JP4143252B2 - Image forming apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image forming apparatus such as a copying machine, a printer, or a facsimile, and more specifically, an image forming unit that forms a toner image on an image holding member by an electrophotographic method, and a toner formed on the image bearing member. An intermediate transfer member on which an image is primarily transferred; a primary transfer means for primarily transferring a toner image formed on the image bearing member onto the intermediate transfer member; and a toner image primarily transferred onto the intermediate transfer member. The present invention relates to an image forming apparatus including a secondary transfer unit that performs secondary transfer on a recording medium.
[0002]
[Prior art]
As an image forming apparatus of this type, a toner image is formed on an image holding member by an image forming unit comprising a charging device, an exposure device, a developing device, etc., and the toner image formed on the image bearing member is a primary transfer unit. An electrophotographic system configured to perform a primary transfer onto an intermediate transfer body by the transfer and then secondary transfer the toner image primarily transferred onto the intermediate transfer body onto a recording medium such as transfer paper or an OHP sheet. Is known (for example, Japanese Patent Laid-Open No. 2-163779).
[0003]
In the image forming apparatus, first, an optical image of a document image is exposed by the exposure device onto the surface of the image carrier uniformly charged to a predetermined potential by the charging device, whereby the image holding device is exposed. An electrostatic latent image of the original image is formed on the body. Subsequently, 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. Then, the toner image primarily transferred onto the intermediate transfer body is secondarily transferred collectively onto the recording medium by the secondary transfer unit. Thereafter, in preparation for the next image formation, the residual toner remaining on the image carrier without being primary transferred onto the intermediate transfer member, and on the intermediate transfer member without being secondarily transferred onto the recording medium. The remaining residual toner is removed by a cleaning means such as a blade or a fur brush. The toner image secondarily transferred onto the recording medium is fixed on the recording medium in a fixing step.
[0004]
[Problems to be solved by the invention]
However, in this type of conventional image forming apparatus, it is easy for a dust image to be generated only at the front end portion of the toner image on the recording medium in the fixing step. The cause of the occurrence of such a dust image is that, after the recording medium enters the fixing step, after the heating body of the fixing device and the recording medium come into contact, the adhesiveness between the heating body and the recording medium is stabilized. This is probably because a gap corresponding to the discharge distance of the Pasche was formed between the heating body and the recording medium, and a slight discharge was generated.
[0005]
As a method for preventing the occurrence of a dust image at the leading end of the toner image on the recording medium, the adhesion between the intermediate transfer member or the heating member and the recording medium is improved to eliminate the occurrence of the minute discharge. A method is conceivable.
However, it is relatively easy to increase the adhesion between the intermediate transfer member or the heating member and the central portion of the recording medium, but the adhesion between the intermediate transfer member or the heating member and the leading end of the recording medium is improved. Thus, it has been difficult to prevent the occurrence of a dust image at the leading edge of the recording medium. This is because the leading edge of the recording medium proceeds with slight vibration due to air resistance, and the vibration of the leading edge of the recording medium is pulled back until just after entering the intermediate transfer member or the heating member, or This is presumably because the leading edge of the recording medium starts to vibrate due to a shock when entering the intermediate transfer member or the heating member.
[0006]
The present invention has been made in view of the above-described problems, and an object of the present invention is to provide an image that can prevent the generation of a dust image at the front end portion of a toner image on a recording medium by a method different from the above method. A forming apparatus is provided.
[0007]
[Means for Solving the Problems]
In order to achieve the above-mentioned object, the invention according to claim 1 is characterized in that a toner image formed on an image carrier is primarily transferred from the image carrier to an intermediate transfer member and then transferred from the intermediate transfer member to a recording medium. Configuration for secondary transfer Because , Fixing means for fixing the toner image to the recording medium by bringing the image forming surface of the recording medium onto which the toner image has been transferred into contact with a heating member. In the image forming apparatus, when the toner image is primarily transferred from the image carrier onto the intermediate transfer member, a 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 switched to a transfer bias higher than the primary transfer bias applied to the other part of the toner image, and the primary transfer bias applied to the other part of the toner image is applied to the tip part of the toner image. The transfer bias is switched to a lower transfer bias than the next transfer bias.
