuit 121H
Is an input / output controller that inputs various data via the driver circuit and drives various components via the driver circuit 121I.

Second I / O controller 121J; this is a filter circuit 121K
Is an input / output controller that inputs various data through the driver circuit and drives various components through the driver circuit 121L.

III. Specific Configuration of Apparatus FIG. 5 is an enlarged sectional view of a main part of an example of the image forming apparatus of the present invention.

The image forming apparatus includes a photosensitive drum 70, a charger 72, a first latent image forming unit 74, and a first developing unit 80 (hereinafter referred to as a first developing unit) which are sequentially disposed along the outer periphery of the photosensitive drum 70. ), A second latent image forming means 76, a second developing means 90 (hereinafter, referred to as a second developing device), and a pre-transfer charger 78, and a visible image formed on the photosensitive drum 70. Is the transfer unit
The data is transferred to the paper 16 at 30.

In order to record a color image in the image forming apparatus configured as described above, first, as shown in FIGS. 6 (a) to 6 (f), the mode of the console panel 11 is switched to the color mode. The photosensitive drum 70 is uniformly charged by 72, and the surface potential of the photosensitive drum 70 is charged to one polarity (minus) (see FIG. 6A). In this state, after a first latent image is formed on the photosensitive drum 70 by the first latent image forming means 74 (see FIG. 6B), a predetermined developing bias voltage is applied to the first developing device 80. Is applied, a first toner image is formed by reversal development (see FIG. 6 (c)). Next, a second latent image is formed on the photosensitive drum 70 by the second latent image forming means 76 (see FIG. 6D), and a predetermined developing bias voltage is applied to the second developing device 90. As a result, a toner image is formed on the photosensitive drum 70 by regular development (see FIG. 6E), and the polarity of both toner images is made uniform by the pre-transfer charger 78 (see FIG. f)), a color image is recorded by transferring both toner images onto the sheet at a time in the transfer unit 30.

In this case, the first and second latent image forming means 74, 76
Is formed of a laser diode, and receives a signal from the image information reading unit 14 to form a predetermined latent image on the photosensitive drum 70.
Writes on top. In this case, the first latent image forming means 74 has a method of writing an image portion, and the second latent image forming means 76 has a method of writing a background portion. It doesn't have to be
The latent image forming means 74 writes a background part and the second latent image forming means 76 writes an image part, or the latent image forming means 74 and 76 write an image part or a background part. You may.

The first developing device 80 has a two-component developing toner for color (red). The first developing device 80 includes a developing roller 81 provided on the photosensitive drum 70 side and a partition 82 on the rear side of the developing roller 81. A first screw auger 83a and a second screw auger 83b, a third screw auger 83c provided via a receiving plate 84 above the screw augers 83a and 83b, and an upstream side of the opening of the housing 80a. A film-shaped sealing member 85 that is attached to the photosensitive drum 70 and is in contact with the photosensitive drum 70, a blade 86 that regulates the thickness of the developer conveyed by the developing roll 81, and a toner box 87 that is attached to the rear side of the housing 80a. A toner auger 87a and a toner auger 87b are provided. In this case, the developing roll 81 is formed of a non-magnetic conductive member such as an aluminum alloy or stainless steel.

The second developing device 90 is formed by a developing device having a black two-component developer.
A magnet roll 91 disposed on the 70 side, upper and lower developer transfer paddles 92, 93 disposed on the back side of the magnet roll 91, and a back side of the developer transfer paddles 92, 93. And auger 94 and 95 for developer transport and stirring.

The first and second developing devices 80 and 90 can alternately move toward and away from the photosensitive drum 70 by a retract mechanism 100 provided on the back.

The retract mechanism 100 includes the first and second developing devices 8.
0, 90 are moved toward and away from the photosensitive drum 70, and both developing units 80, 90 have the same structure.
The zero retract mechanism 100 will be described with reference to FIG.

The retract mechanism 100 includes a crank portion 102 formed on a crankshaft 101 rotated by a drive source (not shown), a yoke cam 103 following the crank portion 102, and elasticity in a direction in which the crank portion 102 and the yoke cam 103 are always in contact. It comprises a return spring 104 for urging the force and separating the developing device 80, and pressure springs 105, 105 for urging the elastic force in a direction in which the developing device 80 always approaches the photosensitive drum 70 side.

In this case, the yoke cam 103 is provided with a guide elongated hole 109 for guiding a guide pin 108 slidably fitted to a forked arm 107 provided on the holding portion 106 of the developing device 80. 8 and 9).

The crank part 1 of the crankshaft 101 formed as described above
8 and 9 are engaged as shown in FIGS. 8 and 9, and when the developing device 80 is separated, when the crankshaft 101 rotates counterclockwise from the state shown in FIG. The yoke cam 103 is moved in the separating direction (right side in FIG. 8) against the elastic force of 105, and the developing device 80 is reliably separated from the photosensitive drum 70. In order to bring the developing device 80 close to the photosensitive drum 70, when the crankshaft 101 is rotated counterclockwise from the state shown by the imaginary line in FIG. 8, the yoke cam 103 comes close to the elastic force of the return spring 104. Direction (left side in FIG. 8)
80 is brought close to the photosensitive drum 70 side and is set in a developing state. In this case, the pressing spring 105 is connected to the developing device 80.
From the holding portion 106 that holds the housing 80a of the housing 80a.
The holding portion 106 and the movable member 110 are respectively fitted to a pair of guide shafts 111, 111 which project toward the rear side (the right side in FIG. 7) and are slidably fitted to the U-shaped movable member 110.
It is formed by a coil-shaped compression spring that is compressed between them. The return spring 104 is formed of a coiled tension spring having one end attached to the fixed side of the copying machine main body 10 and the other end attached to the movable body 110. The holding portions 106 are provided on both sides of the U-shaped movable body 110.
The guide pins 108 which are inserted into the guide long holes 109 of the fork arm 107 and the yoke cam 103 are respectively provided to protrude.

In FIG. 7, reference numeral 112 denotes a positioning roll provided on both sides of the first and second developing devices 80 and 90.
Is a guide pin slidably supported in a guide groove 114 of a guide frame 115 provided in the copying machine main body 10.

The pre-transfer charger 78 has a grid electrode 78b formed in an opening of a shell 78c having a U-shaped cross section in which a wire electrode 78a is stretched.
Is provided, an AC high voltage is applied to the wire electrode 78a, and a DC control voltage is applied to the grid electrode 78b.

The transfer unit 30 is provided with a transfer corotron 33 in which a wire electrode 32 is stretched inside a shell 31 having a U-shaped cross section that opens toward the photosensitive drum 70 side, and is disposed downstream of the transfer corotron 33 and the photosensitive drum. It comprises a stripping corotron 36 in which two wire electrodes 35, 35 are stretched in parallel with each other in a shell 34 having a U-shaped cross section opening toward 70.

