JP2002108117A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus which can prevent the generation of a dust image at the tip part of a toner image on a recording medium. SOLUTION: When transferring a primary toner image on an intermediate transfer belt 121 from a photoreceptor drum 10, a primary transfer bias applied to the tip part of the toner image is switched to a transfer bias higher than the primary transfer bias applied to other portions of the toner image at timing proper for the toner image tip part. Thereby the transfer bias applied only to the tip part of the recording medium where the dust image generates is intensified to prevent the generation of the dust image, and deterioration in the transfer ratio caused by inversion of polarity of toner is prevented because a proper transfer bias is applied to the portions other than the tip part of the recording medium.

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, and a facsimile, and more particularly, to an image forming means for forming a toner image on an image carrier by an electrophotographic method, An intermediate transfer member on which a toner image formed on the image carrier is primarily transferred; a primary transfer unit for primarily transferring the toner image formed on the image carrier onto the intermediate transfer member; The present invention relates to an image forming apparatus including secondary transfer means for secondary-transferring a primary-transferred toner image onto a recording medium.

[0002]

2. Description of the Related Art As this type of image forming apparatus, a toner image is formed on an image carrier by an image forming means including a charging device, an exposure device, a developing device and the like, and the toner formed on the image carrier is formed. After the image is primarily transferred onto the intermediate transfer member by the primary transfer means, the toner image primarily transferred onto the intermediate transfer member is secondarily transferred collectively onto a recording medium such as transfer paper or an OHP sheet. A known electrophotographic image forming apparatus is known (for example, JP-A-2-163779).

In the image forming apparatus, first, a light image of a document image is exposed by the exposure device on the surface of the image carrier uniformly charged to a predetermined potential by the charging device. An electrostatic latent image of the document image is formed on the image carrier. Next, the electrostatic latent image is visualized by supplying toner to the image carrier by the developing device, and a toner image is formed on the image carrier. The toner image formed on the image carrier is primarily transferred onto the intermediate transfer member by the primary transfer unit. And
The toner image primarily transferred onto the intermediate transfer member is collectively and secondarily transferred onto the recording medium by the secondary transfer means. Thereafter, in order to prepare for the next image formation, the residual toner remaining on the image carrier without being primary-transferred onto the intermediate transfer body, and on the intermediate transfer body without being secondary-transferred onto the recording medium The remaining toner is removed by a cleaning unit including a blade and a fur brush.
The toner image secondarily transferred onto the recording medium is fixed on the recording medium in a fixing step.

[0004]

However, in the conventional image forming apparatus of this type, a dust image is liable to occur only at the leading end of the toner image on the recording medium in the fixing step. The reason why such a dust image is generated is that when the recording medium enters the fixing step, after the heating element of the fixing device comes into contact with the recording medium, the adhesion between the heating element and the recording medium becomes stable. It is considered that there is a gap between the heating element and the recording medium, which corresponds to the discharge distance of the Pachet, and a minute discharge is generated.

As a method of preventing the generation of a dust image at the leading end of a toner image on a recording medium, the adhesion between the intermediate transfer member or the heating member and the recording medium is increased to generate the minute discharge. Can be considered.
However, although it is relatively easy to increase the adhesion between the intermediate transfer body or the heating element and the central portion of the recording medium, the adhesion between the intermediate transfer body or the heating element and the tip of the recording medium is improved. Therefore, it has been difficult to prevent the generation of a dust image at the leading end of the recording medium. This is because the tip of the recording medium advances while slightly vibrating due to air resistance, and the vibration of the tip of the recording medium is delayed until immediately after entering the intermediate transfer body or the heating body, or It is considered that the leading edge of the recording medium starts to vibrate due to a shock at the time of entering the intermediate transfer member or the heating member.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and an object of the present invention is to prevent generation of a dust image at a leading end portion of a toner image on a recording medium by a method different from the above method. To provide an image forming apparatus capable of performing the above.

[0007]

In order to achieve the above-mentioned object, a first aspect of the present invention is to provide a method for performing primary transfer of a toner image formed on an image carrier from an image carrier to an intermediate transfer body. A timing corresponding to a tip of the toner image when the toner image is primarily transferred from the image carrier to the intermediate transfer member in an image forming apparatus configured to perform secondary transfer from the intermediate transfer member to a recording medium. Thus, the primary transfer bias applied to the leading end portion of the toner image is switched to a transfer bias higher than the primary transfer bias applied to the other portion of the toner image.