[0008]
The generation of a dust image at the front end of the toner image on the recording medium is reduced as the primary transfer bias is increased when the toner image is primarily transferred from the image carrier onto the intermediate transfer member. This is a primary transfer process in which a toner image is primarily transferred from the image bearing member onto an intermediate transfer member, and charge injection of the toner image into the toner occurs to stabilize the charge amount of the toner. It is considered that a stable amount of toner is maintained after the secondary transfer to the recording medium, the electrostatic force between the recording medium and the toner becomes strong, 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 a permissible level or not at all, an overtransfer state exceeding the appropriate transfer bias occurs, and charge injection into the toner becomes too large, resulting in a transfer rate to the recording medium. The malfunction that it falls is generated. Therefore, in the image forming apparatus of the present invention, when the toner image is primarily transferred from the image carrier onto the intermediate transfer member, the toner image is applied to the leading edge of the toner image at a timing corresponding to the leading edge of the toner image. The primary transfer bias to be applied is switched to a transfer bias higher than the primary transfer bias applied to the other part of the toner image. As a result, the transfer bias of only the leading end portion of the recording medium on which the above-described dust image is generated is strengthened to prevent the occurrence of the dust image, and an appropriate transfer bias is applied to the portion other than the leading end portion of the recording medium. Thus, a decrease in transfer rate due to toner polarity reversal is prevented.
[0009]
According to the second aspect of the present invention, the toner images of a plurality of colors formed on the image holder are sequentially transferred from the image carrier to the intermediate transfer member, and then transferred to the recording medium. A secondary transfer color mode, and a toner image of a specific one of the plurality of color toner images that has been primarily transferred to the intermediate transfer member and then primarily transferred to the intermediate transfer member. And a one-color mode in which the intermediate transfer member is immediately secondarily transferred to a recording medium without giving a primary transfer action during the next rotation of the intermediate transfer member. Because , Fixing means for fixing the toner image to the recording medium by bringing the image forming surface of the recording medium onto which the toner image has been transferred into contact with a heating member. In the image forming apparatus, when the toner image is primarily transferred from the image carrier onto the intermediate transfer member, a 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 part of the toner image, and the primary transfer bias applied to the other part of the toner image is applied to the tip part of the toner image. The primary transfer bias is applied to the front end portion of the toner image in the one-color mode, and the transfer bias is switched so as to be lower than that, and the primary transfer bias applied to the front end portion of the toner image in the color mode is applied. It is characterized by switching to a lower transfer bias.
[0010]
In the case of an image forming apparatus that forms a color image of two or more colors, the toner image that has been primarily transferred first is excluded except for the final color toner image that is finally primarily transferred onto the intermediate transfer member. It undergoes primary transfer action multiple times. Accordingly, the toner image that has been primarily transferred onto the intermediate transfer member has the same effect as that of increasing the primary transfer bias, and the above-described dust image is unlikely to occur at the tip of the toner image. . On the other hand, the toner image that has been primarily transferred first has less margin for overtransfer by the amount of the primary transfer effect that has been received multiple times. Specifically, when four-color toner images are sequentially superimposed and primarily transferred as in the color mode, the primary transfer bias applied to the leading end of each color toner image is 20% less than the appropriate bias. When it is raised, the leading edge portion of the toner image that is primarily transferred first is subjected to the action of the primary transfer bias that is higher than usual four times, so that the image damage due to overtransfer is considerably received. On the other hand, in the case of a monochromatic toner image that does not perform color superposition as in the one-color mode, the primary transfer bias applied to the tip of the toner image is set to twice the appropriate bias. The image is hardly damaged by the overtransfer. As described above, when primary transfer is performed by sequentially superimposing four-color toner images, the primary transfer bias of the leading end portion is strengthened, although only the leading end portion of each color toner image is set to be in an overtransfer state. Therefore, it is desirable to prevent the occurrence of both the dust image and the overtransfer state. Therefore, in this image forming apparatus, the primary transfer bias applied to the front end portion of the toner image in the color mode is lower than the primary transfer bias applied to the front end portion of the toner image in the one color mode. Switch to bias. That is, the amount of increase in the transfer bias applied to the leading end portion of the toner image is different between the color mode and the one-color mode. As a result, in the color mode, the amount of increase in the transfer bias applied to the tip portion of the toner image of each color is suppressed to a small level, so that the image damage due to the overtransfer is reduced and the tip portion of the toner image of each color is reduced. Is subjected to the primary transfer action a plurality of times, so that the generation of a dust image at the tip of the toner image is also reduced. On the other hand, in the one-color mode, the transfer bias applied to the front end portion of the toner image is increased. However, image damage due to the overtransfer caused by this is small, and the dust image is prevented.
