JP2920022B2 - Image forming device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention generally relates to an electrophotographic or electrostatic recording type image forming apparatus.
The recording material is electrostatically attracted to and held on a recording material carrier such as a transfer sheet or a transfer belt of a transfer drum, and an electric field is applied to the recording material, thereby forming a visible image formed on the image carrier. The present invention is suitably embodied in a color image forming apparatus such as a multicolor electrophotographic copying machine or a color printer which obtains a color image by transferring an image (toner image) onto the recording material.

[0002]

2. Description of the Related Art FIG. 24 schematically shows a conventional digital full-color electrophotographic copying machine which is an example of a color image forming apparatus. In this example, the image carrier 1 serving as a photosensitive drum rotates in the direction of the arrow shown in the figure, and is uniformly charged by the charger 2. Next, image exposure is performed by a laser beam E modulated by an image signal of a document (not shown), and the photosensitive drum 1 is exposed.
An electrostatic latent image is formed thereon. This latent image is generated by a predetermined developing device fixed at a developing position of the rotary developing device 3 in advance.
Color development is performed to form a toner image. This toner image is fed from a recording material cassette (not shown), and is transferred to a recording material 6 carried and conveyed on a transfer drum 8. The color image is formed on the recording material 6 by repeating such an image forming process a desired number of times.

A high-voltage power supply and related members around the transfer drum 8 in such a color copying machine, and an outline of the operation thereof will be further described with reference to FIG.

Prior to the above-described copying operation, first, the inner static eliminator 15 and the outer static eliminator 14 operate. The high-voltage power supplies HV6 and HV5 connected to the inner static eliminator 15 and the outer static eliminator 14 are both AC power supplies, and a recording material carrier wound on the outer periphery of the transfer drum 8 for carrying and transporting the recording material 6. That is, the transfer sheet 81 is discharged to approximately 0V.

Next, when the feeding of the recording material 6 is started, the suction charger 10a is operated by the high voltage power supply HV1. In this case, the power supply HV1 has the same polarity as the polarity of the transfer charger 4 described later. At this time, the opposing roller 10b
And the recording material 6 is attracted to the transfer sheet 81.

Next, when the recording material 6 is conveyed to the transfer position, the transfer charger 4 starts discharging by the high voltage power supply HV2 to transfer the first color toner image. Thereafter, when the toner images of the second to fourth colors are transferred with the rotation of the transfer drum 8, the recording material 6 is separated from the transfer drum 8 by the separation claws 18. At this time, the inner static eliminator 15 and the outer static eliminator 14 operate, and the transfer sheet 81 is also neutralized.

In this conventional example, a system in which a bias having a polarity opposite to that of the transfer charger 4 is applied to the adsorption charger 10a has been proposed.

As another prior art, Japanese Patent Application Laid-Open No. 63-118780 filed by the present applicant discloses a full-color copying machine using a transfer belt 8 as shown in FIG. A technique for simultaneously performing the charge removal of the recording material 8 and the suction of the recording material 6 has been proposed. As shown in FIG. 26, an AC bias power supply 80 is connected to the corona discharger 59 at the suction position, and An AC bias power supply 81 and a DC bias power supply 82 are connected to the electric device 62.

[0009]

However, FIG.
4. In the conventional apparatus shown in FIG. 25, (1) the number of high-voltage power supplies around a recording material carrier such as a transfer sheet or a transfer belt of a transfer drum is large, and the cost is high. (2) Since a corona discharger is used in transfer charging, the transfer efficiency becomes very unstable, causing image defects. There was a drawback.

When transfer charging is performed by a corona discharger, the transfer efficiency is determined by the amount of charge injected toward the recording material carrier among the charges discharged from the wires of the corona discharger. is there. However, the ratio of the amount of charge injected in the direction of the recording material carrier largely depends on the impedance in the direction of the recording material carrier, that is, the potential of the recording material carrier, the water content of the recording material carrier due to the environment, and the like. , Tends to be unstable. As a result, the transfer efficiency becomes unstable.

Further, according to the conventional apparatus having such a configuration, there is a problem that the recording material carrier is charged up as the multiple transfer proceeds, and the transfer efficiency gradually decreases. In order to avoid this problem, it is necessary to use a recording material carrier having low resistance, which limits the materials that can be used. Further, in order to suppress charge-up of the recording material carrier, before the toner image is transferred by the transfer charger 4, the side opposite to the side on which the recording material 6 is carried by the adsorption charger 10a or the like. May be charged to the same polarity as the polarity of the toner image. In this case, at the time of transfer of the toner image, the opposite side of the side of the recording material carrier charged by the adsorption charger 10a carrying the recording material 6 is used. There is a problem that the current flowing from the side portion to the ground through the transfer charger 4 becomes excessive, and transfer causes defective transfer such as toner scattering.

Accordingly, an object of the present invention is to reduce the cost of the apparatus by reducing the number of high-voltage power supplies as compared with the conventional apparatus, and to further reduce the cost of the recording material carrier such as a transfer sheet and a transfer belt. The problem of charge-up at the time of transfer is solved, and the charging potential of the recording material carrier is suppressed to a very low level, whereby toner scattering is reduced, and stable and good transfer can be performed. An object of the present invention is to provide an image forming apparatus capable of obtaining the following.

