JP2009098170A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus wherein a transfer condition is set while taking into consideration hollow character and reverse-transfer in a color mode, and the hollow character and transfer performance are improved in a monochrome mode. <P>SOLUTION: The image forming apparatus 1 has a transfer mode comprising the color mode to execute the transfer while all of a plurality of photoreceptors 5 come in contact with an intermediate transfer belt 11 and the monochromatic mode to execute the transfer while at least one of the plurality of photoreceptors 5 comes in contact with the intermediate transfer belt 11, but the remaining photoreceptors 5 are separated from the intermediate transfer belt 11. In the image forming apparatus 1, a plurality of chargers 6 have at least one roller-charging systems or at least one corona-charging systems, respectively, and the transfer condition to the photoreceptor 5 coming in contact with the intermediate transfer belt 11 in the monochrome mode is different from that in the color mode. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an electrophotographic image forming apparatus. In particular, the present invention relates to a color image forming apparatus.

  Conventionally, many forms are known as a color image forming apparatus, and a direct transfer method in which toner images formed on a plurality of image carriers are directly transferred and fixed onto a recording medium to be conveyed and fixed to an intermediate transfer member. There is an intermediate transfer method (indirect transfer method) in which multiple transfer is performed and then transferred to a recording medium and fixed. In any case, in an electrophotographic image forming apparatus, an image carrier is charged by a charging unit in an image forming unit, exposed using an LD or LED, an electrostatic latent image is written, and a toner image is formed by a developing unit. ing. For example, an intermediate transfer type color image forming apparatus has a plurality of image forming units, and each image carrier is in contact with the intermediate transfer member at a different position. The intermediate transfer member is configured as, for example, an endless intermediate transfer belt that is stretched around a plurality of rollers. In the primary transfer unit, a transfer electric field formed between the image carrier and the intermediate transfer belt is used. The secondary transfer means also uses a transfer electric field formed between the intermediate transfer belt and a recording medium (for example, paper). The transfer unit is required to faithfully and stably transfer the toner image to the intermediate transfer body before and after the transfer. That is, in order to realize the performance required for the primary transfer unit and the secondary transfer unit, it is necessary to perform stable transfer with high transfer efficiency. The electrostatic latent images formed on the respective image carriers are developed using charged toners of different colors, and usually at the transfer position where the respective image carriers and the intermediate transfer belt are in contact with each other. A transfer bias is applied to the toner image, and a toner image on each image carrier is sequentially transferred to an intermediate transfer member by a transfer electric field generated by the transfer bias to form a color image.

In the recent image forming apparatuses, the main means for charging is a roller charging system using a roller and a corona charging system using a charging charger.
The corona charging method includes a discharge electrode such as a wire electrode and a shield electrode surrounding the discharge electrode, and charges the surface of the object to be charged to a predetermined potential by a corona shower generated by applying a high voltage to the discharge electrode and the shield electrode. is there. In the corona charging method, even if the applied bias is only a direct current component, the surface of the photoreceptor is charged uniformly enough for practical use, and since it is non-contact, there is little contamination with toner or lubricant. On the other hand, a lot of ozone is generated and a high voltage is required.
On the other hand, the roller charging method is often employed because of the advantages of low ozone and low power consumption. In the roller charging method, charging is performed by placing a charging roller in contact with or near the surface of the photoreceptor. In order to uniformly charge a predetermined potential, the applied bias generally superimposes a DC component and an AC component. Since the AC bias is applied, more discharge is generated than in the corona charging method, so that the surface of the photoconductor is turned over or the photoconductor surface is easily damaged by discharge products. In order to avoid this, a lubricant can be applied to the surface of the photoconductor to avoid the problem of turning over the surface of the photoconductor, but this time, the lubricant adheres to the charging roller and becomes uniform on the photoconductor. It becomes impossible to charge. Therefore, the lubricant application amount is optimized so that both the surface of the photosensitive member is turned over and the lubricant is fixed to the charging roller. However, there is no lubricant coating amount that is completely compatible, and the life of the charging roller is used to some extent.