[0011]
According to a third aspect of the present invention, toner images of a plurality of colors formed on the image holder are sequentially superimposed on the intermediate transfer member from the image carrier, and then transferred to the recording medium from the intermediate transfer member. For secondary transfer Because , A fixing device for fixing the toner image on the recording medium by bringing an image forming surface of the recording medium onto which the toner image has been transferred into contact with a heating element; In the image forming apparatus, when the toner image of each color is primarily transferred from the image carrier onto the intermediate transfer member, the toner image is applied to the front end portion of the toner image at a timing corresponding to the front end of the toner image of each color. The primary transfer bias is set higher than the primary transfer bias applied to the other part of the toner image, and the primary transfer bias applied to the other part of the toner image is applied to the leading end part of the toner image. The transfer bias is switched so as to be lower than the next transfer bias, and the transfer bias is changed for each toner image in accordance with the toner image of each color.
[0012]
In the case of an image forming apparatus that forms two or more color images, the degree of occurrence of the above-described dust image varies depending on the toner image of each color. Therefore, in this image forming apparatus, the primary transfer bias applied to the tip portion of the toner image is switched to a different transfer bias for each toner image in accordance with the toner image of each color. Thus, for example, the toner image of a color that is likely to generate a dust image can be efficiently reduced by increasing the transfer electric field. Further, in the case of a toner image having a color that hardly generates a dust image, it is possible to reduce the damage of the image due to the overtransfer by not increasing the primary transfer bias too much.
[0013]
According to a fourth aspect of the present invention, in the image forming apparatus according to the third aspect, the primary transfer bias applied to the leading end portion of the toner image of each color is applied to the toner image of the color that is primarily transferred to the intermediate transfer body first. It is characterized by being made smaller.
[0014]
In the case of an image forming apparatus that forms two or more color images, the toner image having the color that comes first in the primary transfer order has a higher number of primary transfer operations and is more likely to be in the overtransfer state. Therefore, in this image forming apparatus, the primary transfer bias applied to the tip portion of the toner image of each color is made smaller for the toner image of the color that is first transferred to the intermediate transfer body. As a result, the primary transfer bias applied to the front end portion of the toner image of each color becomes smaller as the toner image of the previous color in the primary transfer order and larger as the toner image of the later color, and the above-described dust image is efficiently obtained. And the image damage caused by the overtransfer is small.
[0015]
According to the fifth aspect of the present invention, the toner images of a plurality of colors formed on the image holder are sequentially superimposed on the intermediate transfer member from the image carrier, and then transferred to the recording medium from the intermediate transfer member. A secondary transfer color mode, and a toner image of a specific one of the plurality of color toner images that has been primarily transferred to the intermediate transfer member and then primarily transferred to the intermediate transfer member. And a one-color mode in which the intermediate transfer member is immediately secondarily transferred to a recording medium without giving a primary transfer action during the next rotation of the intermediate transfer member. Because , A fixing device for fixing the toner image on the recording medium by bringing an image forming surface of the recording medium onto which the toner image has been transferred into contact with a heating element; In the image forming apparatus, only in the one-color mode, 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 higher than the primary transfer bias applied to the other portion. The primary transfer bias applied to the other part of the toner image is switched to a bias lower than the primary transfer bias applied to the front end part of the toner image.
[0016]
In the color mode, the above-described dust image does not occur in a toner image to which an appropriate transfer bias is applied at least twice, and a dust image also occurs in the final color toner image to which the transfer bias is applied only once. There may not be. Conversely, in this color mode, if the primary transfer bias is increased until the dust image is at a permissible level or not at all, an overtransfer state exceeding the appropriate transfer bias occurs, the polarity of the toner is reversed, and the transfer rate is increased. May fall. Therefore, in this image forming apparatus, only in the one-color mode, 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 is applied to the other portion. Is switched to a higher transfer bias. As a result, even when the primary transfer bias is increased, only the primary transfer bias that is applied to the leading end portion of the one-color mode toner image with little image damage due to the overtransfer can be increased. The occurrence of the overtransfer is prevented.
