JP2003156947A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent image deterioration from being caused by the reverse transfer and toner image disturbance. SOLUTION: In a tandem structure in which a plurality of photoreceptor drums 20 are juxtaposed, a primary transfer bias roller 30 is disposed opposite each photoreceptor drum 20 in a corresponding transfer part. The shaft 30a of each primary transfer bias roller 30 is provided, via bearings, with butting members 53 which are in contact with the surface of the corresponding photoreceptor drum 20. Thus, an empty space g is created between the photoreceptor drum 20 and the intermediate transfer belt 29.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile, and the like.
The present invention relates to an image forming apparatus that forms a color image by superimposing a toner image formed on an image carrier on a transfer body.

[0002]

2. Description of the Related Art In recent years, a large amount of color documents have been handled even in offices, and there is a demand for higher speed full color printers and full color copying machines than ever before. In general, a color laser printer, which has become widespread in recent years, has a plurality of developing devices arranged in contact with one photoconductor so that a toner image of each color is formed for each rotation of the photoconductor, and the image is printed from the photoconductor. The so-called one-drum system, in which a color toner image is created by sequentially transferring to an intermediate transfer member or paper held on a transfer drum or the like, is the mainstream. An intermediate transfer method in which toner images of a plurality of colors are superposed on the above-mentioned intermediate transfer member, and then collectively transferred to paper, and a color toner image is directly transferred to the paper held on a transfer drum or the like in order to directly create a color toner image. There is a transfer method. The direct transfer method has a simple structure and is low cost, but since it is transferred multiple times to paper, the conditions of resistance and moisture content of the paper change and stable image formation is difficult. The intermediate transfer method requires only one transfer of the image to the paper, so the image quality is stable.
It has characteristics such as good compatibility with paper types.

However, in any of the methods, in order to obtain a color image using four colors, the photoconductor has to rotate four times, and the productivity cannot be improved. Therefore, in order to correspond to the speedup, the number of photoconductors is increased by the number of colors (usually 3 or 4), each developing device is arranged correspondingly, and the paper continuously contacts the photoconductor. A so-called tandem system or in-line system that can obtain a color image has also been proposed. For example, in JP-A-53-74037 (corresponding U.S. Pat. No. 4,162,843), a plurality of photoconductors are stacked in order to speed up color image output.
An image forming apparatus is described in which a transfer material (paper) is conveyed by a belt-shaped conveying means, and toner images are sequentially transferred in multiplex. In this tandem system, if the peripheral speed of the photoconductor is the same as the one-drum system, it is possible to print at a speed four times or more. However, as described above, in the case of the direct transfer method in which the photoconductor is directly transferred to the paper, there are many problems such as instability during the transfer onto the paper and alignment during the paper conveyance. Therefore, a so-called tandem intermediate transfer method, which is a tandem method and uses an intermediate transfer member, has been developed.
No. 2159 and other publications have been proposed.

However, the normal intermediate transfer member system,
Problems have also been pointed out in the case of a tandem type image forming apparatus in which a plurality of such photoconductors are provided and toner images are sequentially transferred in multiple transfer onto an intermediate transfer body. For example, an image forming unit having a photoreceptor as a latent image carrier, a primary charger as a charging unit, an image exposure unit as a latent image forming unit, a developing unit as a developing unit, and a transfer unit is for cyan toner,
In a full-color image forming apparatus in which four toner images are used, one for magenta toner, one for yellow toner, and one for black toner, and the toner images are sequentially transferred in multiple by the transfer means of the image forming unit for each color toner on the transfer material conveyed by the transfer belt. The toner that has already been transferred onto the intermediate transfer member may be transferred to the photoconductor (hereinafter, referred to as reverse transfer) when the second and subsequent colors are multiplexed and transferred, and the toner may return. When such reverse transfer occurs, when the waste toner from the photoconductor drum cleaner is reused in the developing device, a problem of toner color mixing in the developing device occurs. The color mixture in the developing device poses a serious problem in multicolor image formation. The occurrence of reverse transfer means that the toner image on the intermediate transfer body is disturbed, which causes deterioration of image quality.

In order to solve the same problem, Japanese Patent Laid-Open No. 9-146334 proposes to reduce the reverse transfer by setting the contact angle of the latent image carrier with water to 85 degrees or more. However, it cannot be said that the above problems have been sufficiently solved.

[0006]

SUMMARY OF THE INVENTION The main object of the present invention is to provide an image forming apparatus which can prevent image deterioration due to reverse transfer and disturbance of a toner image.

[0007]

In order to achieve the above object, in the present invention, reverse transfer does not occur unless the toner image already transferred to the transfer member comes into contact with the photosensitive member at the next transfer. Focusing on the above, it was decided to prevent image deterioration due to reverse transfer at the time of multiple transfer by performing transfer while maintaining the photoconductor and the intermediate transfer member in a non-contact state.

Specifically, in the invention described in claim 1, the toner image formed on the image carrier is transferred to the transfer body by the transfer means, and the toner images of at least two colors are superposed on the transfer body. An image forming apparatus that also forms a color image has a configuration in which the image carrier and the transfer body are provided so as to transfer a toner image while maintaining a non-contact state. Here, the transfer body includes the concepts of an intermediate transfer body and a recording material.

According to a second aspect of the invention, in the image forming apparatus according to the first aspect, the transfer body is an intermediate transfer body for receiving a toner image from the image carrier and transferring it to a recording medium. A structure is adopted in which a plurality of toner images are superposed on a transfer body and then collectively transferred to the recording medium.

According to a third aspect of the invention, in the image forming apparatus according to the second aspect, the intermediate transfer member has a drum shape.