In the above-mentioned corona charging method, since it is non-contact charging, there is little deterioration to the charging charger by the lubricant or toner, and the hazard to the photosensitive member is also small. Therefore, the lubricant can be applied sufficiently and sufficiently to prevent the hazard to the photosensitive member, and it is not necessary to consider the contamination of the lubricant and toner on the charging means. Accordingly, although the ozone generation amount and power consumption are inferior to those of the charging roller, the lifetime is superior to that of the charging roller.
In consideration of the characteristics of these charging methods, there is a method in which a charging charger and a charging roller are selectively used for each toner image forming unit. That is, a long-life charging charger is used for a frequently used toner image forming unit (for example, black), and a low-ozone and low-power charging roller is used for an infrequently used image forming unit (for example, color). This method is used. As a result, it is possible to achieve low ozone and low power consumption, which are required from the viewpoint of environmental properties, while reducing the maintenance frequency of the image forming apparatus.

By the way, in a color image forming apparatus, there is an apparatus that can select a color mode using an image forming unit of all colors and a single color mode using only one image forming unit. In addition, some apparatuses have a color reduction mode that uses several image forming units.
In such a color image forming apparatus capable of switching between the color mode and the monochromatic mode, an image forming unit that is not used in the monochromatic mode may be formed apart from the intermediate transfer belt (for example, Patent Document 1). When not separated, the photosensitive member of the image forming unit that is not used is also in contact with the intermediate transfer belt, and therefore the photosensitive member must be driven to rotate in order to avoid problems due to friction. Rotating the photoconductor when not in use, contamination of the photoconductor surface, friction and wear of the cleaning blade, power consumption due to charging bias application, developer deterioration due to rotation of the developing roller, etc. Various problems occur. As a result, the life of the image forming unit is shortened. In order to avoid this, image formation is performed by separating an image forming unit that is not used in the single color mode from the intermediate transfer belt. Since the photosensitive member of the image forming unit that is not used can be stopped, the service life can be greatly extended.
Furthermore, a technique is disclosed in which, in an image forming unit used in the monochrome mode, transfer conditions are made different between the color mode and the monochrome mode (for example, Patent Documents 2 and 3). In the single color mode as compared with the color mode, the speed of the photosensitive member relative to the intermediate transfer belt is decreased, and the pressing force to be pushed up to the photosensitive member and the offset amount of the transfer member are optimized. It is a content that the trouble such as banding that occurs in the single color mode can be avoided by optimizing the transfer conditions.

JP 2001-242680 A JP 2007-33938 A JP 2005-181698 A

However, in such an image forming apparatus, there is a case in which transfer omission occurs. The void is a phenomenon in which a part of the toner cannot be transferred and the color is lost when the toner is overlapped or a thin line is transferred. This is because when the transfer is performed by applying a pressure between the photosensitive member and the intermediate transfer belt, a part of the toner image is lost due to stress concentration on a part of the toner. If the pressing force is too weak, a toner that is difficult to transfer, such as a toner with a high charge amount or a toner with a large particle diameter, is not transferred. In order to avoid this, the pressing force applied to the photoconductor and the intermediate transfer belt needs to be in an appropriate range.
In addition, there is a phenomenon of reverse transfer in which when the toner on the intermediate transfer belt transferred by the upstream photoconductor passes through the downstream photoconductor, the toner returns from the intermediate transfer belt to the photoconductor. When reverse transfer occurs, the amount of toner consumption increases, which directly increases the cost. In view of these circumstances, it is desired to provide an image forming apparatus that can prevent abnormal images such as missing transfer, maximize transfer efficiency, and reduce reverse transfer.
By making a difference (hereinafter simply referred to as a linear speed difference) between rotational speeds of the photosensitive member and the intermediate transfer belt, hollowing out can be prevented. Furthermore, it has been newly clarified that there is a significant difference in the effect of improvement between the charging roller and the charging charger when the linear velocity difference is given.

  Therefore, the present invention has been made in view of the above problems, and the problem is that, in the color mode, transfer conditions are set in consideration of the void and reverse transfer, and in the monochrome mode, the void and toner yield are reduced. An object of the present invention is to provide an improved image forming apparatus.