The occurrence of a dust image at the leading end of the toner image on the recording medium is reduced as the primary transfer bias when the toner image is primarily transferred from the image carrier to the intermediate transfer member is increased. Is done. This is a primary transfer step in which a toner image is primary-transferred from the image carrier onto an intermediate transfer body. In the primary transfer step, charge injection of the toner image into the toner occurs, and the charge amount of the toner is stabilized. It is considered that a stable amount of toner is maintained even after the secondary transfer to the recording medium, the electrostatic force between the recording medium and the toner is increased, and the toner is less likely to be scattered even by the minute discharge. However, if the primary transfer bias is increased until the dust image is at an acceptable level or completely disappears, an over-transfer state exceeding an appropriate transfer bias occurs, the charge injection into the toner becomes too large, and the transfer rate to the recording medium is reduced. The problem of lowering occurs. Therefore, in the image forming apparatus of the present invention, when the toner image is primarily transferred from the image carrier to the intermediate transfer member, the toner image is transferred to the leading end of the toner image at a timing corresponding to the leading end of the toner image. The applied primary transfer bias is switched to a transfer bias higher than the primary transfer bias applied to other portions of the toner image. As a result, the transfer bias only at the leading end of the recording medium where the dust image is generated is increased, and the occurrence of the dust image is prevented, and an appropriate transfer bias is applied to portions other than the leading end of the recording medium. Thus, a decrease in the transfer rate due to the polarity reversal of the toner is prevented.

According to a second aspect of the present invention, a plurality of color toner images formed on an image bearing member are sequentially superimposed on the intermediate transfer member from the image carrier and primary-transferred, and then recorded from the intermediate transfer member. A color mode in which secondary transfer is collectively performed on a medium, and a specific one-color toner image of the plurality of color toner images is primarily transferred to the intermediate transfer body, and then primary-transferred to the intermediate transfer body. The image forming apparatus has a one-color mode in which the transferred toner image is immediately transferred to a recording medium in a one-color mode without giving a primary transfer effect at the time of the next rotation of the intermediate transfer member. When the toner image is primarily transferred onto the intermediate transfer member, a primary transfer bias applied to the leading end of the toner image is applied to another portion of the toner image at a timing corresponding to the leading end of the toner image. To be higher than the primary transfer bias, and The primary transfer bias applied to the leading end of the toner image in the color mode is switched to a lower transfer bias than the primary transfer bias applied to the leading end of the toner image in the one-color mode. .

In the case of an image forming apparatus for forming a color image of two or more colors, the primary transfer is performed first except for the final color toner image which is finally transferred first on the intermediate transfer member. The toner image undergoes the primary transfer action a plurality of times. Therefore, with respect to the toner image previously primary-transferred onto the intermediate transfer member, the same effect as when the primary transfer bias is strengthened is obtained, and the dust image hardly occurs at the leading end of the toner image. . Conversely, the toner image that has been primary-transferred first has less room for overtransfer because it is subjected to the primary transfer action a plurality of times. More specifically, when the four color toner images are sequentially superimposed and primary transferred as in the color mode, the primary transfer bias applied to the leading end of each color toner image is reduced by 20% from the appropriate bias. In other words, the leading end portion of the toner image firstly transferred first receives the action of the primary transfer bias higher than usual for all four times, so that the image is considerably damaged by overtransfer. In contrast, the above 1
In the case of a single-color toner image in which color superposition is not performed as in the color mode, even if the primary transfer bias applied to the leading end of the toner image is twice the proper bias, the image due to the over- Little damage is seen. As described above, when primary transfer is performed by sequentially superposing the four color toner images,
Although only the front end portion of each color toner image is used, 1
Since the excessive transfer state is caused by increasing the secondary transfer bias, it is desired to prevent the occurrence of both the dust image and the excessive transfer state. Therefore, in this image forming apparatus, the primary transfer bias applied to the leading end of the toner image in the color mode is lower than the primary transfer bias applied to the leading end of the toner image in the one-color mode. Switch to bias. That is, the color mode and the 1
The amount by which the transfer bias applied to the leading end portion of the toner image is increased differs between the color mode. Thereby, in the color mode, the amount of increasing the transfer bias applied to the leading end portion of the toner image of each color is suppressed to a small amount.
The image damage due to the overtransfer is reduced, and the leading end portion of the toner image of each color is subjected to the primary transfer action a plurality of times, so that the generation of the dust image at the leading end portion of the toner image is also reduced. On the other hand, in the one-color mode, the transfer bias applied to the leading end portion of the toner image is increased, but the image damage due to the overtransfer is small, and the dust image is prevented.