According to a fourth aspect of the invention, in the image forming apparatus according to the second aspect, the intermediate transfer member has an endless belt shape.

According to the invention of claim 5, claims 1 to 4 are provided.
In the image forming apparatus described in any one of 1 to 3, a plurality of image carriers are provided.

According to a sixth aspect of the invention, the first to fifth aspects of the invention are provided.
In the image forming apparatus according to any one of items 1 to 5, the gap formed between the image bearing member and the transfer member is equal to or larger than the thickness of the toner image previously transferred onto the transfer member. Is taking.

The gap formed between the image bearing member and the transfer member may be 100 μm or less.

The distance of the gap may be variable according to the transfer conditions.

When a plurality of image carriers are provided,
It is also possible to adopt a configuration in which the distance of the gap is different for each transfer portion with respect to each image carrier.

It is also possible to adopt a structure in which the distance of the gap becomes larger at the transfer portion located on the downstream side in the rotational direction of the transfer member.

A structure may be employed in which a gap is not provided in the transfer portion located on the most upstream side in the rotational direction of the transfer body.

The gap between the image carrier and the transfer body is formed by providing a predetermined step on the surface of the image carrier and bringing the surface of the transfer body into contact with the step. It may be configured.

The gap between the image carrier and the transfer member is formed by providing a predetermined step on the surface of the transfer member and bringing the surface of the image carrier into contact with the step. It may be configured.

The stepped portion may be formed by winding a film-shaped member or a wire-shaped member around the surface of the image bearing member or the transfer member in the vicinity of both ends thereof in the circumferential direction.

The film-shaped member or the wire-shaped member may be spirally wound, or the ends may be wound so as to cross each other obliquely on the same plane.

The stepped portion may be provided at a position corresponding to the cleaning area of the image carrier or the transfer body to be contacted.

The stepped portion may be provided outside the image area of the image carrier or at a position corresponding to the outside of the image area.

The stepped portion may be provided outside the charged area of the image carrier or at a position corresponding to the outside of the charged area.

The stepped portion may have a cleaning means for cleaning the surface of the stepped portion before entering the transfer portion.

When the transfer body has an endless belt shape, the transfer means may be a roller-like member provided so as to face the image carrier so as to sandwich the transfer body.

The transfer member may be provided in a state of being wound around a roller member which is the transfer means.

The winding structure of the transfer member may be formed by providing a belt pressing member on either one or both of the front and rear portions of the transfer portion.

[0030]

BEST MODE FOR CARRYING OUT THE INVENTION A first embodiment of the present invention will be described below with reference to FIGS. First, an outline of the overall configuration of a tandem type color copying machine as an image forming apparatus according to this embodiment will be described with reference to FIG. The color copying machine 1 includes an image forming unit 2, an image reading unit 3 provided on the image forming unit 2, an automatic document feeder (ADF) 4 provided on the image reading unit 3, The image forming unit 2 has a paper feeding unit 5 provided below the image forming unit 2. The post-processing device 6 is provided on the left side surface of the apparatus main body, and the large-capacity paper feeding unit 7 and the manual feed tray 8 are provided on the right side surface of the apparatus main body.
Is provided and the whole is systematized.

The image forming unit 2 includes an image writing unit 9,
An image forming unit 10, a primary transfer unit 11, a secondary transfer unit 12, a fixing device 13, a reversing unit 14, and the like are provided. The paper feeding section 5 is provided with paper feeding cassettes 15 and 16 for accommodating papers as recording media, and the paper feeding means 1 for separating and feeding the papers one by one in order from the top.
7. It has a pair of transport rollers 18 for transporting the fed sheet. To explain the operation of forming an image, first,
Image processing is performed by an image processing unit (not shown) based on the image signal read by the image reading unit 3. In the image processing section,
The image signal is converted into black (Bk), yellow (Y), magenta (M), and cyan (C) color signals for image formation and transmitted to the image writing unit 9. The image writing unit 9 includes, for example, a laser light source, a deflector such as a rotary polygon mirror, a scanning imaging optical system, and a laser scanning optical system including a mirror group, and a large number of LEDs arranged one-dimensionally or two-dimensionally. Each of the image forming units 10 includes an LED array, an LED writing system including an image forming optical system, and the like, and has four writing optical paths corresponding to the respective color signals. Image writing according to each color signal is performed on a photosensitive drum (described later).

The electrostatic latent image formed on each photoconductor drum is visualized as a toner image by a corresponding developing device (described later). The toner images of the respective colors are sequentially superposed and transferred onto an intermediate transfer belt (described later) of the primary transfer unit 11. The sheets fed from the sheet feeding cassettes 15 and 16, the large-capacity sheet feeding unit 7 or the manual feed tray 8 are sent to the registration roller pair 19 and temporarily stopped there. After the skew displacement is corrected, the sheet is sent to the secondary transfer portion at the timing when the leading edge of the superimposed image on the intermediate transfer belt coincides with the predetermined position in the transport direction, and the secondary transfer unit 12 performs the superimposed image (color image). Image) is transferred. The sheet after the transfer is sent to the fixing device 13, where the toner is melted by heat and pressure and fixed. The sheet of paper which has been fixed is conveyed to the post-processing device 6, where it is subjected to processes such as sorting and binding. In the case of double-sided printing, it is sent to the reversing unit 14 after fixing, reversed, and then sent to the transfer section again.