The downstream side of the transfer unit 30 is a peeling claw for peeling the paper 16 adhered to the photosensitive drum 70 from the photosensitive drum 70 after the transfer.
A photosensitive drum 18 is provided downstream of the peeling claw 18.
A cleaning device 19 for removing residual toner adhered to 70 is provided. In this case, the cleaning device
19, a cleaning blade 19a for scraping toner adhered to the photosensitive drum 70, a rotating brush roll 19b provided upstream of the cleaning blade 19a, and a toner receiver 19c provided upstream of the rotating brush roll 19b,
A film seal for preventing toner scattering which is attached to the photosensitive drum 70 side of the toner receiver 19c and comes into contact with the photosensitive drum 70
19d.

IV. Features of the equipment 1) Automatic density control (ADC) in the first stage developing means
The first feature of the multicolor image forming apparatus of the present invention is that the first stage developing means (first developing device 80) includes an image disorder preventing means by automatic density control (ADC). That is.
That is, the image disturbance preventing means includes a reference pattern image forming section 150 for forming a reference pattern latent image corresponding to the first toner image density on the photosensitive drum 70, and a reference pattern density detecting section for detecting the density of the reference pattern image. It comprises a detecting unit 160 and a developing bias control unit 170 for controlling the developing bias of the first developing device 80 based on a signal from the reference density detecting unit 160.

In this case, the reference pattern image forming unit 150 includes a first latent image forming unit 74 that forms a reference pattern latent image, and a first developing device 80 that visualizes the reference pattern latent image. The reference pattern density detecting section 160 is formed by a photo sensor arranged on the upstream side of the cleaning device 19.

As the photosensor 160, a light emitting element / light receiving element which is generally used is used. Generally, a shorter focal length has a wider angle latitude, and a smaller angle between the light emitting element and the light receiving element has a smaller angle latitude. Higher concentrations can be detected and the angle latitude is wider. In addition, the photosensor 160 differs in the amount of reflected light and the amount of absorbed light depending on the type of toner used (difference in color, surface condition, etc.), the wavelength of the light emitting element, and the like. It is necessary to decide whether to measure or to measure by diffusion. In the reflection method, angle accuracy and directivity are difficult, but the sensitivity is good in any color of the comparative example. In the diffusion method, the angle latitude is wide, and high sensitivity can be obtained with a high reflectance on the toner surface, but low sensitivity is obtained with a black toner or the like that absorbs light. In addition, a method of improving directivity by a slit is also conceivable, and this method can be simpler and cheaper than that by a lens, and if the slit position is accurate, the optical axis of the element is slightly shifted. Also, since unnecessary light is canceled, sensitivity is good. However, there is a drawback that the S / N ratio decreases because the amount of received light decreases.

Note that it is preferable that the light emitting element emits infrared light that is resistant to contamination.

Next, a method for preventing image disorder of the first developing device 80 in the image forming apparatus of the present invention configured as described above will be described with reference to FIG.

First, when a predetermined mode of the console panel 11 is selected by turning on the main switch of the copier, the LED in the sensor is turned on (step A), and a signal from the upper image reading unit 14 is transmitted to the image forming unit (IPS). The signal from the IPS is transmitted to the first latent image forming means 74, and a reference pattern latent image is formed on the photosensitive drum 70 uniformly charged by the charger 72 from the first latent image forming means 74. Formed (step B). Next, the reference pattern image is developed on the photosensitive drum 70 by the first developing device 80 (Step C). In this case, the second developing device 90 may be driven, but the second developing device 90 may be driven.
Is preferably stopped or retracted from the photosensitive drum 70. The reference pattern image thus formed on the photosensitive drum 70 is optically detected by the photo sensor 160 (Step D). In this case, the paper transport unit 50 is stopped. The reference pattern density signal detected by the photosensor 160 is transmitted to the CPU and is compared with a reference value previously input as data (step E). A predetermined applied bias voltage is determined, and the applied bias voltage is applied to the first developing device 80 (Step F). Accordingly, the first developing device 80 applies a developing bias suitable for an image density that does not cause image disturbance to the first latent image in the subsequent normal image forming process, and the negatively charged red toner is exposed. The image is developed on the drum 70 and the occurrence of image disturbance can be prevented in the second latent image developing step of the post-processing step.

For the reference pattern latent image, the relationship between the development density and the development applied bias potential under different environments of the developer used needs to be accurately determined.
A mesh pattern of an arbitrary size of 10 to 30% is used, and a detection density is made good in a region where the development density is not saturated. The reference pattern image visualized with respect to the reference pattern latent image formed in this way is detected by the photo sensor 160, the reflectance thereof is detected, and the detection signal is compared with data measured in advance by the CPU. In this case, an appropriate calculation is performed based on the reflectance of the reference density image visualized based on the graph showing the relationship between the optical density, the reflectance of the reference density, and the bias voltage shown in FIG. It is converted to a first developing bias voltage. That is, in the case of a low reflectivity A% first developing bias voltage -A ₁ (V) becomes also,
When the reflectivity is high at B%, the first developing bias voltage is −B ₁ (V) with a higher negative charge.

2) First Developing Density Control by Density Control Member The second feature of the apparatus of the present invention is that the first developing device 80 and the second developing device
A density control member, which is means for preventing image disturbance, is provided between the image forming apparatus and the developing device 90, and a bias is applied to the density control member so that the first developing device 80 forms the first image formed on the photosensitive drum 70.
The purpose is to remove the excess density of the developed image. In other words, if the density of the toner image formed by the first development is high, the first development image may be rubbed during the second development and image disturbance may occur. By disposing a density control member to which a bias having a polarity different from that of the first developing toner is applied between the photosensitive drum 70 and the photosensitive drum 70,
Excess toner in the upper layer of the first developed image toner is removed to maintain the first developed image density at a predetermined value or less.

For example, as shown in FIG. 12, the density control member is formed by a conductive roll 200 (hereinafter, referred to as a bias roll) disposed at a position downstream of the developing roll 81 in the first developing device 80. Can be. This bias roll
200 is made of a non-magnetic conductive material such as aluminum alloy and stainless steel, and
The photosensitive drum 70 is rotatably disposed with a slight gap between the photosensitive drum 70 and the photosensitive drum 70 in the same direction (clockwise in FIG. 12) by a drive motor (not shown). Further, a cleaning blade 202 attached to the housing 80a is in contact with the upper surface of the bias roll 200, so that the toner adhered to the bias roll 200 can be scraped and collected in the housing 80a of the first developing device 80. I have. Further, a bias power supply 204 is connected to the bias roll 200, and a bias voltage having a polarity (plus) different from the polarity (minus) of the first developing toner is applied.

In FIG. 12, the other parts are the same as those described in FIG. 5, and the same parts are denoted by the same reference numerals and description thereof will be omitted.