According to a third aspect of the present invention, a plurality of color toner images formed on an image bearing member are sequentially superimposed on the intermediate transfer member from the image bearing member and primary-transferred, and then recorded from the intermediate transfer member. In an image forming apparatus configured to perform secondary transfer collectively on a medium, when the toner images of the respective colors are primarily transferred from the image bearing member onto the intermediate transfer member, at a timing corresponding to the leading end of the toner image of each color. , Applied to the tip of the toner image
Applying the next transfer bias to the other part of the toner image 1
The transfer bias is set to be higher than the next transfer bias, and the transfer bias is switched to a different transfer bias for each toner image in accordance with the toner image of each color.

In the case of an image forming apparatus for forming a color image of two or more colors, the degree of occurrence of the dust image differs for each color toner image. Therefore, in this image forming apparatus, the primary transfer bias applied to the leading end of the toner image is switched to a different transfer bias for each toner image according to the toner image of each color. Accordingly, for example, by increasing the transfer electric field for a toner image of a color in which a dust image is more likely to be generated, the occurrence of the dust image can be efficiently reduced. In addition, in the case of a toner image of a color in which the dust image is unlikely to be generated, the primary transfer bias is not set so large that the image damage due to the overtransfer can be reduced.

According to a fourth aspect of the present invention, in the image forming apparatus according to the third aspect, a primary transfer bias applied to a leading end portion of the toner image of each color is applied to a color which is first primary-transferred to the intermediate transfer member. The smaller the toner image, the smaller the toner image.

In the case of an image forming apparatus for forming a color image of two or more colors, the number of primary transfer operations increases as the color of the toner in the primary transfer order becomes earlier, and the overtransfer state may occur. Is high. Therefore, in this image forming apparatus, the primary transfer bias applied to the leading end portion of the toner image of each color is reduced as the toner image of the color that is primarily transferred to the intermediate transfer member earlier. As a result, the primary transfer bias applied to the leading end of the toner image of each color becomes smaller for the toner image of the first color and becomes larger for the toner image of the later color in the primary transfer order, so that the above-mentioned dust image is efficiently formed. In addition, image damage due to the overtransfer is reduced.

According to a fifth aspect of the present invention, a plurality of color toner images formed on an image bearing member are sequentially superimposed on the intermediate transfer member from the image carrier and primary-transferred, and then recorded from the intermediate transfer member. A color mode in which secondary transfer is collectively performed on a medium, and a specific one-color toner image of the plurality of color toner images is primarily transferred to the intermediate transfer body, and then primary-transferred to the intermediate transfer body. The image forming apparatus has a one-color mode in which the transferred toner image is immediately and secondarily transferred to a recording medium without giving a primary transfer effect at the time of the next rotation of the intermediate transfer member. The primary transfer bias applied to the front end portion of the toner image is switched to a transfer bias higher than the primary transfer bias applied to other portions at a timing corresponding to the front end of the toner image. .

In the color mode, the toner image to which the proper transfer bias is applied two or more times does not generate the dust image, and the toner image of the final color to which the transfer bias is applied only once is not dust. Images may not be generated. Conversely, in the color mode, if the primary transfer bias is increased until the dust image is at an allowable level or completely disappears, an over-transfer state exceeding an appropriate transfer bias occurs, the polarity of the toner is inverted, and the transfer rate is reduced. It may decrease. Therefore, in this image forming apparatus, only in the one-color mode, the primary transfer bias applied to the front end portion of the toner image at the timing corresponding to the front end of the toner image is changed to the primary transfer bias applied to the other portion. The transfer bias is switched to a higher transfer bias. This gives 1
Even if the secondary transfer bias is increased, only the primary transfer bias applied to the leading end portion of the one-color mode toner image with less damage to the image due to the overtransfer is increased, and the overtransfer in the color mode is reduced. The occurrence is prevented.

According to a sixth aspect of the present invention, a plurality of color toner images formed on an image bearing member are sequentially superimposed on the intermediate transfer member from the image carrier and primary-transferred, and then recorded from the intermediate transfer member. In an image forming apparatus configured to perform secondary transfer collectively on a medium, a primary image that is applied to a leading end portion of the toner image at a timing corresponding to the leading end of the toner image of the selected toner image among the plurality of color toner images. The transfer bias is switched to a transfer bias higher than the primary transfer bias applied to other portions.