[0017]
According to the sixth aspect of the present invention, the toner images of a plurality of colors formed on the image holder are sequentially superimposed on the intermediate transfer member from the image carrier, and then transferred onto the recording medium. For secondary transfer Because , A fixing device for fixing the toner image on the recording medium by bringing an image forming surface of the recording medium onto which the toner image has been transferred into contact with a heating element; In the image forming apparatus, a primary transfer bias to be applied to the leading end portion of the toner image at a timing corresponding to the leading end of the selected toner image among the plurality of color toner images is applied to other portions. The transfer bias is switched to a transfer bias higher than the primary transfer bias, and the primary transfer bias applied to other portions of the toner image is switched to a transfer bias lower than the primary transfer bias applied to the leading end portion of the toner image. It is what.
[0018]
In this image forming apparatus, the primary transfer bias applied to the leading end portion of the toner image of the selected toner image is applied to the other portion at a timing corresponding to the leading end of the selected toner image. 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 portion of the toner image can be increased only when the toner image of the color that is likely to generate the dust image is primarily transferred onto the intermediate transfer member. Compared with the case where the primary transfer bias applied to the tip portion of the toner image is increased, the overtransfer of the toner image is reduced. For example, if the above-described dust image can be prevented if an appropriate transfer bias is applied twice or more, 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. As a result, overtransfer of the toner image is reduced as compared with a case where the primary transfer bias applied to the leading end portion of all the toner images is increased.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an embodiment in which the present invention is applied to a full-color electrophotographic copying machine (hereinafter simply referred to as “copying machine”) that forms 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, there are an image writing device 13 composed of an optical unit, a photosensitive member cleaning device 111 as a cleaning unit for the photosensitive drum 10, a charging charger 12 as a charging unit, and the like around the photosensitive 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 full-color electrophotographic copying machine includes a secondary transfer unit 130 as a secondary transfer unit, a fixing unit 145 as a fixing unit, and a paper supply unit that feeds a transfer material as a recording medium. A paper device, a control device as control means for controlling the operation of each device and each unit, and the like are also provided.
[0020]
The photoconductor cleaning device 111 includes a fur brush 111a and a photoconductor cleaning blade 111b, and a recorded image and a pattern image formed on the photoconductor drum 10 are primarily applied to the intermediate transfer belt 121 of the intermediate transfer unit 120. After the transfer, the residual toner and the pattern image of the recorded image remaining on the surface of the photosensitive drum 10 are removed. The fur brush 111a is provided with a solid lubricant 111c as a lubricant made of zinc stearate or the like so as to come into contact with 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.
[0021]
The revolver developing unit 110 includes a Bk developing unit 115 that supplies black toner, a C developing unit 116 that supplies cyan toner, an M developing unit 117 that supplies magenta toner, and a Y developing unit 118 that supplies yellow toner. Each developing device is selectively positioned sequentially at a developing position facing the photosensitive drum 10 as the revolver developing unit 110 rotates, and the toner of each color is sequentially applied to the photosensitive drum 10. Supply.
[0022]
In the intermediate transfer unit 120, an intermediate transfer belt 121 serving as an intermediate transfer member includes a primary transfer bias roller 122 serving as a charge applying unit, a ground roller 123 serving as a pre-primary neutralizing unit, a tension roller 124, and a secondary transfer unit. It is stretched by a counter 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 primary current or a predetermined primary transfer bias under constant current control from a primary transfer power source (not shown).
[0023]
Around the intermediate transfer belt 121, there is a secondary transfer unit 130 as a secondary transfer unit for secondary transfer of the recorded image primarily transferred onto the intermediate transfer belt 121 to a transfer material, and the recorded image An intermediate transfer belt cleaning device 129 as a cleaning unit that removes residual toner of the recorded image and the pattern image remaining on the intermediate transfer belt 121 after being secondarily transferred to the transfer material from the intermediate transfer belt 121; It is arranged.