The features of the present invention, which is a means for solving the above problems, are listed below.
An image forming apparatus according to the present invention includes a plurality of image forming units each having an image carrier and a charging unit, a transfer member that rotates in contact with the image carrier, and the transfer member as the image carrier. An image forming apparatus having a transfer member to be pressed, wherein the image forming apparatus includes a first transfer mode in which transfer is performed in a state where all of the plurality of image bearing members and the transfer target body are in contact with each other; In an image forming apparatus having a transfer mode comprising: a second transfer mode in which at least one of the image carriers is in contact with the transfer target and the remaining image support is transferred in a state of being separated from the transfer target. The plurality of charging means has at least one of a roller charging method and a corona charging method, respectively, and a transfer condition to the image carrier in contact with the transfer target in the second transfer mode is: The first transfer mode is different from the first transfer mode. That.
The image forming apparatus of the present invention is further characterized in that the charging means of the image forming portion that is in contact with the transfer target in the second transfer mode is a corona charging method.
The image forming apparatus of the present invention further provides a difference between a linear velocity at which the image carrier moves and a linear velocity at which the image transfer member moves at a transfer position where the image carrier transfers to the image transfer member. .
In the image forming apparatus of the present invention, the transfer condition of the transfer unit is a pressing force of a pressing unit that presses against the image carrier.
Further, in the image forming apparatus of the present invention, there is only one image forming portion in contact with the second transfer mode in the previous period, and the toner color is black.
In the image forming apparatus of the present invention, in the second transfer mode, the image carrier that is in contact with the transfer target is provided on the most downstream side in the rotation direction of the transfer target.

Further, in the image forming apparatus of the present invention, the charging means of the image forming unit for forming a color toner image uses a roller charging method.
Further, the image forming apparatus of the present invention further uses the roller charging method, the charging means is a charging roller, and the applied voltage superimposes a DC component and an AC component.
Further, the image forming apparatus of the present invention is a non-contact charging method in which the charging roller realizes charging without contacting the image carrier.

  In the image forming apparatus as the means for solving the above, the transfer condition is changed between the first transfer mode and the second transfer mode, and in the first transfer mode, the second transfer is performed under the optimum conditions for reverse transfer. In the mode, the optimum conditions for the toner yield can be obtained, so that it is possible to realize an image forming apparatus that consumes less toner as a whole.

  The best mode for carrying out the present invention will be described below with reference to the drawings. Note that it is easy for a person skilled in the art to make other embodiments by changing or correcting the present invention within the scope of the claims, and these changes and modifications are included in the scope of the claims. The following description is an example of the best mode of the present invention, and does not limit the scope of the claims.

Embodiments of the present invention will be specifically described below with reference to the drawings.
FIG. 1 is an overall configuration diagram showing an image forming apparatus embodying the present invention, and here, a color copying machine is shown as an example. This color copying machine is a tandem type electrophotographic apparatus using an intermediate transfer belt 11, a paper feeding table 2 at the bottom, a copying machine main body 1 above it, and a scanner 3 and automatic document feeding at the top. A device (ADF) 4 is provided.

The copying apparatus main body 1 is provided with a transfer device 9 provided with an endless intermediate transfer belt 11 at substantially the center. The intermediate transfer belt 11 is stretched by a driving roller 14, driven rollers 15 and 16, and a tension roller 70. In FIG. 1, the cleaning device 8 that is driven to rotate clockwise and the left side of the driven roller 15 removes residual toner remaining on the surface after image transfer, and the transfer device 9 performs the next image. Prepared for formation.
Photosensitive members 5Y, 5M, 5C, and 5K (hereinafter, colors are specified) along the moving direction above the linear intermediate transfer belt 11 that is stretched between the driving roller 14 and the driven roller 15. If not necessary, the photosensitive member 5 is simply provided in a counterclockwise direction in FIG. 1, and a known exposure device 21 and developing device 7 are provided therearound. The charging device is provided with a charging roller 6a for yellow, magenta and cyan, and a charging charger 6b for black. The charging roller 6a and the charging charger 6b can be arbitrarily selected according to the design concept. Primary transfer rollers 9Y, 9C, 9M, and 9B constituting the primary transfer means (hereinafter simply referred to as the primary transfer roller 9 when the color need not be specified) are provided. Around the photosensitive member 5, a charging device, a photosensitive member cleaning device, a static eliminating device, and a lubricant application device are provided, and these constitute one image forming unit, in particular, the photosensitive member and the charging device. In many cases, other devices can be appropriately selected and combined.
On the other hand, a secondary transfer device 22 constituting secondary transfer means is provided below the intermediate transfer belt 11. The secondary transfer device 22 is configured to press the intermediate transfer belt 11 and contact the driven roller 16. The secondary transfer device 22 collectively transfers the toner images on the intermediate transfer belt 11 onto a sheet P as a recording medium fed between the intermediate transfer belt 11 and the secondary transfer device 22.
A fixing device 25 for fixing the toner image formed on the sheet P is provided on the downstream side of the secondary transfer device 22 in the sheet conveyance direction, and a pressure roller 27 is in pressure contact with the endless fixing belt 26. The sheet P after the image transfer is conveyed to the fixing device 25 by an endless conveyance belt 24 that is stretched between the pair of rollers 23 and 23. The secondary transfer device 22 may be a transfer system using a transfer roller or a non-contact charger. A sheet reversing device 28 for reversing the sheet P when an image is formed on both the front and back surfaces of the sheet is provided below the secondary transfer device 22.