Another object of the present invention is to use a high-resistance sheet or belt such as PET (polyethylene terephthalate) or PC (polycarbonate) as a recording material carrier, thereby expanding the range of material selection. It is an object of the present invention to provide an image forming apparatus capable of performing the above.

[0014]

The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention provides:
An image carrier for carrying a toner image, a movable recording material carrier for carrying a recording material, and electrostatically transferring the toner image on the image carrier to a recording material carried on the recording material carrier An image forming apparatus, wherein the recording material carrier is capable of holding different potentials at different positions in a moving direction. The charging device has a charging unit that charges the opposite side of the recording medium supporting member to the same polarity as the polarity of the toner image.When transferring the toner image to the recording material by the transfer unit, the transfer unit includes: An image forming apparatus, wherein a portion of the recording material carrier charged by the charging unit, which is opposite to a side on which the recording material is carried, can be contacted, and is grounded via a resistance element. is there.

According to one embodiment of the present invention, the charging means adsorbs the recording material on the recording material carrier.

According to another embodiment of the present invention, the charging means includes an inner charging means and an outer charging means provided to face inward and outward with the recording material carrier interposed therebetween. The inner charging means charges the opposite side of the recording material carrier from the side carrying the recording material to the same polarity as the polarity of the toner image, and the outer charging means carries the recording material of the recording material carrier. The opposite side can be charged to the opposite polarity to the polarity of the toner image. According to another embodiment, a DC voltage or a voltage obtained by superimposing a DC voltage and an AC voltage is applied to at least one of the inner charging unit and the outer charging unit.

According to another embodiment of the present invention, the step of transferring the toner image on the image carrier to the recording material carried on the recording material carrier is repeated to form a toner image of a plurality of colors on the recording material. It can be formed. After transferring the toner image of the first color on the image carrier to the recording material, transferring the toner image of the second color following the first color on the image carrier to the recording material Before the recording, the charging unit can charge the opposite side of the recording material carrier from the side carrying the recording material to the same polarity as the toner image. According to another embodiment, the charge amount per unit weight of the first color toner is larger than the charge amount per unit weight of the second color toner. Further, according to another embodiment, a plurality of the image carriers are provided for respectively carrying the plurality of color toner images.

[0018] According to another embodiment of the present invention, the transfer means includes a conductive brush, and the resistance element includes:
It can be a constant voltage element.

[0019]

Next, an image forming apparatus according to the present invention will be described in more detail with reference to the drawings. The first aspect of the present invention will be described in Examples 1 to 3, and the second aspect of the present invention will be described in Example 4.
7 will be described.

(First Embodiment of the Invention) Next, Examples 1 to 3 will be described.
According to the image forming apparatus of the first aspect of the present invention, the charging means located upstream of the transfer means arranged at the transfer position is like a transfer sheet or a transfer belt entering the transfer position. The surface of the recording material carrier opposite to the recording material carrying surface is charged to the same polarity as the toner image prior to and every time the toner image is transferred.

Embodiment 1 FIG. 1 is a block diagram showing a first embodiment of an image forming apparatus according to the present invention, in which the present invention is applied to a digital full-color electrophotographic copying machine.

The outline of the operation of the copying machine will be described with reference to FIG. 1. The image carrier 1 serving as a photosensitive drum rotates in the direction of the arrow in the figure and is charged uniformly by a charger 2. Next, image exposure is performed by a laser beam E from an exposure device 17 modulated by an image signal of a document (not shown), and an electrostatic latent image is formed on the photosensitive drum 1. This latent image is supplied to a rotary developing device 3 equipped with a yellow developing device 3a, a magenta developing device 3b, a cyan developing device 3c, and a black developing device 3d.
The first color is developed by a predetermined developing device (yellow developing device 3a in FIG. 1) fixed at a developing position in advance to form a toner image.

On the other hand, the recording material 6 fed from the recording material cassette 60 via the paper supply roller 13a and the paper supply guides 13b and 13c is extruded in a direction along the transfer drum 8. At this time, the suction roller 10b is driven by a driving source (not shown).
As a result, a bias having the same polarity as the toner, that is, a negative bias in this embodiment, is applied to the attraction charging brush 10a almost simultaneously. Thereby, the recording material 6 is attracted to the transfer drum 8.

The transfer drum 8 rotates in the direction shown by the arrow in synchronization with the photosensitive drum 1, and the toner image developed by the yellow developing device 3a is transferred onto the recording material 6 by the transfer charging brush 4 at the transfer position. Transcribed. The transfer drum 8 continues to rotate, and prepares for transfer of the next color, magenta in this embodiment.

The photosensitive drum 1 includes a cleaning member 5.
And then charged again by the charger 2 and exposed in the same manner as described above by the next magenta image signal. During this time, the developing device 3 rotates and the magenta developing device 3b is fixed at a predetermined developing position to perform a predetermined magenta development, and the magenta toner image is transferred onto the recording material 6 on the transfer drum 8 in an overlapping manner. .