In this image forming apparatus, the primary transfer bias applied to the leading end portion of the toner image at the timing corresponding to the leading end of the toner image of the selected toner image among the plurality of color toner images is changed. The transfer bias is switched to a transfer bias higher than the primary transfer bias applied to the portion. As a result, the primary transfer bias applied to the leading end of the toner image is increased only when the toner image of the color in which the dust image is likely to be generated is primarily transferred onto the intermediate transfer body.
Over-transfer of the toner images is reduced as compared with the case where the primary transfer bias applied to the leading end portions of all the toner images is increased. For example, if the above-mentioned dust image can be prevented when an appropriate transfer bias is applied two or more times, only the primary transfer bias applied to the leading end portion of the final color toner image to which the transfer bias is applied only once is increased. Thus, over-transfer of the toner image is reduced as compared with a case where the primary transfer bias applied to the leading end portions of all the toner images is increased.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment in which the present invention is applied to a full-color electrophotographic copying machine (hereinafter simply referred to as "copier") for forming an image using an intermediate transfer member will be described. FIG. 1 is a schematic configuration diagram of a printer unit of the copying machine. In FIG. 1, an image writing device 13 composed of an optical unit, a photoconductor cleaning device 111 as a cleaning device for the photoconductor drum 10, a charging charger 12 as a charging device, and the like are provided around a photoconductor drum 10 as an image carrier. A revolver developing unit 110 including a rotary developing device as a developing unit, an intermediate transfer unit 120 as an intermediate transfer unit, and the like are provided.
The printer unit of the full-color electrophotographic copying machine has a secondary transfer unit 130 as a secondary transfer unit, a fixing unit 145 as a fixing unit, and a paper supply unit as a paper supply unit for supplying a transfer material as a recording medium. A paper device, a control device as control means for controlling the operation of each of the above devices and units, and the like are also provided.

The photoconductor cleaning device 111 includes a fur brush 111 a and a photoconductor cleaning blade 11.
1b, the recorded image and the pattern image formed on the photosensitive drum 10 are transferred to the intermediate transfer unit 12
After the primary transfer to the No. 0 intermediate transfer belt 121, the residual toner of the recorded image and the pattern image remaining on the surface of the photosensitive drum 10 are removed. Fur brush 111
In a, a solid lubricant 111c as a lubricant made of zinc stearate or the like is provided so as to contact the brush tip. The fixing unit 145 includes a heating roller 145a, a pressure roller 145b, a pair of paper discharge rollers 145c, and the like.

The revolver developing unit 110 includes a Bk developing device 115 for supplying black toner, a C developing device 116 for supplying cyan toner, an M developing device 117 for supplying magenta toner, and a Y developing device for supplying yellow toner. Each of the developing units is selectively positioned at a developing position facing the photosensitive drum 10 sequentially by the rotation of the revolver developing unit 110, and the toner of each color is transferred to the photosensitive drum. 10 sequentially.

The intermediate transfer unit 120 includes an intermediate transfer belt 121 serving as an intermediate transfer member, a primary transfer bias roller 122 serving as a charge applying unit, an earth roller 123 serving as a pre-primary transfer charge removing unit, and a tension roller 124. It is configured to be stretched by a secondary transfer opposing roller 126 and a driving roller 127 as belt driving means. Here, each roller is made of a conductive material, and each roller other than the primary transfer bias roller 122 is grounded. The primary transfer bias roller 122 is supplied with a constant current or a predetermined primary transfer bias controlled at a constant current from a primary transfer power source (not shown).

Around the intermediate transfer belt 121, a secondary transfer unit 130 as secondary transfer means for secondary transferring a recording image primarily transferred on the intermediate transfer belt 121 to a transfer material is provided. Intermediate transfer belt cleaning device as cleaning means for removing the residual toner of the recorded image and the pattern image remaining on the intermediate transfer belt 121 after the recorded image is secondarily transferred to the transfer material from the intermediate transfer belt 121 129 are provided.
The intermediate transfer belt cleaning device 129 includes a lubricant application roller 129a, an intermediate transfer belt cleaning blade 129b, and a solid lubricant 129c that contacts the lubricant application roller 129a and applies a lubricant made of zinc stearate or the like. It has. Further, while the recorded image is primarily transferred onto the intermediate transfer belt 121, the fur brush 129a and the intermediate transfer belt cleaning blade 129b are separated from the surface of the intermediate transfer belt 121 to transfer the recorded image. After the secondary transfer to the intermediate transfer belt, the fur brush 129a and the intermediate transfer belt cleaning blade 129b are
The device 1 is configured to be freely contactable and detachable by a contacting / separating mechanism (not shown) so as to contact the surface of the device 1.