When a bias voltage is applied to the bias roll 200 configured as described above and rotated, excess toner in the upper layer of the first developed image toner formed on the photosensitive drum 70 is adsorbed on the bias roll 200 and removed. Is done. Accordingly, the first developed image density, that is, the pile height can be maintained at a predetermined value or less, and in particular, image disturbance can be prevented even in an environment in which development efficiency is increased, such as when a copying machine is started. At the same time, it is possible to prevent a so-called thick line. Further, by bringing the bias roll 200 close to the photosensitive drum 70, there is almost no air flow passing through the gap, and it is possible to prevent the cloud toner from the first developing device 80 from scattering into the apparatus.
Further, the bias roll 200 is connected to the first developing device 80 and the second developing device 9.
Since it is located between 0, it is also possible to prevent the toner scattered from the first developing device 80 from being mixed into the second developing device 90.

Next, the result of a test in which the bias roll 200 is used to remove excess toner from the first developed image will be described.

In conducting the test, ◎ Photosensitive drum 70 Negatively charged system (OPC: Organic) photoconductor Process speed: 156 mm / sec ◎ First developing unit 80 ☆ Configuration of developing roll 81 ・ Magnet with stainless steel sleeve of 23 mm in diameter Roll ・ Flux density of main pole (developing magnetic pole) 1,000 gauss ☆ Parameters ・ Trimming gap: 0.6mm ・ Gap between photosensitive drum 70 and developing roll 81: 0.4mm ・ Rotation speed of developing roll 81 3 times that of photosensitive drum 70・ Applied voltage to bias roll 200 −400 to +1000 (V) ☆ Developer (red) ・ Toner (negative) Styrene / acrylic copolymer Red dye (Lisole Scarlet) melt-kneaded with an average particle size of 11.5 μm Red toner Carrier (positive) A carrier having a ferrite core coated with methyl terpolymer and having an average particle size of about 80 μm was used.

As a result of the above test, the results shown in FIG. 13 were obtained.
By setting the voltage to 0 V / 0.4 mm, the state in which the bias roll 200 is not used is as follows: ・ Reflection density (SAD): 1.3 ・ Pile height: 32 μm ・ Image disorder grade: G5 Reflection density (SAD): 1.2 ・ Pile height: 25 μm ・ Image disorder grade: G3, thus preventing image disorder in the second development.

Note that the evaluation of the image disorder glands G1 to G5 decreases in the order of G1>G2>G3>G4> G5, and the case of G3 or less is an allowable range where image disorder does not occur. Further, the collection efficiency increases in proportion to the electric field strength (V / mm), and the collection effect is slightly reduced by changing the peripheral speed ratio of the bias roll 200 to the photosensitive drum 70. When the density was low, the influence on the image disorder was small. In FIG. 13, zone A is at high temperature and high humidity (28 ° C., 85%), and zone B is at normal temperature and humidity (22 ° C., 85%).
55%).

FIG. 14 shows a case where a bias blade is used in place of the bias roll 200. That is, a case where an L-shaped bent bias plate 206 is attached to the opening of the housing 80a on the downstream side of the developing roller 81 of the first developing device 80, and a bias power supply 204 is connected to the bias plate 206. . In this case, the bias plate 206 is formed of a non-magnetic conductive member such as an aluminum alloy or stainless steel similar to the bias roll 200. The gap between the bias plate 206 and the photosensitive drum 70 is set to 0.4 mm, and the voltage applied to the bias plate 206 is +500 to +1000 V / 0.4 mm.

By using the bias plate 206 instead of the bias roll 200 as described above, the same effect as described above can be obtained.

Excess toner adhering to the bias plate 206 can be recovered by spikes of the toner on the developing roll 81. In addition, the toner is attracted to the non-image portion of the photosensitive drum 70 from the bias plate 206 and recovered by the cleaning device 19. It is also possible.

The other parts in FIG. 14 are the same as those in FIG. 12, and the same parts are denoted by the same reference numerals and description thereof will be omitted.

3) Prevention of Image Disturbance by Improving Development Efficiency in First Stage The third feature of the apparatus of the present invention is that the efficiency of the first developed image formed on the photosensitive drum 70 by the first developing device 80 is increased. The purpose is to prevent image disorder by strengthening the electrical coupling force of the developed image on the photosensitive drum 70. That is, the means for applying the developing bias of the first developing device 80 is constituted by the AC and DC superimposed bias applying section 210 (see FIG. 5). The toner of high toner charge (high tribo) and low toner concentration (TC) can be developed with high efficiency, and the electric coupling force of the developed image on the photosensitive drum 70 can be strengthened. 2 First in development
Image disturbance of the developed image can be prevented.

As described above, a test was performed to show the effect when the bias voltage applied to the developing roll 81 of the first developing device 80 was set to the AC and DC superimposed bias application voltage.
The results shown in FIG. 15 were obtained.

In the test, ◎ Photosensitive drum 70 Negatively charged system (OPC: Organic) photoconductor Process speed: 156 mm / sec ◎ First developing unit 80 ☆ Configuration of developing roll 81 ・ Magnet with stainless steel sleeve of 23 mm in diameter Roll ・ Magnetic flux density of main pole (developing magnetic pole) 1,000 gauss ☆ Parameter ・ Trimming gap: 0.6 mm in this embodiment, comparative example: 1.0 mm ・ Gap between photosensitive drum 70 and developing roll 81 This embodiment: 0.4 mm・ Comparative example: 0.8mm ・ Rotation speed of developing roll 81 3.09 times of photosensitive drum 70 ・ Magnet setting angle: +5 ゜ ・ Bias applied voltage AC voltage: Vp-p = 2.0KV, Frequency: 2.4KHz DC voltage: -450V ☆ Developer (red) ・ Toner (negative) Styrene / acrylic copolymer Red toner with average particle diameter of 11.5 μm obtained by melting and kneading red dye (Risol Scarlet) ・ Carrier (positive) Ferrite core is co-formed with methyl terpolymer A coated carrier having an average particle size of about 80 μm was used.

As a result of the above test, by setting the gap between the developing roll 81 and the photosensitive drum 70 to be narrow (for example, 0.4 mm),
High development efficiency of the reflection density (SAD) of the first development can be achieved.
That is, in order to obtain SAD1.0, the toner concentration (TC) was 3.2% in the comparative example, but was
It was 2.3% lower, about 1% lower, and tribo increased about 10%. The reproducibility of the fine line was almost constant in both the present example and the comparative example.

Therefore, the gap between the developing roll 81 and the photosensitive drum 70 is made as narrow as possible, and the first developing bias application voltage is set to an AC and DC superimposed bias application voltage, thereby providing a high tribo and low TC toner. Can be developed with high efficiency, the electric coupling force of the developed image on the photosensitive drum 70 can be strengthened, and the image disorder of the first developed image in the second development can be prevented. .

4) Prevention of Image Disturbance by Specifying Magnetic Flux Density of Magnet Member in Second and Subsequent Stages A fourth feature of the apparatus of the present invention is that by defining the magnetic flux density of the magnet member of the developing means in the second and subsequent stages, The object of the present invention is to prevent image distortion of a first-stage or previous-stage developed image in a developing step by a developing unit of a second or subsequent stage. That is, the image disturbance preventing means is constituted by a pair of repulsive magnetic poles N1 and N2 on the side of the magnet roll 91 of the second developing device 90 facing the photosensitive drum 70, and the repulsive magnetic poles N1 and N2 are provided.
1 and N2 are made to have the same polarity and repel each other, so that the degree of freedom of the carrier in the center of the region between the repulsion magnetic poles N1 and N2 is increased, so that the contact ratio of the toner to the photosensitive drum 70 is increased. That is, the image is prevented from being disturbed.