Subsequently, the same process as described above is performed for cyan and black, respectively. When the transfer of four colors to the recording material 6 is completed, the recording material 6 is separated from the transfer drum 8 by the separation claws 18. Then, the sheet is sent to the fixing device 7 by the transport belt, and a series of full-color printing operations is completed, thereby forming a full-color print image.

FIG. 2 is a schematic diagram of a copying machine showing the transfer drum 8, the photosensitive drum 1 and the high-voltage power supply HV related thereto in FIG. FIG. 3 shows an operation timing chart of the copying machine.

When the recording material 6 is fed along the transfer drum 8, a DC bias is applied to the suction charging brush 10a from the high voltage power supply HV1. The high-voltage power supply HV1 is a constant current transformer, and the output value is set to -50 μA in this embodiment. At this time, the suction roller 10b contacts the transfer drum 8, and presses the recording material 6 against a transfer sheet 81 that is a recording material carrier wound around the transfer drum 8.
Further, a positive charge having a polarity opposite to that of the bias is induced on the surface of the recording material 6. As a result, the recording material 6 is electrostatically attracted to the transfer drum 8. FIG. 4A shows the charge model at this time. The transfer sheet 81 is a dielectric sheet. In this embodiment, a film in which carbon black is dispersed in PC (polycarbonate) and has a thickness of about 150 μm and a volume resistivity of about 3 × 10 ¹⁶ Ωcm is used.

Next, first, the Y (yellow) toner of the first color is transferred onto the recording material 6 carried on the transfer sheet 81. At this time, the transfer charging brush 4, which is made conductive, is used. Since the transfer sheet 81 is grounded, the rear surface potential of the transfer sheet 81 is set to substantially zero potential. At this time, since a positive charge having a polarity opposite to that of the toner is induced on the back surface of the transfer sheet 81, the toner is peeled off from the photosensitive drum 1 and is transferred onto the recording material 6. As a result, the negative charge on the back surface of the transfer sheet 81 is reduced due to such a phenomenon, but does not completely reach zero potential, and the negative charge rich state continues.

FIG. 4B shows a charge model after the transfer of the Y toner of the first color.

Next, prior to the transfer of the second color, the suction charging brush 10a operates again, and a DC bias is applied from the high voltage power supply HV1. However, at this time, since the unfixed toner has adhered to the recording material 6, the suction roller 10b does not contact the transfer drum 8. At this time, since the charging of the attraction charging brush 10a is not intended to attract the recording material 6 and the transfer sheet 81, it is not appropriate to call it as attraction charging. I will call it.

FIG. 4 shows a charge model after the transfer assisting charging.
It is shown in (c). A negative charge is injected into the back surface of the transfer sheet 81, while a positive charge corresponding to the negative charge is injected into the surface of the recording material 6 from the air, as shown in FIG. 4C.

Next, the transfer of the M (magenta) toner of the second color is performed. Also at this time, similarly, since the transfer charging brush 4 is grounded, the negative charge of the back surface potential of the transfer sheet 81 is reduced to approach zero potential. However, negative charges still remain. At this time, the M toner is transferred to the recording material 6 by the injection of the positive charge. The charge model is shown in FIG.

Similarly, auxiliary charge before transfer of C (cyan) toner of the third color, transfer of C (cyan) toner of the third color, auxiliary charge before transfer of Bk (black) toner of the fourth color, Bk of the fourth color The transfer of the (black) toner is performed, and the charge models at this time are shown in FIGS.
(G) and (h).

Recording material 6 after transfer of four color toner images
Is separated from the transfer drum 8 by the separation claw 18. Further, at the time of continuous copying, the next recording material 6 is immediately fed, but the operation is repeated as described above. The charge model is also shown in FIG. 4 as (a) → (b) → (c) → (d) →
The cycle is (e) → (f) → (g) → (h) → (a).

The present inventors measured the surface potential of the back surface of the transfer sheet 81 using a measuring device as shown in FIG.

An outline of this measuring device will be described. First, aluminum was vapor-deposited on a part of the back surface of the transfer sheet 81 of the transfer drum 8, and a monitor aluminum plate 101 connected by a metal wire was prepared so as to have the same potential. A surface voltmeter 102 and a recorder 103 for recording the output from the surface voltmeter 102 are connected.
Was brought closer to the monitor aluminum plate 101, and the potential on the back surface of the transfer sheet 81 was indirectly measured.

FIG. 6 shows the measurement results of the surface potential of the back surface of the transfer sheet 81 during the copying operation.

It can be seen that the transfer sheet 81 is charged to about -3.0 KV after the suction and auxiliary charging, and the potential drops to about -1.0 KV after the transfer. In addition, the transfer potential of the transfer sheet 8
This is because the charge on one back surface is self-decayed (leaked).

As can be understood from the measurement results, in the adsorption step, the toner is charged with the same polarity as the toner, and in the transfer step, the transfer is performed by the induced current without applying a bias voltage (flow). By doing so, the transfer sheet 81 can be used in an extremely low potential range, and can be repeatedly used stably without charge-up even in continuous copying.