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 such as zinc stearate. 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, and the recorded image is transferred. After the secondary transfer to the material, the fur brush 129a and the intermediate transfer belt cleaning blade 129b are configured to be freely contacted and separated by a contact / separation mechanism (not shown) so as to contact the surface of the intermediate transfer belt 121.
[0024]
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. The secondary transfer roller 131 is configured to contact and separate from the intermediate transfer belt 121 by a contact / separation mechanism that does not.
[0025]
Next, an 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. The intermediate transfer belt 121 is driven in a clockwise direction at substantially the same speed as the photosensitive drum 10.
Next, the image writing device 13 exposes the surface of the photosensitive drum 10 with the recording image light of the color image information of the document read by the scanner unit (not shown) and the image light of the pattern image.
As a result, an electrostatic latent image of the color-separated four-color recording images and the pattern images for the respective colors corresponding to the recording images of the respective colors is sequentially formed on the photosensitive drum 10 at a predetermined timing. . The recorded electrostatic images of the respective colors and the electrostatic latent images of the pattern images are sequentially sorted according to the colors corresponding to the toner colors by the toners of the respective colors supplied from the developing units 115, 116, 117, and 118 of the revolver developing unit 110. Developed.
[0026]
Here, the density variation of the recorded image is corrected by the pattern image. FIG. 6 shows an example of means for correcting the density fluctuation of the recorded image 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, image light of a reference density pattern is exposed to the surface of the photoreceptor 1 outside the recorded image forming area where the recorded image is formed 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, a developer 5 is supplied to the photosensitive member 1 by the developing roller 4 of the developing device, and a pattern image 6 composed of a toner image is formed on the photosensitive member 1. Then, the reflection density of the pattern image 6 is detected by a reflection type photosensor (hereinafter referred to as “P sensor”) 9 composed of the light emitting element 7 and the attending element 8, and based on the output value of the P sensor 9. Thus, the toner density of the developer 5 in the developing device is controlled.
[0027]
The recorded images and pattern images of the respective colors developed on the photosensitive drum 10 in this way have a predetermined timing on the surface of the intermediate transfer belt 121 charged to a predetermined surface potential by the primary transfer bias roller 122. The primary transfer is performed while sequentially overlapping.
Meanwhile, during this time, the transfer material is fed from the paper feeding device. The transfer material is conveyed toward a secondary transfer nip portion between the intermediate transfer belt 121 and the secondary transfer roller 131 by a pair of conveyance rollers 151, and the leading ends of the pair of registration rollers 152 are nipped. Is temporarily stopped.
[0028]
Then, at the timing when the leading edge of the image forming area of the transfer material coincides with the leading edge of the color recording image that is superimposed on the surface of the intermediate transfer belt 121 and is primarily transferred, the transfer material is supplied again by the registration roller 152. Paper. Thus, the transfer material is nipped and conveyed by the secondary transfer nip portion formed by the secondary transfer roller 131 contacting the intermediate transfer belt 121. While the transfer material is nipped and conveyed by the secondary transfer nip portion, a predetermined secondary transfer bias is applied to the secondary transfer roller 131 from a secondary transfer bias power source (not shown). Only the color recording image is secondarily transferred onto the material.
Further, when the leading edge of the transfer material on which only the color recording image is secondarily transferred as described above passes through the secondary transfer nip portion, it is disposed downstream of the secondary transfer roller 131 in the transfer material conveyance direction. The separation charge charger 161 serving as a separation unit applies a charge for separation to the transfer material on which the color recording image is transferred, and the charge that electrostatically attracts the transfer material to the intermediate transfer belt 121 is removed. Thus, the transfer material is separated from the intermediate transfer belt 121.