As shown in FIG. 4, the image forming unit 10 includes
Black (Bk), yellow (Y), magenta (M), cyan (C) photoconductor drums 20Bk,
20Y, 20M, and 20C are provided. Ordinary OPC photoconductors are used for these photoconductors. Around the photosensitive drum 20Bk, the laser light 2 from the charging device 21Bk and the image writing unit 9 is sequentially arranged in the counterclockwise direction.
A 2Bk exposure unit, a black two-component magnetic brush developing type developing device 23Bk, a primary transfer unit 11, a cleaning device 24Bk, a charge removing device 25Bk, and the like are provided. The other photoconductor drums 20Y, 20M, and 20C have the same configuration, and only the alphabet corresponding to the toner color is different.

The primary transfer unit 11 includes the support roller 2
6, 27, 28 and their supporting rollers 26, 27, 2
It is provided at an endless intermediate transfer belt 29 as a transfer member that is wound around 8 and a position (transfer portion) that faces the photoconductor drums 20Bk, 20Y, 20M, and 20C with the intermediate transfer belt 29 interposed therebetween. It has a primary transfer bias roller 30 as a transfer means, a belt cleaning device 31 for cleaning the surface of the intermediate transfer belt 29, and the like. The secondary transfer unit 12 has a drive roller 32, a secondary transfer bias roller 33 that is a driven roller, and a conveyor belt 34 that is wound around the drive roller 32 and the secondary transfer bias roller 33. . The fixing device 13 has a belt fixing system configuration, and includes a heating roller 35 having a heat source inside, a fixing roller 36, a fixing belt 37, and a heating roller pressure roller 38 that is in pressure contact with the fixing roller 36 with the fixing belt 37 interposed therebetween. It has a pair of discharge rollers 39 for discharging the sheet after fixing.

The function and configuration of the image forming unit 10 will be described in detail with reference to FIG. Before writing an image, the surface of the photosensitive drum is about -7 with the charging device 21 provided upstream of the writing portion of each of the photosensitive drums 20Bk, 20Y, 20M, and 20C.
It is charged to 00V. In this embodiment, a conductive rubber roller is used as the charging device 21, and this rubber roller is installed in a non-contact manner with a distance of about 50 μm from the photosensitive drum 20. An AC voltage of about 1 kHz and a peak-to-peak voltage of 2 kV is applied to the rubber roller, and its center value is set to about -800V. As a result, the photosensitive drum 20 is uniformly charged to about -700V. The charging device is not limited to this, contact charging for charging a conductive rubber roller so as to contact the photosensitive drum 20, AC + DC
No charging, no AC bias, only DC bias is about -140
Examples thereof include a DC bias roller charging method for charging the photosensitive drum 20 by applying 0 V, a corona charging method using a corotron or a scorotron which has been conventionally used, and a brush charging method.

After writing on the photosensitive drum 20, the electrostatic latent image is developed in the developing process. The developing device will be described by taking the yellow developing device 23Y as a representative. The developing roller 40Y, the doctor blade 41Y, the two screws 42Y and 43Y, the toner concentration sensor 44Y,
It has an outer case 45Y and the like. The positional relationship between the developing roller 40Y and the screws 42Y and 43Y is such that the screws 42Y and 43Y are located obliquely downward from the developing roller 40Y.
The two screws 42Y and 43Y are arranged in parallel in the horizontal direction. Two screws 42 on the outer case 45Y
A partition plate 46Y that divides Y and 43Y into two chambers is provided. The inside and the front of the partition plate 46Y are notched so that the developer can circulate between the two screws 42Y and 43Y. In addition, the photosensitive drum 20 of the outer case 45Y
A portion facing Y is opened, and a part of the developing roller 40Y is exposed from this opening. As described above, the outer case 45Y is provided with the space above the screw 43Y slightly beside the developing roller 40Y with a little more space.
It surrounds Y, the screws 42Y and 43Y, and the doctor blade 41Y.

The developing roller 40Y has a rotatable non-magnetic developing sleeve 40Ya and a magnet 40Yb as a magnetic field generating means fixedly arranged inside the developing sleeve 40Ya.
The developer is a two-component developer consisting of non-magnetic toner and magnetic carrier. The developer is conveyed while being agitated by screws 42Y and 43Y whose feed directions are opposite to each other, and is constantly circulated in the two chambers. The developer that is being stirred and conveyed and circulated is supplied to the developing sleeve 40Y by the screw 43Y, is held on the surface in the form of a magnetic brush by the magnetic force of the magnet 40Yb, and is pumped up in the rotating direction of the developing sleeve 40Ya. The developer drawn on the magnetic brush is cut into a proper amount by the doctor blade 41Y and sent to the developing unit facing the photoconductor drum 20Y. The developer remaining after being cut off by the doctor blade 41Y drops by gravity to the outside of the magnetic brush on the surface of the developing sleeve 40Ya, is returned to the screw 43Y, and is supplied to the developing sleeve 40Ya while being stirred and conveyed again. .

On the other hand, the developer sent to the developing section is visualized by transferring the toner to the electrostatic latent image on the photosensitive drum 20Y. An AC voltage of 2.25 kHz and a peak-to-peak voltage of about 1 kV is applied to the developing sleeve 40Ya with a center value of -500V. Due to this developing bias, the toner moves due to the potential difference from the exposed area (charge potential of about -150 V) on the photoconductor 20Y. The developer that has not been used for visualization is returned to the inside of the outer case 45Y and separated from the developing sleeve 40Ya at a portion where the magnetic force of the magnet 40Yb does not work to be collected by the screw 43Y. In this way, the developer is screw 43Y and screw 4
Developing sleeve 40Y while being stirred and conveyed at 2Y and circulating
It is supplied to a and collected. When the image is repeatedly output, the toner density becomes low, so the toner density sensor 44Y
The toner is replenished (not shown) so as to obtain a constant density while being detected by. The other developing devices 23Bk, 23M, and 23C have the same configuration and function.