Next, a method for separating the intermediate transfer belt 11 will be briefly described.
FIG. 2 is an enlarged view of a main part of a tandem type in which image forming units including a photoconductor are arranged. In the color mode for forming a color image, the photoconductor 5 and the primary transfer roller 9 are in contact with each other to form an image. For this reason, the “first transfer mode in which transfer is performed in a state where all of the plurality of image bearing members and the transfer target are in contact” refers to this color mode (hereinafter, unless otherwise specified, the first transfer mode). Is called color mode.) In contrast, in the monochrome mode for forming a monochrome image, the YCM photoreceptors 5Y, 5C, and 5M and the primary transfer rollers 9Y, 9C, and 9M are separated from each other to form an image. For this reason, “a second transfer mode in which at least one of the plurality of image carriers is in contact with the transfer target and the remaining image carriers are separated from the transfer target” is referred to as this single color. Mode (hereinafter, the second transfer mode is referred to as a single color mode unless otherwise specified). In the separation method, the primary transfer rollers 9Y, 9C, 9M of the collar and the driven roller 15 move downward with the fulcrum roller 72 as a fulcrum, and the tension roller 70 is pushed upward by the tension roller spring 71. The belt tension is lowered by the movement of the primary transfer rollers 9Y, 9C, 9M and the driven roller 15, but the belt tension is maintained by the movement of the tension roller 70. This prevents the occurrence of abnormal images due to fluctuations in belt tension between the color mode and the monochrome mode, which can be exemplified by so-called shock jitter and banding. In addition, since the color image forming units that are separated in the single color mode do not need to be driven, the photosensitive member 5 is not rotated and the charging roller 6a is not biased.

When making a color copy with the color copying machine having the above-described configuration, the document is usually set on the document table 30 of the automatic document feeder 4, but when the document is manually set, The automatic feeding device 4 is opened, a document is set on the contact glass 32 of the scanner 3, and the document is pressed against the contact glass 32 by closing the automatic document feeding device 4.
Next, when a start switch (not shown) is pressed, when the document is set on the automatic document feeder 4, the document is automatically fed onto the contact glass 32. When the document is manually set on the contact glass 32, the scanner is immediately scanned. 3 operates, and the first traveling body 33 and the second traveling body 34 start traveling. As a result, light from the light source of the first traveling body 33 is irradiated toward the original, and further, the direction is changed by 180 degrees by the pair of mirrors of the second traveling body 34 and enters the reading sensor 36 through the joint lens 35. The contents of the original are read.
When the start switch is pressed, the intermediate transfer belt 11 starts rotating, and at the same time, the photosensitive members 5Y, 5C, 5M, and 5K also start rotating, and yellow, magenta, cyan, Each black monochrome image is formed. Each single-color image formed on each photoconductor in this way is superposed on the intermediate transfer belt 11 that rotates clockwise in FIG. 1 and sequentially transferred to form a full-color composite color image.