In this embodiment, the structure of the attraction charging means is the same as that shown in FIG.
To FIG. 11. That is, instead of the attraction roller 10b, an attraction charging brush 10b similar to the attraction charging brush 10a can be used (FIGS. 8, 9, and 11).
A and 10b are connected to a high-voltage power supply HV having a predetermined polarity or are grounded. In the present embodiment, the transfer efficiency is determined by the potential of the transfer sheet 81 when entering the transfer position. Then, this transfer sheet 81
Of the transfer sheet 8 by the suction charging brush 10a.
It is determined by the injection current to 1.

This relationship was determined by experiments. This result is shown in the two-limit graph of FIG. One graph shows the relationship between the suction injection current and the induced current of the transfer charging brush, and the other graph shows the relationship between the induced current and the transfer efficiency. In general, a transfer efficiency of 90% or more is a criterion for good transfer. Therefore, in this case, 90% or more is regarded as an appropriate transfer area.

From these relationships, the injection current at the time of adsorption is
Approximately 13 to 23 μA was found to be appropriate.

On the other hand, in the adsorption step, the original purpose is adsorption of the recording material 6, and FIG. 13 shows the result of an experiment to determine the relationship between the adsorption injection current and the adsorption force. Here, the attraction force indicates the attraction force between A4 size paper as the recording material 6 and the transfer sheet 81. Further, it has been known that if the suction force is 600 gr or more, no deviation or lifting of the recording material occurs, so this area was set as an appropriate suction area. As a result, it can be seen that it is sufficient if there is an adsorption injection current of 8 μA or more.

In view of both the transfer and the adsorption characteristics, the set current in the adsorption was determined to be approximately 18 μA.

Embodiment 2 FIG. 14 shows a second embodiment of the image forming apparatus of the present invention, and shows an embodiment in which the present invention is applied to a digital full-color electrophotographic copying machine. This copying machine has substantially the same configuration and operation as the first embodiment shown in FIG. 1, but differs in the method of carrying the recording material 6 on the transfer drum 8, and therefore, the differences will be mainly described. The operation will be briefly described.

The paper feed roller 13 from the recording material cassette 60
a, the recording material 6 fed via the paper feed guides 13b and 13c is extruded in a direction along the transfer drum 8.

As shown in FIG. 15, the transfer drum 8 is provided with a gripper 82 for gripping the recording material 6, and the recording material 6 fed to the transfer drum 8 is driven by a driving source (not shown). ) Is gripped by the gripper 82 driven by the gripper. As a result, the recording material 6 is carried on the transfer drum 8 and transported to the transfer position.

The recording material 6 carried on the transfer drum 8
After the multi-color transfer of four colors is completed by the above-described series of operations, the recording material 6 is separated from the transfer drum 8 by the separation claw 18 and sent to the fixing device 7 by the conveyance belt, thereby completing the series of full-color printing operations. Thus, a full-color print image is obtained.

FIG. 16 is a schematic diagram showing the configuration of the transfer drum 8, the photosensitive drum 1 and the high-voltage power supply HV related thereto in FIG. FIG. 17 shows an operation timing chart of the copying machine. Prior to the feeding of the recording material 6, the high voltage HV2 for the inside front charger and the high voltage HV3 for the outside front charger output, and the back surface of the transfer sheet 81 is charged to the negative polarity of the same polarity as the toner, and the front surface is charged to the positive polarity of the opposite polarity to the toner. I do. The output value of the high voltage power supply is -100 μA for HV2 and +1 for HV3.
It was set to 00 μA.

Next, the recording material 6 is fed to the transfer drum 8, gripped by the transfer drum gripper 82, and transported to the transfer position. Since the transfer charging brush 4 is grounded, it operates to make the back surface potential of the transfer sheet 81 substantially zero. At this time, since a positive charge having a polarity opposite to that of the toner is induced on the rear surface of the transfer sheet, the toner is transferred onto the recording material 6. As a result, the negative charge on the back surface of the transfer sheet 81 is reduced by such a phenomenon, but the potential does not completely reach zero potential and the negative charge still remains.

The charge model after the transfer of the first color toner is:
This is the same as FIG. 4B according to the above-described embodiment.

Next, prior to the transfer of the second color toner, the inner front charger 15 and the outer front charger 14 operate again, and the back surface of the transfer sheet is again charged to the same polarity as the toner.

Thereafter, the transfer of the second color, the transfer of the third color, and the transfer of the fourth color are performed in the same manner.

In the second embodiment, the pre-charging has been described above as being performed by the pair of corona dischargers 14 and 15 as shown in FIG. 18, but prior to each transfer. The configuration is not limited to this configuration as long as it satisfies the function of negatively charging the back surface of the transfer sheet and the fact that the recording material side, that is, the outside is charged without contact (the toner image is disturbed when contacted). For example, FIG.
9 and FIG. 20, that is, a conductive roller or a conductive brush may be arranged on the transfer sheet 81 side.