The intermediate transfer belt 29 is used for each photoconductor drum 20.
Between the Bk, 20Y, 20M, 20C and the respective primary transfer bias rollers 30, the photosensitive drums 20Bk, 20Y, 2
It is installed so as not to come into contact with 0M and 20C. On the intermediate transfer belt 29, the photosensitive drums 20Bk, 20Y,
The toner images of respective colors are sequentially superposed and transferred from 20M and 20C, and the photosensitive drums 20Bk, 20Y and 20M,
It carries the toner image on 20C. As a transfer method, a primary transfer bias roller 30 provided so as to face the photosensitive drum 20 so as to sandwich the intermediate transfer belt 29 generates a transfer electric field to electrostatically transfer. In this embodiment,
A voltage of about 1.5 kV is applied to a stainless steel primary transfer bias roller 30 provided on the back surface of the intermediate transfer belt 29 to generate a transfer electric field. Although various materials can be used for the intermediate transfer belt 29, here, a polyimide belt having high durability and a high Young's modulus, a Pvdf belt having excellent surface smoothness, or a polyurethane resin layer is used. A multilayer structure belt having an elastic surface, which has a polyurethane rubber layer and further has a coating layer containing a fluorine component thereon, is used. Particularly, since the surface of the polyurethane multilayer structure belt has elasticity, it has good adhesion to the paper surface and is excellent in secondary transfer.

Each of the belts has a volume resistance of about 10 ¹⁰ to 10 ¹² Ωcm, and the surface resistance of the portion on which the toner is placed is 10 ¹² Ω /
□ The characteristic values are above and excellent in transferability. However, in the present invention, since it is necessary to accurately form a gap between the intermediate transfer belt 29 and the photosensitive drum 20 during the primary transfer, the surface does not have elasticity, such as polyimide or PV.
A resin intermediate transfer belt such as DF is desirable. After the intermediate transfer belt 29 has passed through the final image forming portion, a color image in which four colors are overlapped is formed on the intermediate transfer belt 29.

After the paper is fed from the paper feed section 5, it is first introduced into the secondary transfer unit 12 through the pair of registration rollers 19, and the image is formed where the intermediate transfer belt 29 and the secondary transfer bias roller 33 come into contact with each other. Is transferred to a sheet, and a color image is formed. The sheet after the image transfer is the fixing device 1
3, the image is fixed and a color image is obtained. After the transfer of the image onto the sheet, the intermediate transfer belt 29 is cleaned of the transfer residual toner by a belt cleaning device 31 provided downstream of the secondary transfer position, and the next image is formed again by the image forming unit.

Next, the cleaning device 24 will be described as a representative of the yellow cleaning device 24Y. The cleaning device 24Y is the photosensitive drum 20Y after the primary transfer.
To remove the toner remaining on the upper surface, an elastic cleaning blade, a fur brush, or a combination thereof is used. In the present embodiment, an elastic body, for example, a cleaning blade 47Y made of polyurethane rubber,
The fur brush 48Y, an electric field roller 49Y arranged in contact with the fur brush 48Y, a scraper 50Y of the electric field roller 49Y, and a recovery screw 51Y. The fur brush 48Y is conductive, and the electric field roller 4
9Y is a metal. A fur brush 48 rotating in a counter direction opposite to the rotation direction of the photoconductor drum 20Y.
The residual toner on the photosensitive drum 20Y is scraped off by Y, and the toner attached to the fur brush 48Y is removed by the electric field roller 49Y rotating in the counter direction with respect to the fur brush 48Y. The electric field roller 49Y is cleaned by the scraper 50Y. At this time, the electric field roller 4
A bias is applied to 9Y, and the residual toner is moved from the photosensitive drum 20Y to the fur brush 48Y and from the fur brush 48Y to the electric field roller 49Y by electrostatic force, and finally scraped off by the scraper 50Y and collected by the recovery screw 51.
Y is collected in a waste toner bottle (not shown). The positional relationship between the cleaning device 24Y and the developing device 23Y is such that the portion of the recovery screw 51Y of the cleaning device 24Y overlaps the upper outer case of the screw 42Y of the developing device 23Y. The other cleaning devices 24Bk, 23M, and 23C have the same configuration and function.

The reverse transcription phenomenon which is the subject of the present invention is
The adhesive force between the photoconductor drum 20 and the toner already transferred onto the intermediate transfer belt 29 becomes stronger by some action than the attraction force in the belt direction due to the transfer electric field, and as a result, a part of the toner is transferred onto the photoconductor drum 20. It is a phenomenon of returning. In order to prevent the reverse transfer, basically, the toner already transferred to the intermediate transfer belt 29 does not come into contact with the photosensitive drum 20 at the time of the next transfer. Does not work and reverse transcription cannot occur. Based on this viewpoint, in the present embodiment, as shown in FIG.
A gap g is provided between the photoconductor drum 20 and the intermediate transfer belt 29.
Is provided to make it in a non-contact state. As shown in FIG. 2, in the gap g, the abutting member 53 provided on the outer circumference of the bearing 52 fixed to both ends of the rotary shaft 30a of the primary transfer bias roller 30 contacts the surface of the photosensitive drum 20. Formed by. g = radius of abutting member 53-radius of primary transfer bias roller 30-thickness of intermediate transfer belt 29.