[0029]
The transfer material thus separated from the intermediate transfer belt 121 is conveyed toward the fixing unit 145 by the transfer material conveying belt 171 disposed between the separation charger 161 and the fixing unit 145. The The transfer material conveying belt 171 is stretched around a driving roller 172 and a driven roller 173, and is moved and rotated counterclockwise in FIG. Further, around the transfer material conveyance belt 171, a charging brush 174 as a charge applying unit for applying a charge for adsorbing the transfer material to the surface of the transfer material conveyance belt 171, and the transfer material conveyance belt 171. Separation-charger that weakens the charge of the transfer material transport belt 171 and separates the transfer material from the surface of the transfer material transport belt 171 in a state where the leading end of the transported transfer material reaches the front of the fixing unit 145 175, a charge removal charger 176 for removing charges applied to the surface of the transfer material conveying belt 171 is provided. The transfer material separated from the surface of the transfer material conveying belt 171 is fixed by the fixing unit 145 to the color recording image that has been secondarily transferred onto the image forming surface (the upper surface in FIG. 1). The paper is discharged onto a paper discharge tray (not shown) provided outside the copying machine by the paper discharge roller 145c.
The separation [static elimination] means is turned on immediately before the recording medium faces the separation means, before the margin time for starting up the operation, and turned off immediately after passing the recording medium, and is the minimum necessary for separating the recording medium. It is timing.
[0030]
Hereafter, the characteristic part of this invention is demonstrated.
FIGS. 2 to 6 show timing charts of the image writing signal, the primary transfer bias, and the secondary transfer bias when they coincide with each other. Here, in the case of time coincidence, the timing is shifted by the layout distance and process speed of each process. That is, in each of the timing charts, the horizontal axis does not indicate an accurate time, and in order to make the explanation easy to understand, each signal is turned on / 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 in the vertical axis direction. Further, in both the conventional example and the embodiment, the primary transfer bias is turned off / on for each color toner image. However, even if the primary transfer bias is applied between the toner images of each color. Conventionally, the size of the primary transfer bias in this portion is larger, smaller, or the same as that of the toner image, but any suitable bias may be used.
[0031]
FIG. 6 shows a timing chart in a conventional copying machine. In this conventional copying machine, the primary transfer bias applied to each color toner image is always the same.
In this case, as described above, a dust image is easily generated only at the front end portion of the toner image on the recording medium in the fixing step.
[0032]
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, it corresponds to the leading end of the toner image. At the timing, 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 the other portion of the toner image.
As a result, the transfer bias of only the leading end portion of the recording medium on which the above-described dust image is generated is strengthened to prevent the occurrence of the dust image, and an appropriate transfer bias is applied to the portion other than the leading end portion of the recording medium. Thus, a decrease in transfer rate due to toner polarity reversal is prevented.
[0033]
FIG. 3 shows a timing chart in another copying machine according to this embodiment.
In the case of a copying machine that forms two or more color images, the degree of occurrence of the above-described dust image varies depending on the toner image of each color. Therefore, in this copying machine, the primary transfer bias applied to the tip portion of the toner image is switched to a different transfer bias for each toner image in accordance with the toner image of each color.
Thus, for example, the toner image of a color that is likely to generate a dust image can be efficiently reduced by increasing the transfer electric field. Further, in the case of a toner image having a color that hardly generates a dust image, it is possible to reduce the damage of the image due to the overtransfer by not increasing the primary transfer bias too much.
[0034]
For example, the toner image having the color that comes first in the primary transfer order has a higher number of primary transfer operations and is likely to be in the overtransfer state.
Therefore, in the timing chart shown in FIG. 3, the primary transfer bias applied to the leading end portion of the toner image of each color is made 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 front end portion of the toner image of each color becomes smaller as the toner image of the previous color in the primary transfer order and larger as the toner image of the later color, and the above-described dust image is efficiently obtained. And the image damage caused by the overtransfer is small. In this example, the one-color mode (monochrome mode) is not relevant.
[0035]
FIG. 4 shows a timing chart in still another copying machine according to the present embodiment.
In the color mode in which the toner images of the respective colors are superimposed and sequentially subjected to primary transfer, the above-described dust image does not occur in the toner image to which an appropriate transfer bias is applied twice or more, and the transfer bias is applied only once. In some cases, a toner image of the final color that is not formed does not generate a dust image. Conversely, in this color mode, if the primary transfer bias is increased until the dust image is at a permissible level or not at all, an overtransfer state exceeding the appropriate transfer bias occurs, the polarity of the toner is reversed, and the transfer rate is increased. May fall.