Embodiment 3 FIG. 21 is a structural view showing a third embodiment of the image forming apparatus according to the present invention, and has four photosensitive drums similar to the full-color electrophotographic copying machine shown in FIG. 1 shows a digital full-color electrophotographic copying machine. Hereinafter, an outline of the operation of the copying machine will be described.

In FIG. 21, first, second, third and fourth image forming units Pa, Pb and Pc are provided in the main body of the copying machine.
And Pd are provided side by side, and images of different colors are formed through processes of latent image, development, and transfer.

The image forming units Pa, Pb, Pc, and Pd each have a dedicated image carrier, that is, the photosensitive drums 1a, 1b,
1c, 1d, and the image forming units Pa, Pb, Pc,
Photosensitive drums 1a, 1b, 1c, 1d formed of Pd
The upper toner image is transferred onto a recording material 6 carried and conveyed on a recording material carrier 8 disposed adjacent to each image forming unit. In this embodiment, the recording material carrier 8 is a transfer belt, and is stretched between the drive roller 12 and the support roller 11. After the transfer, the image on the recording material 6 is heated and pressed by the fixing unit 7 to be fixed.

More specifically, the formation of a toner image on each of the photosensitive drums 1a, 1b, 1c, and 1d by each image forming unit is performed by a charger 2a disposed on the outer periphery of the photosensitive drum.
2b, 2c, 2d, the photosensitive drums 1a, 1b, 1c,
1d is uniformly charged, and the laser beam exposure device 17 scans an image signal to form a latent image. This latent image is stored in a developing device 3 having different color developers.
a, 3b, 3c, and 3d,
A toner image is formed on a, 1b, 1c, and 1d.

On the other hand, the recording material 6 is fed from the recording material cassette 60 to the transfer belt 8 via the registration rollers 13.

The transfer belt 8 is a dielectric resin film such as a polyethylene terephthalate resin film sheet (PET sheet), a polyvinylidene fluoride resin film sheet, or a polyurethane resin film sheet. A belt that is joined and joined to form an endless shape, or a seamless (seamless) belt is used. In the case of a belt having no seams, not only is it difficult to produce a belt having uniform physical properties, but it is not suitable in terms of cost, etc., for mass production such as a belt having a stable circumference and production time. On the other hand, in the case of a belt having a seam, when forming an image on the seam position, changes in physical properties such as unevenness and resistance value of the seam portion are inevitable, and forming an image on the seam disturbs the image. In general, in order to obtain high image quality, a means for detecting a seam position (not shown) is provided, and an image is not formed on the seam.

The recording material 6 is conveyed from the registration rollers 13 onto the transfer belt 8. On the other hand, the transfer belt 8 is charged to the same polarity as the toner by the pre-transfer charging brush 9a.

The toner image on the photosensitive drum 1a is transferred to the recording material 6 under the action of the transfer charging brush 4a. The recording material 6 includes second, third, and fourth image forming units Pb,
Pc and Pd, and similarly, the pre-charging brush 9b
9d, the photosensitive drums 1b, 1c, and 1d on the recording material 6 under the action of the transfer charging brushes 4b to 4d.
The toner image on d is superimposed and transferred.

The recording material 6 on which an image has been formed by the fourth image forming unit Pd is separated from the transfer belt 8 and is conveyed to the fixing unit 7.

After the transfer, the photosensitive drums 1a, 1b, 1c, 1
The developer remaining on the photosensitive drum cleaning part 5a
It is removed by 5b, 5c and 5d, and is ready for the subsequent latent image formation. The developer remaining on the transfer belt 8 is neutralized by a belt static eliminator 9 ′ to remove electrostatic attraction, and is then scraped off by a rotating fur brush 16.

FIG. 22 shows the transfer belt 8 of the copying machine shown in FIG.
FIG. 2 is a schematic configuration diagram showing a high-voltage power supply HV and the like related thereto.

When the main body of the copying machine starts to operate, the output is started by the high-voltage power supply HVg connected to the pre-transfer charging brushes 9a to 9d. In this embodiment, the high-voltage power supply HVg is a constant-voltage power supply, and supplies a current until the voltage reaches a preset voltage. The set value was -3 KV. Due to the pre-transfer charging, the back surface potential of the transfer belt 8 entering the transfer area is a negative potential having the same polarity as the toner.

When the recording material 6 is placed on the transfer belt 8 and enters the first transfer zone, the first color Y (yellow) toner is transferred by the grounded transfer charging brush 4a. This is because the induced current flows through the transfer charging brush 4a in order to neutralize the negatively charged transfer belt 8, as in the above-described two embodiments.

In the same manner, when the transfer of the second to fourth colors is completed, the recording material 6 is separated from the transfer belt 8.

In this embodiment, the transfer charging brushes 4a to 4
Although d is used by directly grounding, the same effect can be obtained by grounding via a varistor (constant voltage element) as a resistance element as in the configuration of FIG. However, when the ground is connected via the resistance element, an excessive current is applied to the ground through the transfer charging brushes 4a to 4d from a portion of the recording material carrier such as a transfer sheet or a transfer belt opposite to the side on which the recording material is supported. Can be prevented from flowing, and a better transfer without toner scattering can be performed.