The secondary transfer unit 130 includes a secondary transfer roller 131 as a secondary transfer member, a secondary transfer roller cleaning blade 132 for removing toner attached to the secondary transfer roller 131, and the like. Yes,
The secondary transfer roller 131 is configured to contact and separate from the intermediate transfer belt 121 by a contact / separation mechanism (not shown).

Next, the image forming operation of the copying machine will be described. In FIG. 1, the photosensitive drum 10 is driven to rotate counterclockwise, and the surface thereof is uniformly charged to a predetermined potential by the charging charger 12. Also,
The intermediate transfer belt 121 is driven clockwise at substantially the same speed as the photosensitive drum 10. Next, the image writing device 13 exposes the surface of the photoreceptor drum 10 with the recording image light of the color image information of the document read by a scanner unit (not shown) and the image light of the pattern image. As a result, on the photosensitive drum 10, the recorded images of four colors separated and the electrostatic latent images of the pattern images for each color corresponding to the recorded images of each color are sequentially formed at a predetermined timing. . The electrostatic latent images of the recorded images and the pattern images of the respective colors are sequentially formed by the toners of the respective colors supplied from the developing units 115, 116, 117, and 118 of the revolver developing unit 110 for each color corresponding to the toner colors. Developed.

Here, in the recorded image, the fluctuation of the density is corrected by the pattern image. FIG. 6 shows an example of a means for correcting the density fluctuation of the recorded image by using this pattern image. In FIG. 6, the surface of the photoreceptor 1 as the image carrier is uniformly charged to a predetermined potential by the charging roller 2 as the charging device. Next, the surface of the photoconductor 1 outside the recording image forming area for forming the recording image is exposed to image light of a reference density pattern by the image writing device 3 as the exposure device, and the electrostatic latent image of the reference density pattern is exposed. Form an image. Thereafter, the developer 5 is supplied to the photoconductor 1 by the developing roller 4 of the developing device, and a pattern image 6 composed of a toner image is formed on the photoconductor 1. Then, the reflection density of the pattern image 6 is detected by a reflection type photosensor (hereinafter, referred to as “P sensor”) 9 including a light emitting element 7 and a student element 8, and based on the output value of the P sensor 9. Thus, the toner concentration of the developer 5 in the developing device is controlled.

The recorded image and the pattern image of each color developed on the photosensitive drum 10 in this manner are applied to the surface of the intermediate transfer belt 121 charged to a predetermined surface potential by the primary transfer bias roller 122, for a predetermined time. The primary transfer is performed while being sequentially superimposed at the timing shown in FIG. On the other hand, during this time, the transfer material is fed from the sheet feeding device. The transfer material is transferred to the intermediate transfer belt 121 and the secondary transfer roller 13 by a pair of transport rollers 151.
The sheet is conveyed toward the secondary transfer nip portion with No. 1 and temporarily stopped with the pair of registration rollers 152 holding the leading ends thereof.

Then, at the timing when the leading end of the image forming region of the transfer material and the leading end of the color recording image superimposed on the surface of the intermediate transfer belt 121 and primary-transferred coincide with each other, the transfer material is transferred by the registration roller 152. Is fed again. As a result, the transfer material is nipped and conveyed by the secondary transfer nip formed by the contact of the secondary transfer roller 131 with the intermediate transfer belt 121. A predetermined secondary transfer bias is applied to the secondary transfer roller 131 from a secondary transfer bias power source (not shown) while the transfer material is nipped and conveyed by the secondary transfer nip. Only the above-mentioned color recorded image is collectively secondarily transferred onto the material. Further, when the leading end of the transfer material on which only the color recording image is collectively secondary-transferred as described above passes through the secondary transfer nip portion, the transfer material is disposed downstream of the secondary transfer roller 131 in the transfer material transport direction. Separation charge is applied to the transfer material on which the color recording image has been transferred by the separation charger 161 serving as a separation unit, and the transfer material is transferred to the intermediate transfer belt 121.
The transfer material is separated from the intermediate transfer belt 121 by removing the charge electrostatically attracted to the intermediate transfer belt 121.