[0036]
Therefore, in this copying machine, 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 only in the one-color mode is higher than the primary transfer bias applied to other portions. Can be switched to a higher transfer bias. As a result, even when the primary transfer bias is increased, only the primary transfer bias that is applied to the leading end portion of the one-color mode toner image with little image damage due to the overtransfer can be increased. The occurrence of the overtransfer is prevented.
[0037]
FIG. 5 shows a timing chart in still another copying machine according to the present embodiment.
In this copying machine, a primary transfer bias applied to the leading end portion of the toner image of the selected toner image is applied to the other portion at a timing corresponding to the leading end of the selected toner image. 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 portion of the toner image can be increased only when the toner image of the color that is likely to generate the dust image is primarily transferred onto the intermediate transfer member. Compared with the case where the primary transfer bias applied to the tip portion of the toner image is increased, the overtransfer of the toner image is reduced.
[0038]
For example, when the above-described dust image can be prevented when an appropriate transfer bias is applied twice or more, as shown in the timing chart of FIG. 5, the leading end portion of the final color toner image to which the transfer bias is applied only once. By increasing only the primary transfer bias applied to the toner image, overtransfer of the toner image is reduced as compared with the case where the primary transfer bias applied to the tip portion of all the toner images is increased. In this example, the one-color mode (monochrome mode) is not relevant.
[0039]
【The invention's effect】
According to the first to sixth aspects of the invention, it is possible to prevent a dust image at the leading end on the recording medium, and to further suppress or eliminate a reduction in transfer rate due to overtransfer, which is a side effect, as much as possible. There is.
[0040]
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 tip portion of the toner image of each color is suppressed to a small level, so that the image damage due to the overtransfer is reduced, and Since the front end portion of each color toner image undergoes a primary transfer action a plurality of times, the generation of a dust image at the front end portion of the toner image is also reduced. On the other hand, in the one-color mode, the transfer bias applied to the front end portion of the toner image is increased. However, there is little damage to the image due to the overtransfer, and the dust image is prevented.
[0041]
According to the third aspect of the present invention, for example, by increasing the transfer electric field for a toner image having a color that tends to generate a dust image, the generation of the dust image can be efficiently reduced. Further, in the case of a toner image having a color that hardly generates a dust image, the primary transfer bias is not increased so much that the image damage caused by the overtransfer can be reduced. There is.
[0042]
According to the fourth aspect of the present invention, the primary transfer bias applied to the front end portion of each color toner image is smaller in the primary transfer order as the previous color toner image and larger in the later color toner image. Thus, there is an excellent effect that the dust image is efficiently reduced and the damage of the image due to the overtransfer is small.
[0043]
According to the invention of claim 5, only the primary transfer bias applied to the front end portion of the one-color mode toner image with little image damage due to the overtransfer is increased even if the primary transfer bias is increased. Thus, there is an excellent effect that the occurrence of the overtransfer in the color mode is prevented.
[0044]
According to the sixth aspect of the present invention, when the above-described dust image can be prevented when an appropriate transfer bias is applied twice or more, the leading end portion of the final color toner image to which the transfer bias is applied only once is applied. By increasing only the primary transfer bias to be applied, there is an excellent effect that overtransfer of the toner image is reduced as compared with the case where the primary transfer bias to be applied to the leading end portion of all toner images is increased. .