For the purpose of the present invention, the gap g provided between the photosensitive drum 20 and the intermediate transfer belt 29 is desired to be larger than the thickness of the toner layer previously transferred to the intermediate transfer belt 29. In order to achieve the specified image density on the paper, it is generally 0.3 mg / cm depending on the toner type.
A toner adhesion amount of about ^{2 to} 1.5 mg / cm ² is required. Converting this to the height by converting the toner specific gravity, it is about 8μm-40μm
It will be high. In this embodiment, 0.7 mg / cm ² , about 15 μ
It was m. Therefore, in order to prevent reverse transfer in this embodiment, the gap g needs to be at least 8 μm or more. However, if the gap (gap g) is too wide, the toner on the photoconductor drum 20 will be transferred to the intermediate transfer belt 29.
The bias voltage for satisfying the electric field strength for transferring to is too high.

FIG. 6 shows a graph in which the electrolytic strength received by the toner is calculated when the distance of the gap g is changed. During the transfer in the normal (conventional) contact state, the distance between the photosensitive drum 20 and the intermediate transfer belt 29 is about 20 μm, which is almost determined by the thickness of the toner. The electric field strength at this time is about 120 from the graph, and when it is separated up to 100 μm, it decreases to about half of 60. The electric field strength weakens slowly with increasing distance, but the wider the distance, the lower the electric field strength.
It is not good for transcription. Table 1 shows the experimental results of measuring the void distance and the transfer rate.

[0046]

[Table 1]

In Table 1, the transfer rate greatly differs depending on the air gap distance of 20 μm and 30 μm. The transferability is due to a slight speed difference during transfer while the photoconductor drum 20 and the intermediate transfer belt 29 are in contact with each other. Is improved. When the voids are widened, this assist is lost and the transfer rate is reduced. In order to achieve a good transfer of 85% or more, the gap is 50μ.
m or less is desirable, and the void is 100μ to achieve 80% or more.
It is preferably m or less. 80% of transfer rate is 98% at the time of contact
Although it seems that it has deteriorated considerably compared to the above, when the color image is taken with this conventional system, the reverse transfer rate is about 8% in each transfer process, so the most upstream color toner will be transferred repeatedly. 20% or more is lost by reverse transfer, and the image is rubbed, which may deteriorate the image quality. On the other hand, in the system of the present embodiment in which reverse transfer does not occur, the initial transfer efficiency is slightly poor, but if the image is not disturbed thereafter, it can be said that it is better because the image quality does not deteriorate.

From the above example, it is not necessary to separate the gap g more than necessary, and it is sufficient that the toner on the intermediate transfer belt 29 does not come into contact with the photosensitive drum 20 at a minimum. Furthermore, when the toner is first transferred to the photoconductor drum 20, there is no concern of reverse transfer, and thus it is not necessary to make non-contact. Further, since it is a color image, the thickness of the toner layer increases as the secondary color, the tertiary color and the toner overlap on the intermediate transfer body, so that the optimum gap g between the photosensitive drum 20 and the intermediate transfer belt 29 is obtained. It is expected that the distance will vary depending on the transcription conditions. Therefore, it is effective to change the gap g in accordance with the transfer conditions. In the case of the tandem type of the present embodiment, the optimum value may be set for each color transfer portion.

As described above, since the thickness of the toner image increases as the transfer proceeds downstream in the tandem type due to color superposition, it is desirable to set the gap g to expand accordingly, and especially the most upstream transfer. Since there is no concern about reverse transfer in the part, it is not necessary to provide the gap g. Tables 2 and 3 show examples of setting voids. Here, in the examples of Table 2, the variation of the void is limited to two types (50 μm and 80 μm),
It was applied to the upstream two stages and the downstream two stages, respectively. This is because cost is emphasized, and cost can also be suppressed by suppressing variations. Further, if it is attempted to optimize each station (transfer unit), the cost is high, but a more efficient design around the transfer is possible. Table 3 shows examples in which the air gap distance is changed for each station.

[0050]

[Table 2]

[0051]

[Table 3]

In this embodiment, the example of using the intermediate transfer belt 29 is described, but the intermediate transfer drum system may be adopted in view of the mechanical layout, required accuracy, size and the like. FIG. 7 and FIG. 8 show the formation configuration of the gap g when there is one photosensitive drum and the intermediate transfer body is a drum (second embodiment). The gap g is defined by the intermediate transfer drum 5
Bearings 55 fixed to both ends of the rotating shaft 54a of No. 4
The abutting member 56 provided on the outer periphery of the photosensitive drum 5
It is formed by abutting on the surface of 7. g = radius of abutting member 56-radius of intermediate transfer drum 54.

FIG. 9 shows a third embodiment as a modified example of the first embodiment. Note that the same parts as those in the first embodiment are denoted by the same reference numerals, and unless otherwise necessary, the description of the configuration and the function that has already been made will be omitted and only the main parts will be described (the same applies to other embodiments below). In the present embodiment, instead of the abutting member 53, an abutting member 58 in which the distance of the gap g is different every 90 ° rotation is used. The abutting member 58 rotates 90 ° for each station, and the distance of the gap g changes. At position 1 corresponding to the most upstream transfer portion, the distance of the gap g is 0, that is, contact, and is 20 μm at position 2, 40 μm at position 3, and 60 μm at position 4.

As described above, there are roughly two machine layouts using the intermediate transfer member. One is a one-drum type that has one intermediate transfer member and one photosensitive drum, and one developing device having a plurality of developing means. One has one intermediate transfer member and a plurality of photosensitive drums. It is a tandem type. In both cases, a drum type or a belt type can be used as the intermediate transfer member. In the present invention, it is necessary to accurately provide a gap between the photoconductor drum and the intermediate transfer member. In this case, it is generally easier to maintain accuracy when the intermediate transfer member is a drum type. However, in the case of a tandem machine with a low degree of freedom in mechanical layout, there is also a problem that the machine becomes large in the drum type, and the point is that the belt type has the freedom of layout and miniaturization. There are merits. Just select the function that you want to realize.