The transfer material separated from the intermediate transfer belt 121 in this manner is directed to the fixing unit 145 by the transfer material transport belt 171 disposed between the separation charger 161 and the fixing unit 145. Transported. The transfer material transport belt 171 is stretched around a drive roller 172 and a driven roller 173, and is moved and rotated counterclockwise in FIG. Further, around the transfer material transport belt 171, the transfer material transport belt 1
A charging brush 174 serving as a charge applying means for applying a charge for adsorbing the transfer material on the surface of the transfer material 71, and the leading end of the transfer material transported by the transfer material transport belt 171 arrives before the fixing unit 145. In this state, the separation-charger 175 that weakens the charge of the transfer material transport belt 171 to separate the transfer material from the surface of the transfer material transport belt 171, and removes the charge applied to the surface of the transfer material transport belt 171. A static elimination charger 176 is provided. Then, the transfer material separated from the surface of the transfer material transport belt 171 is fixed by the fixing unit 145 to the color recording image which is collectively and secondarily transferred to its image forming surface (the upper surface in FIG. 1). The discharge roller 145c
Is discharged onto a discharge tray (not shown) provided outside the copying machine. The separation [static elimination] means is turned on immediately before the recording medium faces the separation means, before a margin time for starting operation, and turned off immediately after passing through the recording medium. It is timing.

The features of the present invention will be described below.
2 to 6 show an image writing signal, a primary transfer bias,
5 shows a timing chart of the next transfer bias at the time of location match. Here, in the case of time coincidence, the timing is shifted by the layout distance and process speed of each step. In other words, in each of the timing charts, the horizontal axis does not indicate the exact time, and in order to make the description easy to understand, each signal is turned on and off based on the color recording image formed on the intermediate transfer belt 121. The timing of turning off is shown, and when the horizontal axis is time, the timing of turning on / off each signal is shifted according to the layout position and operation timing of each member operated by the signal. The magnitudes of the primary transfer bias and the secondary transfer bias are shown along the vertical axis. Further, in both the conventional example and the embodiment, the primary transfer bias is turned on / off for each color toner image. However, even if the primary transfer bias is applied between the toner images of each color. Well,
Conventionally, the magnitude of the primary transfer bias in this portion is larger or smaller than the bias of the toner image,
Alternatively, there is the same, but any suitable bias may be used.

FIG. 6 shows a timing chart of a conventional copying machine. In this conventional copying machine, the primary transfer bias applied to the toner image of each color is always the same. In this case, as described above, in the fixing step, a dust image was likely to occur only at the leading end of the toner image on the recording medium.

Therefore, in the copying machine according to the present embodiment, as shown in FIG. 2, when the toner image is primarily transferred from the photosensitive drum 10 onto the intermediate transfer belt 121, the leading end of the toner image is The primary transfer bias applied to the leading end of the toner image is switched to a transfer bias higher than the primary transfer bias applied to the other portions of the toner image at a timing corresponding to. As a result, the transfer bias only at the leading end of the recording medium where the dust image is generated is increased, and the occurrence of the dust image is prevented, and an appropriate transfer bias is applied to portions other than the leading end of the recording medium. Thus, a decrease in the transfer rate due to the polarity reversal of the toner is prevented.

FIG. 3 shows a timing chart of another copying machine according to this embodiment. In the case of a copying machine that forms a color image of two or more colors, the degree of occurrence of the dust image differs for each color toner image. Therefore, in this copying machine, the primary transfer bias applied to the leading end of the toner image is switched to a different transfer bias for each toner image according to the toner image of each color. This allows, for example,
By increasing the transfer electric field for a toner image of a color in which a dust image is more likely to occur, the occurrence of the dust image can be reduced more efficiently. In addition, in the case of a toner image of a color in which the dust image is unlikely to be generated, the primary transfer bias is not set so large that the image damage due to the overtransfer can be reduced.

For example, the number of times of the primary transfer operation increases as the color of the toner in the primary transfer order comes first, and the possibility of the overtransfer state is high. Therefore, in the timing chart shown in FIG. 3, the primary transfer bias applied to the leading end of the toner image of each color is set smaller for the toner image of the color that is primarily transferred to the intermediate transfer belt 121 first. As a result, the primary transfer bias applied to the leading end of the toner image of each color becomes smaller for the toner image of the first color and becomes larger for the toner image of the later color in the primary transfer order, so that the above-mentioned dust image is efficiently formed. In addition, image damage due to the overtransfer is reduced. In this example, the one-color mode (single-color mode) does not matter.