  On the other hand, the sheet feeding roller 42 of the selected sheet feeding stage in the sheet feeding table 2 rotates, and the sheet P is fed out from the selected sheet feeding cassette 44 in the paper bank 43 and separated into one sheet by the separation roller 45. And conveyed to the paper feed path 46. The fed sheet P is conveyed by the conveying roller 47 to the paper feeding path 48 of the copying machine main body 1 and comes into contact with the registration roller 49 to be temporarily stopped. In the case of manual sheet feeding, the sheet P set on the manual sheet feeding tray 51 is fed out by the rotation of the sheet feeding roller 50, separated into one sheet by the separation roller 52, and conveyed to the manual sheet feeding path 53. It comes into contact with the registration roller 49 and temporarily stops. In any case, the registration roller 49 starts rotating at an accurate timing in accordance with the color image on the intermediate transfer belt 11, and the 嫉妬 P that has been stopped is moved between the intermediate transfer belt 11 and the secondary transfer roller 22. And a color image is transferred onto the sheet P by the secondary transfer device 22 described above. The sheet P on which the color image has been transferred is transported to the fixing device 25 by the secondary transfer device 22 having a transport function, and is overheated and pressurized to fix the transferred image, and then guided to the discharge side by the switching claw 55. The paper is discharged onto the paper discharge tray 57 by the discharge roller 56 and stacked.

  When the double-sided copy mode is selected, the sheet P on which an image is formed on the front surface is conveyed to the sheet reversing device 28 side by the switching claw 55, and is reversed and guided to the secondary transfer position 22 again. After the image is formed on the sheet, the sheet is discharged onto the sheet discharge tray by the discharge roller 56 and stacked. When a black single color image is formed on the intermediate transfer belt 11, the driven rollers 15 and 16 other than the driving roller 14 are moved to place the yellow, magenta, and cyan photoconductors 5 Y, 5 M, and 5 C into the intermediate transfer belt 11. It is trying to keep away from. In the so-called 1-drum type image forming apparatus 1 having only one photoconductor, instead of the tandem type shown in FIG. 1, black is first formed in order to increase the first copy speed. In general, the remaining colors are imaged only when the original is in color.

In such a configuration, the registration roller 49 is usually used while being grounded, but a bias can be applied to remove the paper dust of the sheet P. For example, when a bias is applied using a conductive rubber roller having a total surface of 18 mm and a surface coated with a conductive NBR rubber having a thickness of 1 mm, the volume resistance of the rubber material is about 10 ⁹ Ωcm, and the toner transfer side (surface Side) and a voltage of about +200 V is applied to the back side of the sheet. In general, in the intermediate transfer method, paper dust does not easily move to the photoconductor, so that there is little need to consider paper dust transfer, and there is no problem even if it is grounded. Although a DC bias is generally applied as the applied voltage, an AC voltage having a DC offset component can be applied to charge the sheet more uniformly.
Since the surface of the sheet after passing through the registration roller to which a bias is applied in this way is slightly charged on the negative side, compared with the case where no voltage is applied to the registration roller in the transfer from the intermediate transfer belt to the sheet. The transfer conditions may change and the transfer conditions may be changed.

Next, the relationship between the linear velocity difference and the void in this embodiment will be described. FIG. 3 is a graph showing the relationship between the linear velocity ratio and the hollow rank in each charging method. The “missing rank” is an evaluation of the degree of missing in five levels, with 1 being bad and 5 being the best. The permissible rank of the middle drop rank is rank 3 or higher. Using the rotational speed of the intermediate transfer belt 11 as a reference, the linear speed difference is positive when the speed of rotation of the photosensitive member is fast, and the linear speed difference is negative when the speed of the photosensitive member is slow. ing.
In the case where the charging roller 6a is used, it is better that the linear velocity difference is 0% and the difference in linear velocity is not different. On the other hand, when the charging charger 6b is used, the hollow spot rank is good when the linear speed difference is given, and the hollow spot rank is remarkably improved as compared with the case where there is no linear speed difference. This is because the charging roller 6a applies a bias by superimposing a DC component and an AC component, and the hazard to the surface of the photosensitive member is large. This hazard promotes the wear of the cleaning blade due to minute discharge products on the order of several to several tens of μm adhering to the surface. Further, when transferring, since the local surface frictional force is different, the physical adhesion force between the toner and the photosensitive member is locally increased or decreased, so that a void occurs at a place where the physical adhesion force is increased.
Therefore, even if the linear velocity difference is changed, there is no change in the local physical adhesive force. On the other hand, when the charging charger 6b is used, the hazard on the surface of the photosensitive member 5 is small, so that even if there is no linear speed difference, it is an allowable range. When the linear velocity difference is given, a shearing stress is generated, so that a force of the shearing force of the toner acts in the direction in which the toner moves, that is, the direction in which the toner is transferred from the photosensitive member 5 to the intermediate transfer belt 11. Since shearing occurs in a place where the physical interaction is the smallest, in this embodiment, the shearing occurs near the interface between the photoconductor 5 and the toner. This is because in the case of the charging roller 6a, the contribution of surface hazard is greater than the contribution of shear stress. Therefore, the charging roller 6a and the charging charger 6b have different linear velocity differences and tendency to drop out, and each has an optimum value.