In this case, as shown in FIG. 16, the magnet roll 91 is disposed opposite to the developing area, and the repulsion magnetic poles N1 and N2, which are the main developing poles that rub the magnetic brush against the outer peripheral surface of the photosensitive drum 70. And a separation pole N4 for separating the magnetic brush from the magnet roll 91, and transport poles N3 and S1 for absorbing the developer to form a magnetic brush and transporting the magnetic brush to the development area. These magnetic poles are usually provided on a magnet roll 91 made of ferrite by a magnetizing method, but a rare earth magnet may be used instead of the ferrite magnet. As shown in FIG. 17, the repulsive magnetic poles N1 and N2 have a U-shaped cross section in which a concave groove 222 is formed on the surface of the main pole (N pole) along the axial direction of the magnet roll 91. The magnet body 220 is formed by a magnet roll body 91a.
, And a rotating sleeve 91b is arranged on the outer periphery of the magnet roll main body 91a to form the magnet roll 91.

The repulsion magnetic poles N1 and N2 configured as described above increase the degree of freedom of the carrier at the center of the region between the repulsion magnetic poles N1 and N2, so that the contact ratio of the toner to the photosensitive drum 70 increases, that is, high-efficiency development is performed. In addition to the above, the rubbing of the magnetic brush can be reduced to prevent the first developed image from being disturbed. However, in order to increase the density of the developed image in the second development to a certain value or more, it is necessary to increase the magnetic force and the repulsion difference between the repulsion magnetic poles N1 and N2, and to prevent image disorder of the first development image. On the contrary, there is a trade-off problem that the magnetic force and the repulsion difference between the repulsion magnetic poles N1 and N2 must be reduced. Therefore, it is necessary to solve this problem. Therefore, in the present invention, when the peripheral speed of the magnet roll 91 with respect to the photosensitive drum 70 is in the range of 0.6 to 1.4, the repulsion difference between the two repulsion magnetic poles N1 and N2 is set to 350 to 100 gauss, whereby the image of the first developed image is formed. Disturbance can be prevented, and the density of the second developed image can be kept constant.

Next, a description will be given of the result of a test of the relationship between the image disorder of the first developed image and the density of the second developed image when the repulsion difference between the repulsion magnetic poles N1 and N2 is changed.

In conducting the test, ◎ Photosensitive drum 70 Negative charging system (OPC: Organic) photoconductor Process speed: 156 mm / sec ◎ Second developing unit 90 ☆ Configuration of developing roll 81 ・ Stainless steel sleeve with 24.5 mm diameter Magnet roll of repulsive magnetic pole-Magnetic flux density of main pole (developing magnetic pole) 1,200 gauss ☆ Parameter-Trimming gap: 0.5 mm-Gap between photosensitive drum 70 and magnet roll 91: 0.6
mm ・ Rotation speed of magnet roll 91 0.2 to 1.2 times that of photosensitive drum 70 ☆ Developer (black) ・ Toner (positive) Styrene / acrylic copolymer, carbon black, positive charge control agent are melt-kneaded / crushed / classified Average particle size 1
1.5 μm black toner Carrier (negative) 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.

As a result of the above test, the SA of the second development when the repulsion difference between the repulsion magnetic poles N1 and N2 is 250 Gauss and when the repulsion difference is 450 Gauss
The relationship between D, peripheral speed ratio and image disturbance is as shown in FIGS. 18 (a) and (b) and FIGS. 19 (a) and (b). When the repulsion difference between the repulsive magnetic poles N1 and N2 is 250 gauss, Means that the peripheral speed ratio is 1.0
In the case of, the SAD (black) is 1.4, and the image disorder of the first developed image is the red SAD
Is 1.4 in G4, whereas when the repulsion difference is 450 Gauss, the SAD (black) is 1.5 when the peripheral speed ratio is 1.0, and the image disorder of the first developed image is G7 in 1.4 in red SAD. . In addition, the same test was conducted for other cases of the repulsion difference between the repulsion magnetic poles N1 and N2, and the results are as shown in Table 1 below.

Therefore, the magnet roll for the photosensitive drum 70
When the peripheral speed of 91 is in the range of 0.6 to 1.4, both repulsive magnetic poles N1 and N2
By setting the repulsion difference of the first developed image to 350 to 100 Gauss, preferably 200 to 300 Gauss, it is possible to prevent image disturbance of the first developed image and to keep the density of the second developed image constant.

5) Prevention of Carrier Attachment by Specifying Magnetic Flux Density of Magnet Member in Second and Subsequent Stages The fifth feature of the apparatus of the present invention is that the carrier adhered to photosensitive drum 70 during the second and subsequent stages of development is transferred. Sometimes paper
The purpose of the present invention is to improve the image quality by preventing carrier adhesion that causes white spots in the transferred image by being transferred onto the transfer image 16.

In the embodiment shown in 4) above, the magnetic force A of the repulsive magnetic poles N1 and N2
(The upstream side in the rotation direction of the photosensitive drum 70) and B (the photosensitive drum 70
(The downstream side in the rotation direction of the toner) is the same, so that the carrier adhered to the photosensitive drum 70 during the second development has the same polarity as the toner when the polarity of the different polarity toner is made uniform by the pre-transfer charging by the pre-transfer charger 78. As a result, the carrier may be transferred onto the paper 16 at the time of transfer, and there is a possibility that a white spot may occur in the transferred image.

Therefore, in the present invention, as shown in FIG. 20, the magnetic force B (1,400 gauss) of the repulsive magnetic pole N2 is made larger than the magnetic force A (1,200 gauss) of the repulsive magnetic pole N1, so that the developing area (1,000 gauss) can be obtained. By applying a large restraining force to the carrier having an increased degree of freedom at the end of the developing area, the number of carriers that breaks the restraint from the magnet roll 91 and transfers to the photosensitive drum 70 is reduced, and carrier adhesion is prevented. It was made. The opening angle between the repulsive magnetic poles N1 and N2 is 24 °.

In this case, the magnetic force A of the repulsive magnetic pole N1 and the magnetic force B of the repulsive magnetic pole N2
A good result was obtained by setting the relationship between the magnetic forces A and B to B−A ≧ 0.1B and 800 gauss <A <1,400 gauss, except that the repulsion difference was the same as in 4) above (Table). -2).

6) Improvement of the image quality of a single-color image by switching the peripheral speed ratio of the developing carrier of the developing means of the second and subsequent stages The sixth characteristic of the apparatus of the present invention is that By changing the peripheral speed ratio with respect to the photosensitive drum according to the multi-color copying and the single-color copying of the second and subsequent developments alone, it is possible to improve the maintenance of the density and the image quality during the mono-color copying. Things.