Generally, when setting the gap, in addition to the method shown in the above embodiment, for example, the shaft of the photosensitive drum,
The distance from the shaft of the other party (for example, the intermediate transfer drum) can be accurately set, or a member such as a bearing is provided on the shaft of the photoconductor drum, and a difference in outer circumference between the bearing and the photoconductor drum is provided. There is a method of pressing it against and providing a void. However, 100 as treated by the present invention
With such a method, it may be difficult to accurately set a minute void of μm or less because the manufacturing accuracy, the mounting accuracy, and the like are sensitively reflected. Therefore, if a step is accurately formed in advance in the vicinity of both ends of the surfaces that come into contact with each other and the step is brought into contact with the other party and driven, the accuracy between the surface of the intermediate transfer body and the surface of the photosensitive drum is improved. The gap can be maintained well, and the influence of eccentricity of the drive of the photoconductor drum can be minimized.

This embodiment (fourth embodiment) is shown in FIG.
10 and 1 as a modification of the second embodiment shown in FIG.
It will be described based on 1. The stepped portion is also costly and difficult to be accurately manufactured by a method such as ordinary cutting. Therefore, a step can be formed with high accuracy by winding and pasting a film-shaped member having a uniform thickness in advance on the surface. As shown in FIG. 10, film-shaped members 60 are circumferentially wound around both ends of the photoconductor drum 57, whereby stepped portions are formed on the surface of the photoconductor drum 57. The material of the film member 60 is preferably a resin such as polyethylene, polystyrene, polyimide or nylon, and a material having a smooth surface and good thickness accuracy is desirable. As a material for attaching the film member 60 to the photosensitive drum 57, a room temperature solidifying type adhesive having a constant thickness or an adhesive tape is preferable. Adhesion strength can be obtained by melt adhesion, but the thickness is likely to be out of order, so caution is required. In addition, care must be taken when winding the film member 60, and considering that the intermediate transfer drum 54 and the photoconductor drum 57 rotate continuously, the influence of the joint in the circumferential direction must be minimized. In the present embodiment, as shown in FIG. 11, the end portion 60a of the film member 60,
60b are wound on the same plane so as to cross each other obliquely. As a result, even if the photoconductor drum 57 rotates, a part of the film-shaped member 60 always comes into contact with the other party (intermediate transfer drum 54), and the influence of the joint is hardly expressed. As shown in FIG. 12, the wire member 61 may be spirally wound around the surface of the photosensitive drum 57 (fifth embodiment). In this case as well, even if the photosensitive drum 57 rotates, a part of the wire-shaped member 61 always comes into contact with the other party (intermediate transfer drum 54), and the influence of the joint is hardly expressed. The film-shaped member 60 and the wire-shaped member 61 forming the stepped portion may be provided on the surface of the intermediate transfer drum 54. Figure 13
An example (sixth embodiment) in which the film member 60 is provided on the surface of the intermediate transfer drum 54 is shown. The stepped portion may be provided on either the intermediate transfer member or the photosensitive drum. It is better to choose one that can be designed with higher accuracy and at lower cost.

Since the gap g provided between the photosensitive drum and the intermediate transfer member is minute, if foreign matter adheres to a stepped portion or the like for maintaining the gap, the gap distance may be greatly changed and the image may be affected. Be careful because there is. Therefore, it is ideal that the stepped portion for holding the void is applied to a portion free from foreign matter. If the step portion is provided on the intermediate transfer member, the step portion is provided so as to be located within the cleaning area of the photoconductor cleaning means (the cleaning device 24 in the first embodiment), and the step portion is provided on the photoconductor drum. If it is present, the intermediate transfer member cleaning means (belt cleaning device 31 in the case of the first embodiment) is installed so as to be located in the cleaning area,
It is preferable to apply a step to the surface that has become normal by the cleaning means.

In some cases, the toner is developed in the developing section, and a carrier larger than the toner is attached to the photosensitive drum. Therefore, it is desirable that the step portion is located outside the width of the developing area. Similarly, when a charging roller or the like is used, foreign matter may adhere to the photosensitive drum at the charging section. Further, if the step member is installed on the photoconductor drum, there is a problem that it is difficult to charge the photoconductor drum. Therefore, it is desirable that the step portion is installed outside the width of the charging device. Further, not only the partner with which the step portion is abutted, but also the surface of the step portion needs to be clean. Therefore, it is effective to clean this part before applying. It is effective to perform this cleaning by providing a urethane blade, a fur brush, a web, etc. immediately before entering the transfer portion. In the first embodiment, as shown in FIG. 4, when a step portion is formed on the intermediate transfer belt 29, the surface of the step portion is a belt immediately before entering the transfer portion of the most upstream black photosensitive drum 20Bk. It is cleaned by the cleaning device 31.

In order to move the toner from the photosensitive drum to the intermediate transfer member, it is necessary to apply an electrostatic bias to give a transfer electric field to the toner. Therefore, in the intermediate transfer drum, a bias is installed on the drum core, and in the intermediate transfer belt system, a bias applying member is installed on the back side of the intermediate transfer belt. In the case of the intermediate transfer belt, unlike the drum, it is necessary to provide means for accurately managing the gap. Therefore, the precision of the gap can be maintained by disposing a highly accurate roller so as to sandwich the belt at a position facing the photosensitive drum. There are brush bias method and blade bias method in addition to the roller method for applying the transfer bias, but in both cases, the pressing pressure from the back surface of the belt varies depending on the location, and it is not possible to accurately maintain the gap. The bias roller (primary transfer bias roller 30) is made of metal (SUS) in the first embodiment, but a resin or a highly accurate rubber roller may be used.