FIG. 4 shows a timing chart of still another copying machine according to this embodiment. In the color mode in which the toner images of the respective colors are superimposed and primary transferred sequentially, the toner image to which the proper transfer bias is applied two or more times does not generate the dust image, and the transfer bias is applied only once. In some cases, a dust image does not occur even in the final color toner image that is not processed. Conversely, in the color mode, if the primary transfer bias is increased until the dust image is at an allowable level or completely disappears, an over-transfer state exceeding an appropriate transfer bias occurs, the polarity of the toner is inverted, and the transfer rate is reduced. It may decrease.

Therefore, in this copying machine, only in the one-color mode, the primary transfer bias applied to the leading end of the toner image at the timing corresponding to the leading end of the toner image causes the primary transfer bias to be applied to the other portions. The transfer bias is switched to a transfer bias higher than the transfer bias. As a result, even when the primary transfer bias is increased, only the primary transfer bias applied to the leading end portion of the toner image in the one-color mode in which the image is less damaged due to the overtransfer is increased. The occurrence of the overtransfer is prevented.

FIG. 5 shows a timing chart of still another copying machine according to this embodiment. In this copying machine, the primary transfer bias applied to the leading end portion of the toner image is applied to the other portion at the timing corresponding to the leading end of the toner image of the selected toner image among the plurality of color toner images. The transfer bias is switched to a transfer bias higher than the primary transfer bias. As a result, the primary transfer bias applied to the leading end of the toner image is increased only when the toner image of the color in which the dust image is likely to be generated is primarily transferred onto the intermediate transfer member. Over-transfer of the toner image is reduced as compared with the case where the primary transfer bias applied to the leading end of the toner image is increased.

For example, if the above-mentioned dust image can be prevented when an appropriate transfer bias is applied two or more times, the final color toner image to which the transfer bias is applied only once as shown in the timing chart of FIG. By increasing only the primary transfer bias applied to the leading end of the toner image, excessive transfer of the toner image is reduced as compared with the case where the primary transfer bias applied to the leading end of all toner images is increased. In addition,
In this example, the one-color mode (single-color mode) does not matter.

[0039]

According to the first to sixth aspects of the present invention, it is possible to prevent a dust image on the leading end portion of the recording medium, and to minimize a reduction in the transfer rate due to overtransfer, which is a side effect.
Alternatively, there is an excellent effect that it can be eliminated.

In particular, according to the second aspect of the present invention, in the color mode, the amount of increase in the transfer bias applied to the leading end of the toner image of each color is kept small, so that the image damage due to the overtransfer is reduced. In addition, since the leading end of the toner image of each color is subjected to the primary transfer operation a plurality of times, the occurrence of a dust image at the leading end of the toner image is also reduced. On the other hand, in the one-color mode, the transfer bias applied to the leading end portion of the toner image is increased. However, there is little damage to the image due to the overtransfer, and there is an excellent effect that the dust image is prevented.

According to the third aspect of the present invention, for example,
By increasing the transfer electric field for a toner image of a color in which a dust image is more likely to occur, the occurrence of the dust image can be reduced more efficiently. In addition, in the case of a toner image of a color in which the dust image is unlikely to occur, the primary transfer bias is not increased so much that the image damage due to the overtransfer can be reduced. There is.

According to the fourth aspect of the present invention, the primary transfer bias applied to the leading end of the toner image of each color is one.
The next transfer order is smaller for the toner image of the first color and larger for the toner image of the second color, so that there is an excellent effect that the dust image is efficiently reduced and the image damage due to the overtransfer is small. .

According to the fifth aspect of the present invention, even if the primary transfer bias is increased, the toner image in the one-color mode in which the image is less damaged by the overtransfer is applied to the leading end.
Only the next transfer bias can be increased, and there is an excellent effect that the occurrence of the overtransfer in the color mode is prevented.

According to the sixth aspect of the present invention, if the dust image can be prevented when the proper transfer bias is applied two or more times, the final color toner image to which the transfer bias is applied only once is applied. By increasing only the primary transfer bias applied to the front end portion, an excellent overtransfer of the toner image is reduced as compared with the case where the primary transfer bias applied to the front end portion of all toner images is increased. effective.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a printer unit of a full-color electrophotographic copying machine as an image forming apparatus according to an embodiment of the present invention.

FIG. 2 is a time chart showing on / off timings of main signals of the copying machine according to the embodiment.

FIG. 3 is a time chart showing on / off timings of main signals of another copying machine according to the embodiment.

FIG. 4 is a time chart showing on / off timings of main signals of still another copying machine according to the embodiment.