That is, in the image forming apparatus using the general superimposing method, when the single-color copy mode by the second developing device 90 is selected in addition to the multi-color mode, the peripheral speed ratio of the magnet roll 91 is suitable for preventing image disorder. In this case, the image quality is not sufficient because the image is set in the state (peripheral speed ratio: 1.0). Therefore, the magnet roll of the second developing unit 90
A driving means is connected to the photosensitive drum 70 (specifically, the rotating sleeve 91b) via a switching means so that the rotating sleeve 91b can be moved relative to the photosensitive drum 70 according to the time of red and black color copying and the time of black single color copying of the second color. By changing the peripheral speed ratio, it is possible to prevent image disturbance during color copying and maintain the first and second image quality densities, and to improve the characteristic of maintaining the density of the second developed image during monochromatic copying. Things.

In this case, as the switching means for the peripheral speed ratio, for example,
As shown in FIG. 21, a DC or AC servomotor 230 having a speed control unit can be used, and the peripheral speed ratio of the rotary sleeve 91b is 1.0 during color copying by the speed control unit, and the peripheral speed ratio during monochromatic copying. The ratio is set between 1.2 and 1.3. Accordingly, as shown in FIG. 21, the rotating sleeve 91b mounted on the drive shaft 232 of the servomotor 230 is rotated at a peripheral speed ratio of 1.0 in the color mode, and within a range of the peripheral speed ratio of 1.2 to 1.3 in the monochromatic mode. Rotated by In the DC servo motor, the means for switching the speed includes, for example, a field control method, an armature control method, or a direct-acting armature control method. The speed can be switched by changing the power supply frequency.

The driving means and the speed switching means do not necessarily need to be servomotors. For example, as shown in FIG. 22, a drive shaft 232 of a drive motor 231 and a driven shaft 234 on which a rotating sleeve 91b is mounted are connected with a key 236 for a clutch. At the same time, the key 236 is engaged with the drive gear 240 mounted on the drive shaft 232 via the bearing 238 so that the key 236 can be engaged and disengaged. The driven shaft 232 is engaged with the driven gear 242 to be mounted. In the color mode, the drive shaft 232 and the driven shaft 234 are connected by the key 236. In the monochrome mode, the rotation of the drive shaft 232 is transmitted to the drive gear 240 by the key 236 to drive the driven gear. The power may be transmitted to the driven shaft 234 and the rotating sleeve via the 242. Instead of this transmission mechanism, for example, the drive shaft 232 and the driven shaft 234 are connected via a two-line gear transmission system and a one-way clutch, and the peripheral speed ratio of the rotary sleeve 91b is switched by forward / reverse rotation of the drive motor. You may make it.

Second developing device having switching means configured as described above
At 90, when the mode of the console panel 11 is set to the color mode, a signal from a mode switching operation unit (not shown) is transmitted to the CPU, and in response to a command from the CPU, the magnet roll 91 causes the first developed image to be reproduced. The rotation is performed at a peripheral speed ratio (1.0) within a range where image disturbance does not occur, and the second development is performed.
Prevents image distortion of a developed image. Also, console panel 11
Is set to the monochrome mode (black) of the second development,
Similarly, a signal from the mode switching operation unit is transmitted to the CPU, and C
In response to a command from the PU, the magnet roll 91 has a peripheral speed ratio of 1.2
The black toner of the single color copy is developed by the second developing unit 90 by setting any one of the ranges of 1.3 to 1.3.

Therefore, at the time of color copying, the magnet roll 91 of the second developing device 90 is rotated at a relatively low speed to prevent the image disorder of the first developed image, and at the time of monochromatic copying, the magnet roll 91 is rotated at a high speed. In addition, it is possible to improve the density maintenance of the second development (70 to 80% of the whole) and to achieve high image quality.

Note that the initial charge potential affects the density maintenance of the second development. However, when an image is formed by the negative-to-positive development process shown in FIG. The contrast can be increased by reducing the non-image portion potential in the same manner as in the mode, so that the sharpness of the line image can be increased.

7) Prevention of Carrier Attachment by Specifying Particle Size Distribution of Magnetic Carriers in Second and Subsequent Developing Means A seventh feature of the device of the present invention is that the two-component developer in the second and subsequent developing means has a magnetic property The purpose is to prevent carrier adhesion by defining the particle size distribution of the carrier within a certain range.

The resin carrier (MT carrier) having a small particle diameter used to prevent image disturbance in the first development has a weak saturation magnetization as compared with a normal core carrier, so that carrier adhesion easily occurs. It is said that the smaller the diameter, the higher the frequency of occurrence of carrier adhesion, and most of the scattered carriers are 10 μm to 20 μm
Occupy. Therefore, it is necessary to reduce the occurrence of carrier adhesion by removing the MT carrier having a small particle size.

The present invention has been made in view of the above circumstances, and when the center particle system of the magnetic MT carrier of the two-component developer in the second developing device 90 is set to 45 μm, the magnetic MT carrier having 50% or less of the center particle size of the carrier is used. By limiting the content to 10% by weight or less of the entire magnetic MT carrier, the occurrence of carrier adhesion is reduced.

Next, a test for confirming the reason for defining the particle size distribution of the MT carrier as described above will be described.

In conducting the test, ◎ Photosensitive drum 70 Negative charging system (OPC: Organic) photoconductor Process speed: 156 mm / sec ◎ Second developing unit 90 ☆ Configuration of developing roll 81 ・ Stainless steel sleeve with 24.5 mm diameter Magnet roll of repulsive magnetic pole-Magnetic flux density of main pole (developing magnetic pole) 1,200 gauss ☆ Parameter-Trimming gap: 0.5 mm-Gap between photosensitive drum 70 and magnet roll 91: 0.6
mm ・ Rotation speed of magnet roll 91 1.0 times that of photosensitive drum 70 ・ Image potential (VH): -600V ・ Non-image potential (VL) -200 to -100V ・ Applied bias voltage (VBias): -300V ☆ Catch up・ Parameters ・ Magnetic force: 1,200 gauss ・ Catch-up bias: + 400V ・ Gap between photosensitive drum 70 and catch-up roll:
0.4mm ☆ Developer (black) ・ Toner (positive) Average particle size 1 obtained by melt-kneading / crushing / classifying styrene / acrylic copolymer, carbon black, and positive charge control agent
1.5 μm black toner Carrier (negative) A polymer carrier core having an average particle size of about 37 μm obtained by melt-kneading / classifying a styrene / acrylic copolymer and magnetite was used.

Carrier particle size distribution of comparative example: FIG. 23 (a) Carrier particle size distribution of this example As shown in FIG.
The MT carrier was reduced to 10% by weight or less of the entire MT carrier by classification / cutting. That is, the center particle size of the MT carrier (45
μm) of 50% or less
It was less than 10% by weight.

As a result of the above test, the carrier adhesion generation amount and the carrier adhesion area coverage (area ratio of carrier adhesion on the copy) between the comparative example and the present example were as shown in FIGS. 24 and 25. That is, by cutting the small particle size carrier, the amount of carrier adhesion was reduced to 1/2 or less.