  In the case of the color mode, since all the four photoconductors 5 are in contact with the intermediate transfer belt 11, it is desirable that the linear velocity difference is the same for all the photoconductors 5. This is to avoid mechanical vibrations and blurs that occur due to differences in linear velocity. When the linear velocity difference is given, the hollowing using the charging roller 6a is out of the allowable range, so the linear speed difference is not given in the color mode. On the other hand, in the single color mode, since the color photoconductors 5 are separated from each other, only the black photoconductor 5K needs to be considered. Therefore, image formation is performed with a linear velocity difference that greatly improves the hollow rank. However, if the linear velocity difference is excessively increased, the transferred characters are stretched or shrunk, so that the character stretches within a permissible range. When used in all colors of the charger 6b, the hollow rank is improved. However, it is not realistic because the generation of environmental pollutants such as ozone and NOx becomes remarkable.

Next, the relationship between the pressing force and the transfer rate in this embodiment will be described. FIG. 4 is a graph showing the relationship between the pressing force, the primary transfer rate from the photoconductor, and the reverse shot rate.
The transfer rate is an index indicating how efficiently the toner on the surface of the photoconductor 5 can be transferred to the intermediate transfer belt 11, and is expressed as (toner adhesion amount per unit area on the intermediate transfer belt 11) / (on the photoconductor). Toner adhesion amount per unit area) × 100. Therefore, a higher transfer rate is preferable.
The reverse transfer rate is an index indicating how much toner moves on the photosensitive member 5 when the toner on the intermediate transfer belt 11 passes through the transfer nip (the amount of toner attached per unit area on the photosensitive member). ) / (Toner adhesion amount per unit area on the intermediate transfer belt 11) × 100. The reverse transfer is a phenomenon in which the toner transferred upstream is transferred to the downstream photoreceptor 5 in the downstream transfer nip, and the reverse transfer rate is preferably smaller.
FIG. 4 shows transfer rates and reverse transfer rates that occur between the black photosensitive member and the intermediate transfer belt. It can be seen that as the pressing force increases, both the transfer rate and the reverse transfer rate increase. This is because the pressing force is increased at the transfer nip portion where the photosensitive member 5 and the intermediate transfer belt 11 are in contact with each other, so that the toner adheres uniformly to the belt and the transfer rate is increased.
Since the multi-color toner passes as it is located downstream, the reverse transfer rate is more important than the transfer rate. In other words, in the present embodiment, toners of three colors of yellow, cyan, and magenta pass through the black transfer nip, so that the contribution of reverse transfer is very large. Therefore, in the case of the color mode, it is advantageous from the viewpoint of toner consumption to set the pressing force low so as to reduce the reverse transfer rate. In contrast, in the case of the single color mode, the color photoconductors 5 are separated from each other, so that it is not necessary to consider reverse transfer. Therefore, the pressing force is increased, the transfer rate is increased, and the toner consumption is reduced.
In this way, the toner consumption can be optimized by changing the pressing force between the color mode and the single color mode.

Next, the relationship between the pressing force and the dropout rank in this embodiment will be described. FIG. 5 is a graph showing the relationship between the pressing force and the hollow rank in the image forming mode. As described above, an acceptable range is a rank 3 or higher. The rank differs greatly between the color mode and the single color mode, but this is such a value because of the experimental result with the optimum linear velocity difference for each mode.
In the case of the color mode, it is preferable that the pressing force is weak. Based on this, the pressing force is about 20 N / m. If the pressing force is weak, it can be understood from FIG. 5 that the transfer efficiency is lowered. As a result, sufficient pressure cannot be applied between the photosensitive member 5 and the intermediate transfer belt 11, so that the hollowing out is likely to deteriorate.
On the other hand, in the single color mode, it is preferable that the pressing force is high because transfer efficiency is increased. However, when the pressing force increases, stress concentrates on the toner sandwiched between the photosensitive member 5 and the intermediate transfer belt 11, and a void occurs. By increasing the pressing force within the permissible range, it is possible to achieve both the midsole and the transfer rate.