As described above, the induced current transfer method (TRIC; Transfer by
The image forming apparatus of the present invention using (Induced Current)
Compared with the conventional device configuration, it requires less high-voltage power supply and so on, so the cost is lower. Since the charging potential of a recording material carrier such as a transfer sheet and a transfer belt can be extremely low, good transfer without toner scattering or the like can be performed. A high-resistance sheet or belt such as PET or PC can be used as the recording material carrier, and the range of material selection is expanded. There is such a benefit.

(Second Embodiment of the Present Invention)
According to the image forming apparatus of the first aspect of the present invention described above, the charging unit located upstream of the transfer unit disposed at the transfer position is similar to a transfer sheet or a transfer belt entering the transfer position. The surface opposite to the recording material carrying surface of the recording material carrying member is charged with the same polarity as the toner image prior to the toner image transfer, and every time the transfer is performed.
Next, according to the image forming apparatus of the second aspect of the present invention described in Embodiments 4 to 7, the charging means is provided on the surface opposite to the recording material carrying surface of the recording material carrying member entering the transfer position. Is charged to the same polarity as the toner image prior to the first transfer operation of the toner image.

Embodiment 4 An image forming apparatus according to a fourth embodiment of the present invention is suitably embodied in a digital full-color electrophotographic copying machine similar to that used in the first embodiment described above. You.

That is, an outline of the operation of the copying machine according to the present embodiment will be described with reference to FIG. 1. The image carrier 1 serving as a photosensitive drum rotates in the direction of the arrow shown in the figure, and is uniformly charged by the charger 2. Next, image exposure is performed by a laser beam E from an exposure device 17 modulated by an image signal of a document (not shown), and an electrostatic latent image is formed on the photosensitive drum 1. This latent image is formed by a yellow developing device 3a, a magenta developing device 3b,
The first color development is performed by a predetermined developing device (yellow developing device 3a in FIG. 1) fixed in advance to the developing position of the rotary developing device 3 equipped with the cyan developing device 3c and the black developing device 3d to form a toner image. You.

On the other hand, the recording material 6 fed from the recording material cassette 60 via the paper supply roller 13a and the paper supply guides 13b and 13c is extruded in the direction along the transfer drum 8. At this time, the suction roller 10b is driven by a driving source (not shown).
As a result, a bias having the same polarity as the toner, that is, a negative bias in this embodiment, is applied to the attraction charging brush 10a almost simultaneously. Thereby, the recording material 6 is attracted to the transfer drum 8.

The transfer drum 8 rotates in the direction shown by the arrow in synchronization with the photosensitive drum 1, and the toner image developed by the yellow developing device 3a is transferred onto the recording material 6 by the transfer charging brush 4 at the transfer position. Transcribed. The transfer drum 8 continues to rotate, and prepares for transfer of the next color, magenta in this embodiment.

The photosensitive drum 1 includes a cleaning member 5.
And then charged again by the charger 2 and exposed in the same manner as described above by the next magenta image signal. During this time, the developing device 3 rotates and the magenta developing device 3b is fixed at a predetermined developing position to perform a predetermined magenta development, and the magenta toner image is transferred onto the recording material 6 on the transfer drum 8 in an overlapping manner. .

Subsequently, the same process as above is performed for cyan and black, respectively. When the transfer of four colors to the recording material 6 is completed, the recording material 6 is separated from the transfer drum 8 by the separation claws 18. Then, the sheet is sent to the fixing device 7 by the transport belt, and a series of full-color printing operations is completed, thereby forming a full-color print image.

The configuration of the copying machine showing the transfer drum 8, the photosensitive drum 1, and the high-voltage power supply HV related thereto in this embodiment is the same as the schematic diagram of FIG. 2 described above.
FIG. 27 shows an operation timing chart of the copying machine according to the present embodiment.

Referring to FIG. 2, in this embodiment, when the recording material 6 is fed along the transfer drum 8, a DC bias is applied to the suction charging brush 10a from the high voltage power supply HV1. The high-voltage power supply HV1 is a constant current transformer, and the output value is set to -80 μA in this embodiment. At this time,
The suction roller 10b contacts the transfer drum 8, and the recording material 6
Is pressed against a transfer sheet 81 which is a recording material carrier wound around the transfer drum 8. Further, a positive charge having a polarity opposite to that of the bias is induced on the surface of the recording material 6. As a result, the recording material 6 is electrostatically attracted to the transfer drum 8. The charge model at this time is shown in FIG. The transfer sheet 81 is a dielectric sheet. In this embodiment, the thickness is about 150 μm in which carbon black is dispersed in PC (polycarbonate), and the volume resistivity is about 3 × 10 ^16.
An Ωcm film was used.

Next, the first color Y (yellow) toner is first transferred to the recording material 6 carried on the transfer sheet 81. At this time, the transfer charging brush 4 which is made conductive is Since the transfer sheet 81 is grounded, the rear surface potential of the transfer sheet 81 is set to zero potential. At this time, a positive charge having a polarity opposite to that of the toner is induced on the back surface of the transfer sheet 81.
The toner is peeled off from the photosensitive drum 1 and is transferred onto the recording material 6. As a result, the negative charge on the back surface of the transfer sheet 81 is reduced due to such a phenomenon, but does not completely reach zero potential, and the negative charge rich state continues.