The carrier adhesion greatly depends on Δ (VBias−VL), and when Δ (VBias−VL) = 100 V, Δ (VBias−V)
In the case of L) = 200V, the amount of carrier generation is about three times. However, the non-image portion potential (VL) and the applied bias (VB
ias) is related to the range in which fog in the white background and image disturbance in the first development do not occur, and is set in consideration of these proper ranges, and the value of Δ (VBias−VL) cannot be extremely reduced. The limit is Δ (VBias−VL) ≧ 100V.

A test similar to the above was carried out by appropriately changing the center particle diameter (25 μm, 45 μm). As a result, a magnetic MT carrier having a diameter of 50% or less of the center particle diameter of the magnetic MT carrier of the two-component developer in the second developing device 90 was obtained. Is defined to be 10% by weight or less of the entire magnetic MT carrier, the occurrence of carrier adhesion can be reduced.

Therefore, carrier adhesion can be prevented in the second development, so that it is not necessary to provide a carrier collecting means such as a catch-up roll for removing the carrier adhered to the photosensitive drum 70 on the downstream side.

8) Prevention of Carrier Attachment by Switching Applied Bias of Carrier Collection Means After Development in Second and Subsequent Stages An eighth feature of the apparatus according to the present invention is that the bias applied to carrier collection means in the second and subsequent stages is equal to that of developing means. Is switched between multi-color copying and single-color copying of the developing means of the second and subsequent stages, so that carrier adhesion during single-color copying can be reduced and the life of the photosensitive drum 70 can be increased. .

That is, in general, in a superposition developing system in which normal development is performed after reversal development, it is necessary to provide a catch-up roll 250 on the downstream side of the second developing device 90 for collecting the carrier adhered to the photosensitive drum 70. However, since the polarity of the first developed image toner and the polarity of the carrier of the second developer are the same during multi-color copying, the bias of the catch-up roll 250 is changed to the carrier of the second developing device 90. Since the polarity cannot be reversed, there is a disadvantage that the effect is small at the time of monochromatic copying of the second developed image.

The present invention has been made in view of the above circumstances, and a total of 80
%, The carrier adhesion during black single-color copying is reduced, the drum life is extended by reducing the scratches on the surface of the photosensitive drum 70, and the developer life is extended by suppressing carrier consumption, and running costs are reduced. Is intended to be reduced.

In this case, as shown in FIG. 26, a bias power supply 2 having two power supplies 254 and 256 having different polarities is provided to a catch-up roll 250 via a switching element 252 as a switching means.
58 is connected. Then, at the time of color copying, the switching element 252 is connected to the negative polarity power supply 254 to apply a negative bias to the catch-up roll 250 to collect the first and second developer carriers adhered to the heating drum 70, Further, at the time of black single-color copying by the second developing device 90, the switching element 252 is connected to a power supply of a positive polarity, applies a positive bias to the catch-up roll 250, and applies a second bias of the negative polarity attached to the photosensitive drum 70.
It collects the carrier of the developer. In this case,
The switching operation of the switching element 252 can be performed by a signal from an operation input detecting unit to the console panel 11.

In the above description, the case where the switching element 252 is connected to the negative polarity power supply 254 at the time of color copying has been described.However, it is not always necessary to connect the switching element 252 to the negative polarity power supply 254. A bias of ± 0 V may be applied.

9) Prevention of Color Mixing with Carrier Recovery after Development at Second Stage The ninth feature of the apparatus of the present invention is that the carrier is recovered by a carrier recovery unit disposed downstream of the second and subsequent development units. To prevent color mixture between the carrier and the color toner of the first development, and to prevent deterioration in image quality.

That is, the catch-up roll is disposed downstream of the developing means of the second and subsequent stages, and the carrier attached to the photosensitive drum 70 is collected by the catch-up roll to prevent carrier adhesion. ). However, when the carrier collected by the catch-up roll 250 is collected in the second developing device 90, the first developing device 80
Since the color toners developed at the same time are also collected in the second developing device 90 at the same time because of the same polarity, there is a problem that the color toners are mixed to cause color mixing, thereby deteriorating the image quality.

Accordingly, in the present invention, as shown in FIG. 27, a collection box 260 is formed on the downstream side of the second developing device 90 with the second developing device 90, and a catch-up roll 250 is provided in the collection box 260. At the same time, by bringing the blade 262 into contact with the surface of the catch-up roll 250, the photosensitive drum 7
After collecting the color toner and the black carrier adhered to 0 with the catch-up roll 250, the carrier collected by the catch-up roll 250 is scraped by the blade 262 and collected in the collection box 260. Things.

The collection box 260 may be formed integrally with the second developing device 90, but as shown in FIG.
90, a groove-shaped guide groove 264 is formed, and the collection box 260 is provided with a dovetail convex 266 that is slidably fitted in the guide groove 264, so that the carrier is filled in the collection box 260. Can be easily taken out.

V. Conclusion In the above embodiment, the case of a digital color copying machine for forming a two-color image of red toner and black toner has been described. However, the present invention is not necessarily limited to this, and the same applies to a color copying machine of three or more colors. Of course, it can be used. In the above embodiment, the case where a color image is formed in the mode of reversal development (negative) → normal development (positive) has been described. However, it is not always necessary to perform the mode in this mode. The same operation can be performed when a color image is recorded in the mode.

In the above embodiment, the case where the latent image carrier is an organic photosensitive drum has been described. However, the latent image carrier does not necessarily need to be a drum. For example, the latent image carrier may be formed in a belt shape.

In the above embodiment, the first latent image forming means 74 and the second
The latent image forming means 76 is formed by a laser diode. However, the latent image forming means 74 and 76 need not necessarily be laser diodes. For example, a liquid crystal light valve comprising a uniform light source and a liquid crystal micro shutter. ,
Arbitrary ones such as light emitting diode (LED) arrays and optical fibers can be used.

Further, in the above-described embodiment, the case where the carrier of the developer is a resin carrier has been described. However, the carrier does not necessarily need to be made of a resin, and may be a metal carrier if the density is low. In this case, a styrene / acrylic copolymer is used as the toner of the developer, but the toner does not necessarily need to be a styrene / acrylic copolymer. For example, resins used in this field such as epoxy and polyethylene are used. All are applicable.

Further, in the above embodiment, since the two-component developer for the reversal development is used in the first developing device 80, when the operation of the device is interrupted due to, for example, a paper jam, the charging device is not charged.
When the charge voltage of 72 and the developing bias are turned off at the same time, the photosensitive drum 70 moves even after the developing bias is turned off, during which a large amount of carrier adheres, causing the surface of the photosensitive drum 70 to be damaged and the cleaning blade 19a to be damaged. There is a possibility that it will end up. In order to prevent this, at least until the photosensitive drum 70 stops, that is, until the uncharged portion comes to the developing position, the timing of the developing bias is slightly delayed and kept ON to prevent carrier adhesion. At the same time, it is possible to prevent adverse effects such as uneven density caused by a decrease in the amount of the developer.