When a roller-shaped bias applying member is used, as shown in FIG. 14, the intermediate transfer belt 29 is placed not on the photosensitive drum 20 side but on the primary transfer bias roller 30 side.
The accuracy of the void is better when the coil is pressed by winding (7th embodiment). In the present embodiment, the intermediate transfer belt 29
Film-like members 6 for forming stepped portions on both sides of the surface of the photoconductor drum, that is, at intervals in the axial direction of the photoconductor drum 20.
2 is pasted. When the photosensitive drum 20 is wound around the photosensitive drum 20, it is difficult to keep a gap between the step portions and the belt portion between the step portions, and there is a risk that the belt may come into contact with the surface of the photosensitive drum 20. By winding the intermediate transfer belt 29 around the primary transfer bias roller 30, the belt portion between the step portions is wound around the primary transfer bias roller 30 side by the tension that stretches the intermediate transfer belt 29. The possibility of contacting the photoconductor drum 20 is suppressed.

If the intermediate transfer belt 29 in the transfer section is wound around the primary transfer bias roller 30 side, for example, if it is a one-drum type, the photosensitive drum and the intermediate transfer belt are adjacent to each other at one place. However, in the tandem type, since there are a plurality of places, it is difficult to adopt this configuration in all the transfer parts. In such a case, the intermediate transfer belt 29 is provided at the transfer portion before and after the transfer portion, and the primary transfer bias roller 3 is provided.
It is preferable to provide a member that changes the direction of the intermediate transfer belt 29 so that the intermediate transfer belt 29 is wound around the 0 side. For example, as shown in FIG. 15, the intermediate transfer belt 29 is pressed so as to be wound around the primary transfer bias roller 30 by a roller 63 as a belt pressing member before and after the transfer portion (eighth embodiment). In this case, the rollers 63 are provided only on both ends of the surface of the intermediate transfer belt 29 outside the image area so as not to touch the transferred toner image. The roller 63 may be provided either on the front side or the back side of the transfer section.

In each of the above embodiments, the tandem type using the intermediate transfer belt and the one-drum type using the intermediate transfer drum are illustrated. However, the one-drum type using the intermediate transfer belt as the intermediate transfer body is a tandem type. A similar configuration can also be applied to a configuration in which an intermediate transfer drum is used for each type, a direct transfer configuration for one drum type and a tandem type.

[0063]

According to the invention of claim 1, the toner image formed on the image carrier is transferred to the transfer body by the transfer means, and the toner images of at least two colors are superposed on the transfer body. In an image forming apparatus that forms a color image by
Since the image carrier and the transfer member are arranged so as to transfer the toner image while keeping the non-contact state, there is no color mixture due to reverse transfer, and the toner image is not disturbed. Images can be obtained.

According to a second aspect of the invention, in the image forming apparatus according to the first aspect, the transfer body is an intermediate transfer body for receiving a toner image from the image carrier and transferring it to a recording medium. Since a plurality of toner images are superposed on the intermediate transfer body and then collectively transferred to the recording medium, the non-contact state can be accurately maintained, and the reverse transfer and the disturbance of the toner image can be further suppressed. .

According to the third aspect of the invention, in the image forming apparatus according to the second aspect, since the intermediate transfer member has a drum shape, the non-contact state can be maintained with high accuracy, The reverse transfer and the disturbance of the toner image can be suppressed with high accuracy.

According to a fourth aspect of the invention, in the image forming apparatus according to the second aspect, since the intermediate transfer member has a shape of an endless belt, the degree of freedom of layout of the apparatus can be increased. Therefore, the device can be made compact.

According to the fifth aspect of the invention, in the image forming apparatus according to any one of the first to fourth aspects, since a plurality of the image bearing members are provided, the reverse transfer and the toner image formation are performed. Higher speed can be realized while having the function of preventing disturbance.

According to the sixth aspect of the invention, in the image forming apparatus according to any one of the first to fifth aspects, the gap formed between the image carrier and the transfer body is first formed. Since the thickness of the toner image transferred onto the transfer body is equal to or larger than that of the toner image, the toner and the image carrier are not physically contacted with each other, and reverse transfer and disturbance of the toner image can be reliably prevented.

When the gap formed between the image bearing member and the transfer member is set to 100 μm or less, good transfer can be achieved without extremely weakening the transfer electric field required for toner transfer. It is possible to prevent reverse transfer and disorder of the toner image while maintaining the rate.

When the distance of the air gap is variable according to the transfer conditions, reverse transfer and disorder of the toner image can be prevented according to the transfer conditions without excess or deficiency.

When a plurality of image carriers are provided,
When the distance of the gap is different for each transfer portion with each image carrier, the function of preventing reverse transfer due to the gap and disturbance of the toner image can be obtained in each transfer portion without waste.

In the case where the distance of the gap becomes larger at the transfer portion located on the downstream side in the rotational direction of the transfer member, the optimum reversal is performed in accordance with the change in the toner layer height during repeated transfer. It is possible to create anti-copying conditions.

If a gap is not provided in the transfer portion located on the most upstream side in the rotational direction of the transfer body, the most upstream transfer efficiency without concern about reverse transfer can be improved.