FIG. 28B shows a charge model after the transfer of the Y toner of the first color.

Thereafter, similarly, the transfer of the second, third, and fourth colors is sequentially performed in such a manner that the negative charge on the back surface of the transfer sheet 81 applied initially is consumed.

The charge models after the transfer of the second color, the transfer of the third color, and the transfer of the fourth color are shown in FIGS. 28 (c), (d), and (d), respectively.
(E).

Recording material 6 after transfer of four color toner images
Is separated from the transfer drum 8 by the separation claw 18. Further, at the time of continuous copying, the next recording material 6 is immediately fed, but the operation is repeated as described above.

The present inventors measured the surface potential of the back surface of the transfer sheet 81 using the measuring device described above and shown in FIG. FIG. 29 shows the measurement result of the surface potential of the back surface of the transfer sheet 81 during the copying operation.

The transfer sheet 81 has a substantially −
It can be seen that the toner is charged to 8.0 KV, and the potential drops by about 1.8 KV after transfer, and approaches zero. In addition, the transfer potential of the transfer sheet 8
This is because the charge on one back surface is self-decayed (leaked).

As described above, the transfer sheet 81 is attracted and charged.
The back surface is charged with the same polarity as the toner, so that all four colors can be transferred only by the induced current without applying a high voltage during transfer.

In this embodiment, the structure of the attraction charging means may be as shown in FIGS. 8 to 11, instead of the structure shown in FIG. That is, instead of the attraction roller 10b, an attraction charging brush 10b similar to the attraction charging brush 10a can be used (FIGS. 8, 9, and 11).
0a and 10b are connected to a high voltage power supply HV having a predetermined polarity or grounded. In the present embodiment, the transfer efficiency is determined by the transfer sheet 81 when entering the transfer position.
Is determined by the potential of And this transfer sheet 8
The potential of 1 is determined by the current injected into the transfer sheet 81 by the suction charging brush 10a.

This relationship was determined by experiments. This result is shown in the two-limit graph of FIG. One graph shows the relationship between the suction injection current and the induced current of the transfer charging brush, and the other graph shows the relationship between the induced current and the transfer efficiency. In general, a transfer efficiency of 90% or more is a criterion for good transfer. Therefore, in this case, 90% or more is regarded as an appropriate transfer area.

It can be seen from this graph that the amount of induced current decreases as the color becomes later. In addition, the range of the suction injection current at which good transfer from the first color to the fourth color can be performed is approximately −5.
It turns out that it is 8-88 microA.

On the other hand, in the adsorption charging step, the recording material 6 is originally adsorbed, and if the adsorbing force is 600 gr or more, as described above with reference to FIG. Since it is known that this area does not occur, it can be understood that this area should be set as an appropriate adsorption area and that an adsorption injection current of 8 μA or more is sufficient. In the present embodiment, in consideration of both the characteristics of the transfer and the suction, the set current in the suction is determined to be approximately −80 μA.

Fifth Embodiment In the fourth embodiment of the present invention described in the fourth embodiment, each developing device of the rotary developing device 3, namely, the yellow developing device 3a, the magenta developing device 3b, the cyan developing device 3c, and the black developing device The toner tribo of each developing unit 3d is substantially constant. However, in the fifth embodiment of the present invention, the toner of each developing unit uses a different tribo and the toner is developed in descending order of the toner tribo. Was performed.

That is, the tribo of the toner used is the temperature 2
Under an environment of 3 ° C. and 60% humidity, the first color (yellow toner) was 23 μc / gr, the second color (magenta toner) was 20 μc / gr, the third color (cyan toner) was 18 μc / gr, and the fourth color (black toner) was 16 μc / gr. .

The relationship between the transfer induction current and the transfer efficiency of each color toner in this embodiment is as shown in FIG. From this figure, it can be seen that low tribo toner can perform proper transfer with a smaller amount of current.

From the results, it can be seen that in this embodiment, the amount of suction and injection current at which the toners of the first to fourth colors can transfer properly is approximately 30 μA to 88 μA. That is, in this embodiment, a wider latitude can be obtained than in the fourth embodiment.

This is because, for example, even when the type of the recording material 6 is changed, or when there is a difference in the manner in which the potential is applied when the adsorption charging is applied due to environmental fluctuation or the like, the first to fourth colors may be used.
It means that more stable transfer can be performed up to the color tone.

Embodiment 6 With reference to FIG. 14 described above, a sixth embodiment of the image forming apparatus of the present invention will be described. This embodiment shows an embodiment in which the present invention is applied to a digital full-color electrophotographic copying machine. This copier has substantially the same structure and operation as the fourth embodiment shown in FIG. 1, but differs in the method of carrying the recording material 6 on the transfer drum 8, and therefore, this difference will be mainly described. The operation will be briefly described.

The paper feed roller 13 from the recording material cassette 60
a, the recording material 6 fed via the paper feed guides 13b and 13c is extruded in a direction along the transfer drum 8.