In the above embodiment, the latent image formation (gradation expression) by the first and second and subsequent stages of latent image forming means is not specified at all. When output (output for reading a density of a document to produce a halftone as well as black and white and multi-leveling a latent image) is output, the image potential (V1) of the first image and the background potential ( VL) (well-type potential) may not be formed, which may cause image disturbance in the second and subsequent development steps. However, at the time of halftone, the amount of the color toner is small, so that the image becomes thin and the image is hardly disturbed.

In order to solve the above problem, it is necessary to reliably form a well-type potential to prevent image disorder in the second and subsequent development steps.

Therefore, in the present invention, the multi-valued image can be used to some extent, but the first-stage latent image formation is binarized (white or black 2
By forming a latent image by the type, a well-type potential is reliably formed, and image disturbance in the second and subsequent development steps of the first image can be prevented. In this case, the second image and the subsequent latent image forming means (specifically, the second latent image forming means 76) are output as multi-level output by a line system, so that the image quality of the black image as the second image is obtained. Improvement can be achieved.

The operation of the image forming apparatus will be described with reference to FIG. 28. After the image information reading unit 14 determines an image signal corresponding to the first image, the image signal is transmitted to the image processing unit 17. The laser drive of the laser diode 74, which is the first latent image forming means, is operated, and the photosensitive drum is driven through the optical system 26 such as the mirror 22, the polygon mirror 23, and the lens 24.
After the first image to be formed on the surface of the first image 70 is expressed in gradation by binary, the first image is formed by the first developing device 80. Next, the laser driving unit of the second laser diode 76 is operated based on a signal corresponding to the second image stored in the image processing unit 17 and determined by the image information reading unit 14, and is transmitted through the optical system 26. Thus, a latent image of a multi-valued second image is formed on the photosensitive drum 70 by a line system, and the second developing device 80
Developed by Then, after being made to have the same polarity by the pre-transfer charger 78, the toner is transferred onto the paper 16 in the transfer unit 30.

As described above, by binarizing the first image, a well-type potential can be reliably ensured, so that image disturbance in the second developing device 80 can be prevented and the floor required for the first image can be prevented. The tone expression can be made inexpensive. In addition, when the second image is multi-valued by a line system, multi-gradation expression can be realized, and image quality can be improved.

The reason why the first image is binarized in this way is that the red image as the first image is generally shaded or underlined with respect to the main black image, which is required for the black image. This is because such a gradation expression is not necessary.

[Effects of the Invention] Since the multicolor image forming apparatus of the present invention is configured as described above, the following effects can be obtained.

1) According to the multicolor image forming apparatus of the first aspect, a range in which a developed image to be toner-developed on the latent image carrier from the first-stage developing unit is not disturbed by the second-stage and subsequent developing units. The first image is not disturbed in the developing process of the second and subsequent developing units, and high image quality can be maintained.

According to a second aspect of the present invention, there is provided a multi-color image forming apparatus, comprising: a reference pattern image forming section for forming a reference density pattern corresponding to a toner image density on a latent image carrier; And a developing bias control unit for controlling the developing bias of the first stage developing means based on a signal from the reference density detecting unit. In the developing step, image disturbance of the first image can be prevented, and the reference panned image forming unit is also used as the first latent image forming unit, and the reference pattern image forming unit is used as the first developing unit. As a result, it is not necessary to separately provide components, and effective use of space can be achieved.

3) The multi-color image forming apparatus according to claim 3, wherein the image disturbance preventing means is provided between the first-stage developing means and the second-stage developing means. Since it is formed by a density control member to which a bias for removing an excessive density of the toner image formed on the latent image carrier from the eye developing means is applied,
The excessive density of the first image can be adjusted to an appropriate state to prevent image disorder, and also prevent the cloud toner from the first-stage developing means from scattering into the machine, and
It is possible to prevent the second stage developing means from being mixed.

4) According to the multicolor image forming apparatus of the fourth aspect, the image disturbance preventing means is constituted by an AC and DC superimposed bias applying section for applying a developing bias to the first-stage developing means. By improving the development efficiency of one image, image disturbance of the first image can be prevented.

According to a fifth aspect of the present invention, in the multicolor image forming apparatus, the magnet member in the developing means of the second and subsequent stages is constituted by a pair of repulsive magnetic poles whose polarities repel each other, and the magnet member with respect to the latent image carrier. Since the peripheral speed and the repulsion difference between the repulsive magnetic poles are set to appropriate values, it is possible to prevent image disturbance in the developing process of the second image and subsequent stages of the first image.

6) According to the multicolor image forming apparatus of the sixth aspect, the magnetic force of both repulsive magnetic poles of the magnet member is formed larger in the downstream side in the rotational direction with respect to the upstream side in the rotational direction of the latent image carrier in a fixed relationship. Therefore, it is possible to prevent the carrier from adhering due to the prevention of the image disturbance according to the above 5).

According to a seventh aspect of the present invention, the peripheral speed of the developing carrier with respect to the latent image carrier in the second and subsequent developing units can be switched between the single color mode and the multicolor mode. Since it is formed, the image density at the time of monochrome copying can be maintained, and the image quality can be improved.

8) According to the multicolor image forming apparatus of the present invention, the small particle size of the two-component developer in the magnetic carrier in the second and subsequent developing units is cut to keep the particle size distribution within a predetermined range. Therefore, it is possible to prevent the carrier from adhering in the second and subsequent development steps, and to improve the image quality.

9) According to the ninth aspect of the present invention, the carrier collecting means is disposed downstream of the developing means of the second and subsequent stages, and the applied bias of the carrier collecting means is set to the multi-color in the single color mode. Since it is formed to be switchable in the mode,
It is possible to prevent carrier adhesion in the second and subsequent development steps and to improve image quality.

10) According to the multicolor image forming apparatus of claim 10, since the collection box for collecting the carrier collected by the carrier collection unit is formed so as to be separated from the development unit, the collection box for the first-stage development unit is formed. It is possible to prevent color mixture between the developer (color toner) and the developer of the second and subsequent developing units, and to improve the image quality.