FIG. 5 is a time chart showing on / off timings of main signals of still another copying machine according to the embodiment.

FIG. 6 is a timing chart showing on / off timings of main signals in a conventional copying machine.

[Explanation of symbols]

 REFERENCE SIGNS LIST 10 photoconductor drum 12 transfer charger 13 image writing device 110 revolver developing unit 111 photoconductor cleaning device 120 intermediate transfer unit 121 intermediate transfer belt 122 primary transfer bias roller 123 ground roller 124 tension roller 126 secondary transfer facing roller 127 drive roller 129 intermediate Transfer belt cleaning device 130 Secondary transfer unit 131 Secondary transfer roller 132 Secondary transfer roller cleaning blade 145 Fixing unit 152 Registration roller 161 Separate charging charger

Claims (6)

[Claims] An image forming apparatus having a structure in which a toner image formed on an image carrier is primarily transferred from the image carrier to an intermediate transfer member, and then secondary-transferred from the intermediate transfer member to a recording medium. Transferring the toner image from the image carrier onto the intermediate transfer member by one
At the time of the next transfer, the primary transfer bias applied to the leading end portion of the toner image at a timing corresponding to the leading end of the toner image is higher than the primary transfer bias applied to the other portions of the toner image. An image forming apparatus characterized by switching to (1). 2. The method according to claim 1, wherein a plurality of color toner images formed on the image bearing member are sequentially superimposed on the intermediate transfer member from the image carrier and primary-transferred, and then collectively transferred from the intermediate transfer member to a recording medium. A color mode in which secondary transfer is performed, and after a specific one color toner image of the plurality of color toner images is primarily transferred to the intermediate transfer body, the toner image primarily transferred to the intermediate transfer body is An image forming apparatus having a one-color mode in which a secondary transfer is immediately performed on a recording medium without giving a primary transfer effect at the time of the next rotation of the intermediate transfer body. The above toner image is
At the time of the next transfer, at a timing corresponding to the front end of the toner image, the primary transfer bias applied to the front end portion of the toner image is set higher than the primary transfer bias applied to the other portions of the toner image, In addition, the primary transfer bias applied to the leading end of the toner image in the color mode is switched to a transfer bias lower than the primary transfer bias applied to the leading end of the toner image in the one-color mode. Image forming apparatus. 3. The method according to claim 1, wherein the toner images of a plurality of colors formed on the image bearing member are sequentially superimposed on the intermediate transfer member from the image carrier and primary-transferred, and then collectively transferred from the intermediate transfer member to a recording medium. In the image forming apparatus configured to perform secondary transfer, when the toner images of the respective colors are primarily transferred from the image carrier onto the intermediate transfer member, the toner images of the respective colors are formed at timings corresponding to the leading ends of the toner images of the respective colors. The primary transfer bias applied to the front end portion is made higher than the primary transfer bias applied to the other portions of the toner image, and is switched to a different transfer bias for each toner image in accordance with the toner image of each color. An image forming apparatus comprising: 4. The image forming apparatus according to claim 3, wherein a primary transfer bias applied to a leading end portion of the toner image of each color is smaller for a toner image of a color that is primarily transferred to the intermediate transfer member earlier. An image forming apparatus. 5. A multi-color toner image formed on an image bearing member is superimposed on an intermediate transfer member from the image bearing member and primary-transferred, and then collectively transferred from the intermediate transfer member to a recording medium. A color mode in which secondary transfer is performed, and after a specific one color toner image of the plurality of color toner images is primarily transferred to the intermediate transfer body, the toner image primarily transferred to the intermediate transfer body is In an image forming apparatus having a one-color mode in which a secondary transfer is immediately performed to a recording medium without giving a primary transfer effect at the time of the next rotation of the intermediate transfer body, the image forming apparatus is provided with An image forming apparatus, wherein at a corresponding timing, a primary transfer bias applied to a leading end portion of the toner image is switched to a transfer bias higher than a primary transfer bias applied to other portions. 6. A multi-color toner image formed on an image carrier is superimposed on an intermediate transfer member from the image carrier in a primary transfer, and then collectively transferred from the intermediate transfer member to a recording medium. In the image forming apparatus configured to perform secondary transfer, a primary transfer bias applied to a leading end portion of the toner image at a timing corresponding to a leading end of the toner image of the selected toner image among the toner images of the plurality of colors is changed. Wherein the transfer bias is switched to a transfer bias higher than the primary transfer bias applied to the portion.