The gap between the image carrier and the transfer member is formed by providing a predetermined step on the surface of the image carrier and bringing the surface of the transfer member into contact with the step. In this case, voids of 100 μm or less, which cannot be obtained by the usual method, can be stably created.

The gap between the image carrier and the transfer member is formed by providing a predetermined step on the surface of the transfer member and bringing the surface of the image carrier into contact with the step. In this case, voids of 100 μm or less, which cannot be obtained by the usual method, can be stably created.

In the case where the step portion is formed by winding a film-shaped member or a wire-shaped member in the circumferential direction on the surface of the image bearing member or the transfer member in the vicinity of both ends thereof, a usual method is used. Cannot be obtained with 100μ
A void of m or less can be stably created.

When the film-like member or the wire-like member is spirally wound, or when the end portions are wound so as to cross each other obliquely on the same plane, a gap due to a seam is formed. Variation can be prevented, and reverse transfer can be stably prevented.

When the stepped portion is provided at a position corresponding to the cleaning area of the image carrier or the transfer member to be contacted, it is possible to prevent the gap variation due to foreign matter and stabilize the reverse transfer. Can be prevented.

When the stepped portion is provided outside the image area of the image carrier or at a position corresponding to the outside of the image area, it is possible to prevent a gap variation due to a foreign substance and stably prevent reverse transfer. can do.

When the stepped portion is provided outside the charged area of the image carrier or at a position corresponding to the outside of the charged area, it is possible to prevent the fluctuation of the void due to the foreign matter and to stably prevent the reverse transfer. can do.

In the case where the cleaning means is provided for cleaning the surface of the step portion before the step portion enters the transfer portion, it is possible to prevent the fluctuation of the air gap due to foreign matter such as dust, and to perform the reverse transfer. Can be stably prevented.

When the transfer body has an endless belt shape, the transfer means is a roller-like member provided so as to face the image carrier so as to sandwich the transfer body. It is possible to improve the air gap accuracy in the intermediate transfer belt in which the air gap accuracy is hard to be obtained as compared with the drum.

When the transfer body is provided in a state of being wound around the roller-shaped member which is the transfer means, the gap accuracy can be further improved and stabilized.

When the winding structure of the transfer member is formed by providing the belt pressing member on one or both of the front and rear portions of the transfer portion, the gap accuracy is further improved and the stability is improved. Can be made.

[Brief description of drawings]

FIG. 1 is a schematic side view showing a non-contact state between a photosensitive drum and an intermediate transfer belt according to a first embodiment of the present invention.

FIG. 2 is a schematic front view showing a non-contact state between a photosensitive drum and an intermediate transfer belt.

FIG. 3 is an enlarged view of a configuration around a photosensitive drum.

FIG. 4 is a schematic front view showing an image forming unit, a primary transfer unit, and a secondary transfer unit.

FIG. 5 is a schematic front view of a color copying machine as an image forming apparatus.

FIG. 6 is a graph showing the relationship between the air gap distance and the electric field strength.

FIG. 7 is a schematic side view showing a non-contact state between a photosensitive drum and an intermediate transfer drum in the second embodiment.

8 is a schematic front view showing a non-contact state between a photosensitive drum and an intermediate transfer drum in the embodiment shown in FIG.

FIG. 9 is a schematic front view showing a non-contact state between a photosensitive drum and an intermediate transfer drum in the third embodiment.

FIG. 10 is a schematic side view showing a non-contact state between a photosensitive drum and an intermediate transfer drum in a fourth embodiment.

FIG. 11 is a view showing a seam portion of a film-shaped member according to a fourth embodiment.

FIG. 12 is a view showing a seam portion of a wire-shaped member according to a fifth embodiment.

FIG. 13 is a schematic side view showing a non-contact state between a photosensitive drum and an intermediate transfer drum in the sixth embodiment.

FIG. 14 is a schematic side view showing a non-contact state between a photosensitive drum and an intermediate transfer belt in a seventh embodiment.

FIG. 15 is a schematic side view showing a non-contact state between a photosensitive drum and an intermediate transfer belt in an eighth embodiment.

[Explanation of symbols]

20 Photoreceptor Drum as Image Carrier 29 Intermediate Transfer Belt 30 as Transfer Body Primary Transfer Bias Roller 31 as Transfer Means Belt Cleaning Device 54 as Cleaning Means Intermediate Transfer Drums 60, 62 as Transfer Body Film Member 61 Wire-like member 63 Roller g as a belt pressing member Gap

Claims (6)

[Claims] 1. A toner image formed on an image bearing member is transferred onto a transfer member by a transfer means, and toner images of at least two colors are transferred onto the transfer member (including the concept of an intermediate transfer member and a recording material). An image forming apparatus for forming a color image by superimposing the image carrier, wherein the image carrier and the transfer body are provided so as to transfer a toner image while maintaining a non-contact state. 2. The image forming apparatus according to claim 1, wherein the transfer body is an intermediate transfer body for receiving a toner image from the image carrier and transferring it to a recording medium, and a plurality of intermediate transfer bodies are provided on the intermediate transfer body. An image forming apparatus, wherein toner images are superposed and then collectively transferred onto the recording medium. 3. The image forming apparatus according to claim 2, wherein the intermediate transfer member has a drum shape. 4. The image forming apparatus according to claim 2, wherein the intermediate transfer member has an endless belt shape. 5. The image forming apparatus according to any one of claims 1 to 4, wherein a plurality of the image carriers are provided. 6. The image forming apparatus according to claim 1, wherein the void formed between the image bearing member and the transfer member is previously transferred onto the transfer member. An image forming apparatus having a thickness of a toner image or more.