In summary, the color mode is set so as to reduce the pressing force so as to optimize the transfer efficiency without giving a linear velocity difference so as to satisfy the hollow rank in all colors. In the single color mode, the color photoconductor 5 and the intermediate transfer belt 11 are separated from each other. In other words, the setting is such that the linear velocity difference is provided to significantly improve the void, the pressing force is increased, the transfer efficiency is increased, and the toner consumption is suppressed. Since the margin of the acceptable rank for hollowing has increased, the pressing force can be increased to increase the transfer efficiency.
Therefore, by changing the transfer condition between the color mode and the single color mode, it is possible to set the optimum condition for reverse transfer in the color mode and the optimum condition for the toner yield in the single color mode. An image forming apparatus with a small amount can be realized.

1 is an overall configuration diagram showing an image forming apparatus embodying the present invention, in which a color copying machine is shown as an example. FIG. 3 is an enlarged view of a tandem main part in which image forming parts including a photoconductor are arranged. It is a graph which shows the relationship between the linear velocity ratio in each charging system, and a hollow rank. It is a graph which shows the relationship between a pressing force, the primary transfer rate from a photoconductor, and a reverse shot rate. It is a graph which shows the relationship between pressing force and a hollow rank by image formation mode.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Image forming apparatus main body 2 Paper feed table 3 Scanner 4 Automatic document feeder (ADF)
5 Photoconductor 6 Charging Device 6a Charging Roller 6b Charging Charger 7 Developing Device 8 Cleaning Device 9 Primary Transfer Device 10 Image Forming Unit 11 Intermediate Transfer Member 21 Exposure Device 22 Secondary Transfer Device 24 Secondary Transfer Belt 25 Fixing Device 42 Paper Feed Roller 44 Paper feed cassette 45 Separation roller 46 Side plate 47 Transport roller 48 Paper feed path 49 Registration roller 56 Ejection roller 57 Ejection tray

Claims (9)

A plurality of image forming units each having an image carrier and a charging unit;
A transfer object rotating in contact with the image carrier;
A transfer member that presses the transferred body against the image carrier,
A first transfer mode for performing transfer in a state where all of the plurality of image bearing members and the transfer target body are in contact with each other;
An image forming apparatus having a transfer mode including a second transfer mode in which at least one of the plurality of image carriers is in contact with a transfer target and the remaining image carriers are separated from the transfer target. ,
The plurality of charging means has at least one of a roller charging method and a corona charging method, respectively, and a transfer condition to the image carrier in contact with the transfer target in the second transfer mode is An image forming apparatus, which is different from the first transfer mode. The image forming apparatus according to claim 1, wherein the charging unit of the image forming unit that is in contact with the transfer target in the second transfer mode is a corona charging method. The image forming apparatus is characterized in that a difference is provided between a linear velocity at which the image carrier moves and a linear velocity at which the image transfer member moves at a transfer position where the image carrier transfers to the image transfer member. Item 3. The image forming apparatus according to Item 1 or 2. The image forming apparatus according to claim 1, wherein the transfer condition of the transfer unit is a pressing force of a pressing unit that presses against the image carrier. 5. The image forming apparatus according to claim 1, wherein only one image forming unit is in contact in the second transfer mode in the first half, and the color of the toner is black. 6. In the second transfer mode, the image carrier that is in contact with the transfer object is provided on the most downstream side in the rotation direction of the transfer object. The image forming apparatus described in 1. The image forming apparatus according to claim 1, wherein the charging unit of the image forming unit that forms a color toner image uses a roller charging method. The image forming apparatus according to claim 7, wherein the charging unit is a charging roller using the roller charging method, and the applied voltage superimposes a DC component and an AC component. The image forming apparatus according to claim 7, wherein the charging roller is a non-contact charging method that realizes charging without being in contact with the image carrier.

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