[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 copier 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]
10 Photosensitive drum
12 Transcription charger
13 Image writing device
110 Revolver development unit
111 Photoconductor cleaning device
120 Intermediate transfer unit
121 Intermediate transfer belt
122 Primary transfer bias roller
123 Earth Roller
124 Tension roller
126 Secondary transfer counter roller
127 Driving 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 Separately charged charger

Claims (6)

The toner image formed on Zotadashi bearing member, after the primary transfer to the intermediate transfer member from the image bearing member, a configuration in which secondarily transferred recording medium from the intermediate transfer member,
In an image forming apparatus comprising fixing means for contacting the image forming surface of the recording medium onto which the toner image has been transferred and a heating body to fix the toner image on the recording medium ,
When the toner image is primarily transferred from the image carrier onto the intermediate transfer member, a 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 applied to the toner image. Switch to a transfer bias higher than the primary transfer bias applied to other parts of
An image forming apparatus, wherein a primary transfer bias applied to another part of the toner image is switched to a transfer bias lower than a primary transfer bias applied to a leading end part of the toner image. A color in which a plurality of color toner images formed on an image carrier are sequentially transferred from the image carrier to an intermediate transfer member, and then transferred to the recording medium, and then secondary-transferred collectively from the intermediate transfer member to a recording medium. Mode,
After the toner image of one specific color among the toner images of the plurality of colors is primarily transferred to the intermediate transfer member, the toner image primarily transferred to the intermediate transfer member is transferred to the intermediate transfer member during the next rotation of the intermediate transfer member. without giving the primary transfer action, a structure having a 1-color mode to transfer secondary immediately recording medium,
In an image forming apparatus comprising fixing means for contacting the image forming surface of the recording medium onto which the toner image has been transferred and a heating body to fix the toner image on the recording medium ,
When the toner image is primarily transferred from the image carrier onto the intermediate transfer member, a 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 applied to the toner image. The primary transfer bias applied to the other part of the toner image is made higher, and the primary transfer bias applied to the other part of the toner image is made lower than the primary transfer bias applied to the tip part of the toner image. The primary transfer bias applied to the leading end portion of the toner image in the color mode is lower than the primary transfer bias applied to the leading end portion of the toner image in the one color mode. An image forming apparatus characterized by switching to A structure in which a plurality of color toner images formed on an image holding member are sequentially transferred from the image carrier to an intermediate transfer member and then primary transferred, and then the intermediate transfer member collectively transfers to a recording medium. Because
In an image forming apparatus comprising fixing means for contacting the image forming surface of the recording medium onto which the toner image has been transferred and a heating body to fix the toner image on the recording medium ,
When the toner image of each color is primarily transferred from the image carrier onto the intermediate transfer member, a primary transfer bias applied to the front end portion of the toner image at a timing corresponding to the front end of the toner image of each color, The primary transfer bias applied to the other part of the toner image is higher than the primary transfer bias applied to the other part of the toner image, and the primary transfer bias applied to the other part of the toner image is higher than the primary transfer bias applied to the tip part of the toner image. An image forming apparatus, wherein the transfer bias is switched so as to be low, and the transfer bias is switched for each toner image according to the toner image of each color. The image forming apparatus according to claim 3.
An image forming apparatus, wherein a primary transfer bias applied to a leading end portion of each color toner image is made smaller as a toner image of a color that is first transferred to the intermediate transfer body first. A color in which a plurality of color toner images formed on an image carrier are sequentially transferred from the image carrier to an intermediate transfer member, and then transferred to the recording medium, and then secondary-transferred collectively from the intermediate transfer member to a recording medium. Mode,
After the toner image of one specific color among the toner images of the plurality of colors is primarily transferred to the intermediate transfer member, the toner image primarily transferred to the intermediate transfer member is transferred to the intermediate transfer member during the next rotation of the intermediate transfer member. without giving the primary transfer action, a structure having a 1-color mode to transfer secondary immediately recording medium,
In an image forming apparatus comprising fixing means for contacting the image forming surface of the recording medium onto which the toner image has been transferred and a heating body to fix the toner image on the recording medium ,
Only at the time of the one-color mode, 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 switched to a transfer bias higher than the primary transfer bias applied to the other portion.
An image forming apparatus, wherein a primary transfer bias applied to another part of the toner image is switched to a transfer bias lower than a primary transfer bias applied to a leading end part of the toner image. A structure in which a plurality of color toner images formed on an image holding member are sequentially transferred from the image carrier to an intermediate transfer member and then primary transferred, and then the intermediate transfer member collectively transfers to a recording medium. Because
In an image forming apparatus comprising fixing means for contacting the image forming surface of the recording medium onto which the toner image has been transferred and a heating body to fix the toner image on the recording medium ,
The primary transfer bias applied to the leading end portion of the toner image at a timing corresponding to the leading end of the selected toner image of the plurality of color toner images is higher than the primary transfer bias applied to the other portion. Switch to high transfer bias,
An image forming apparatus, wherein a primary transfer bias applied to another part of the toner image is switched to a transfer bias lower than a primary transfer bias applied to a leading end part of the toner image.

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