[Brief description of the drawings]

FIG. 1 is a perspective view showing the appearance of a copying machine having the image forming apparatus of the present invention, FIG. 2 is a schematic sectional view thereof, FIG. 3 is a circuit diagram showing an outline of a circuit configuration, and FIG. FIG. 5 is a sectional view showing a first embodiment of the image forming apparatus of the present invention, FIGS. 6 (a) to 6 (f) are explanatory views of a color image forming step, and FIG. 7 is a developing means. 8 and 9 are side views showing the operation of the main part of the retract mechanism, and FIG. 10 is the first view of the present invention.
11 is a flowchart showing a relationship between an optical density, a reference density image, and a first developing bias voltage, and FIG. 12 is a cross-sectional view showing an attached state of an excessive density control member according to the present invention. FIG. 13, FIG. 13 is a graph showing the relationship between the applied voltage of the excess density control member and image disturbance, pile height, reflection density, FIG. 14 is a cross-sectional view showing another embodiment of the excess density control member, and FIG. FIG. 9 is a graph showing a relationship between toner charge, reflection density, toner density, and the like when a superimposed bias application voltage is applied according to the invention;
FIG. 16 is a cross-sectional view of the magnet roll of the second developing device in this relation, FIG. 17 is an enlarged cross-sectional view of the repulsive magnetic pole of the magnet roll, FIGS. 18 (a), (b) and FIGS.
(B) is a magnetic force pattern diagram and a graph showing the relationship between SAD / peripheral speed ratio / image disorder evaluation in the example of the present invention and the comparative example, respectively. FIG. 20 is another embodiment of the repulsive magnetic pole of the magnet roll in the present invention. FIG. 21 is a schematic sectional view showing the peripheral speed switching means of the magnet member according to the present invention, FIG. 22 is a schematic sectional view showing another embodiment of the peripheral speed switching means, and FIG. ) And (b) are graphs showing the carrier particle size distribution before and after the small particle size carrier cut of the two-component developer in the developing means of the second and subsequent stages, and FIG. 24 shows the carrier adhesion in the comparative example and this embodiment. FIG. 25 is a graph showing the relationship between the generation amount and the voltage, FIG. 25 is a graph showing the relationship between the carrier adhesion area coverage and the carrier particle size in the comparative example and this example, and FIG. 26 is the catch-up roll applied bias in the present invention. Switching means FIG. 27 is a cross-sectional view showing the carrier recovery means of the present invention, FIG. 28 is a schematic side view showing another image disturbance preventing means of the present invention, and FIG. Schematic sectional view of an image forming apparatus, FIG.
FIG. 30 is an explanatory diagram showing a procedure of an image forming process of a conventional multicolor image forming apparatus. Symbol Description (70) Photosensitive Drum (Latent Image Carrier) (72) Charging Device (74) First Latent Image Forming Means (76) Second Latent Image Forming Means (78) ... Charger before transfer (80) First developing device (first developing device) (90) Second developing device (second developing device) (91) Magnet roll (magnet member) (150) ) Reference pattern image forming unit (160) Photo sensor (reference pattern density detection unit) (170) Development bias control unit (200) Bias roll (density control member) (204) Bias power supply (206) ... bias plate (concentration control member) (210) ... superimposed bias applying part (220) ... magnet body (222) ... concave groove (230) ... servo motor (231) ... drive motor ( 232) Drive shaft (234) Driven shaft (236) Conversion key (240) Drive gear (242) Driven gear (250) Catch-up roll (carrier recovery means) (252) Switching element (switching means) (254) Negative pole power supply (256) Positive pole power supply (258) Bias power supply (260) …… Carrier collection box

Continuing from the front page (72) Inventor Tohru Yoshida 2274 Hongo, Ebina-shi, Kanagawa Fuji Xerox Co., Ltd. References JP-A-63-110471 (JP, A) JP-A-1-154173 (JP, A) JP-A-62-166363 (JP, A) JP-A-62-49377 (JP, A) 63-127260 (JP, A) JP-A-61-248068 (JP, A) JP-A-61-116366 (JP, U) (58) Fields investigated (Int. Cl. ⁶ , DB name) G03G 13/01 G03G 15/01-15/01 117 G03G 15/08-15/095 G03G 15/06-15/06 102

Claims (4)

(57) [Claims] A latent image carrier, a plurality of latent image forming means for forming a plurality of electrostatic latent images on the latent image carrier based on different image information, and a developing device opposed to the latent image carrier. A plurality of developing means having a carrier and developing each latent image on the latent image carrier with toner held on the developing carrier, and a plurality of latent images on the latent image carrier; Transfer means for directly transferring the toner directly from the carrier to the transfer medium, wherein each developing means includes a magnet member in the developer carrier, and a magnetic brush of a two-component developer comprising a toner and a magnetic carrier on the developer carrier. A multicolor image forming apparatus for developing a latent image on the latent image carrier, wherein an AC and DC superimposed bias is applied as a developing bias of the first stage developing means. Color image forming apparatus. 2. A latent image carrier, a plurality of latent image forming means for forming a plurality of electrostatic latent images on the latent image carrier based on different image information, and a developing device opposing the latent image carrier. A plurality of developing means having a carrier and developing each latent image on the latent image carrier with toner held on the developing carrier, and a plurality of latent images on the latent image carrier; A transfer means for directly transferring the toner directly from the carrier to the transfer medium, wherein at least the second and subsequent developing means include a magnet member in the development carrier and a toner and magnetic carrier on the development carrier. In a multicolor image forming apparatus for forming a latent image on the latent image carrier by forming a magnetic brush of a component developer, a magnetic pole of a magnet member facing the latent image carrier is used in developing means of the second and subsequent stages. Are reciprocating with each other When the peripheral speed of the developing carrier with respect to the latent image carrier is in the range of 0.6 to 1.4, the repulsion difference between the repulsive magnetic poles is determined.
A multicolor image forming apparatus characterized by having a thickness of 350 to 100 Gauss. 3. A latent image carrier, a plurality of latent image forming means for forming a plurality of electrostatic latent images on the latent image carrier based on different image information, and a developing device opposing the latent image carrier. A plurality of developing means having a carrier and developing each latent image on the latent image carrier with toner held on the developing carrier, and a plurality of latent images on the latent image carrier; A transfer means for directly transferring the toner directly from the carrier to the transfer medium, wherein at least the second and subsequent developing means include a magnet member in the development carrier and a toner and magnetic carrier on the development carrier. In a multicolor image forming apparatus that forms a magnetic brush of a component developer to develop a latent image on the latent image carrier, the peripheral speed of the development carrier relative to the latent image carrier in the developing means of the second and subsequent stages is In single color mode and multi-color mode A multicolor image forming apparatus characterized in that the apparatus is switchable. 4. A latent image carrier, a plurality of latent image forming means for forming a plurality of electrostatic latent images on the latent image carrier based on different image information, and a developing device opposing the latent image carrier. A plurality of developing means having a carrier and developing each latent image on the latent image carrier with toner held on the developing carrier, and a plurality of latent images on the latent image carrier; A transfer means for directly transferring the toner directly from the carrier to the transfer medium, wherein at least the second and subsequent developing means include a magnet member in the development carrier and a toner and magnetic carrier on the development carrier. While forming a magnetic brush of a component developer to develop the latent image on the latent image carrier,
In a multicolor image forming apparatus in which the charge polarity of the toner used in the first-stage developing unit and the charge polarity of the carrier used in the second-stage and later developing units are the same, At the downstream side in the rotation direction of the image carrier, a carrier collecting means for collecting the magnetic carrier attached to the latent image carrier is provided, and the polarity of the applied bias of the carrier collecting means is set to a monochromatic mode or a multicolor mode. A multi-color image forming apparatus characterized in that the multi-color image forming apparatus is configured to be inverted.