The transfer drum 8 is provided with a gripper 82 for gripping the recording material 6 as shown in FIG. 15, and the recording material 6 fed to the transfer drum 8 is driven by a driving source (not shown). ) Is gripped by the gripper 82 driven by the gripper. As a result, the recording material 6 is carried on the transfer drum 8 and transported to the transfer position.

The recording material 6 carried on the transfer drum 8
After the multi-color transfer of four colors is completed by the above-described series of operations, the recording material 6 is separated from the transfer drum 8 by the separation claw 18 and sent to the fixing device 7 by the conveyance belt, thereby completing the series of full-color printing operations. Thus, a full-color print image is obtained.

In this embodiment, the transfer drum 8 shown in FIG.
The configuration of the copying machine showing the photosensitive drum 1 and the related high-voltage power supply HV is the same as the schematic diagram of FIG. 16 described above. FIG. 32 shows an operation timing chart of the copying machine.

Prior to the feeding of the recording material 6, the high voltage HV2 for the inner pre-charger and the high voltage HV3 for the outer pre-charger output, and the reverse side of the transfer sheet 81 has the same polarity as the toner and the reverse side has the opposite polarity. It is positively charged. In this embodiment, the output value of the high-voltage power supply is such that HV2 is -500 μA,
3 was set to +500 μA.

Next, the recording material 6 is fed to the transfer drum 8, is gripped by the transfer drum gripper 82, and is conveyed to the transfer position. Since the transfer charging brush 4 is grounded, it operates to make the back surface potential of the transfer sheet 81 substantially zero. At this time, since a positive charge having a polarity opposite to that of the toner is induced on the rear surface of the transfer sheet, the toner is transferred onto the recording material 6. As a result, the negative charge on the back surface of the transfer sheet 81 is reduced by such a phenomenon, but the potential does not completely reach zero potential and the negative charge still remains.

The charge model after the transfer of the first color toner is
This is the same as FIG. 28B according to the above-described embodiment.

Thereafter, the transfer of the second color, the transfer of the third color, and the transfer of the fourth color are performed in the same manner.

In the sixth embodiment, the pre-charging has been described as being performed by the pair of corona dischargers 14 and 15 as shown in FIG. 18 in the above description. The configuration is not limited as long as it satisfies the function of charging the back surface negatively and the fact that the recording material side, that is, the outside is charged without contact (the toner image is disturbed when contacted). For example, the configuration shown in FIGS. 19 and 20, that is, a conductive roller or a conductive brush may be disposed on the transfer sheet 81 side.

Embodiment 7 FIG. 33 is a block diagram showing a seventh embodiment of the image forming apparatus according to the present invention, wherein four photosensitive members similar to those of the full-color electrophotographic copying machine shown in FIG. 21 or FIG. 1 shows a digital full color electrophotographic copier with a drum. Hereinafter, an outline of the operation of the copying machine will be described.

In FIG. 33, the first, second, third and fourth image forming units Pa, Pb and Pc are provided in the main body of the copying machine.
And Pd are provided side by side, and images of different colors are formed through processes of latent image, development, and transfer.

The image forming units Pa, Pb, Pc, and Pd each have a dedicated image carrier, that is, the photosensitive drums 1a, 1b,
1c, 1d, and the image forming units Pa, Pb, Pc,
Photosensitive drums 1a, 1b, 1c, 1d formed of Pd
The upper toner image is transferred onto a recording material 6 carried and conveyed on a recording material carrier 8 disposed adjacent to each image forming unit. In this embodiment, the recording material carrier 8 is a transfer belt, and is stretched between the drive roller 12 and the support roller 11. After the transfer, the image on the recording material 6 is heated and pressed by the fixing unit 7 to be fixed.

More specifically, the formation of a toner image on each of the photosensitive drums 1a, 1b, 1c, and 1d by each image forming unit is performed by a charger 2a disposed on the outer periphery of the photosensitive drum.
2b, 2c, 2d, the photosensitive drums 1a, 1b, 1c,
1d is uniformly charged, and the laser beam exposure device 17 scans an image signal to form a latent image. This latent image is stored in a developing device 3 having different color developers.
a, 3b, 3c, and 3d,
A toner image is formed on a, 1b, 1c, and 1d.

On the other hand, the recording material 6 is fed from the recording material cassette 60 to the transfer belt 8 via the registration rollers 13.

The transfer belt 8 is a dielectric resin film such as a polyethylene terephthalate resin film sheet (PET sheet), a polyvinylidene fluoride resin film sheet, or a polyurethane resin film sheet. A belt that is joined and joined to form an endless shape, or a seamless (seamless) belt is used. In the case of a belt having no seams, not only is it difficult to produce a belt having uniform physical properties, but it is not suitable in terms of cost, etc. for mass production such as a belt having a stable circumferential length and production time. On the other hand, in the case of a belt having a seam, when forming an image on the seam position, changes in physical properties such as unevenness and resistance value of the seam portion are inevitable, and forming an image on the seam disturbs the image. In general, in order to obtain high image quality, a means for detecting a seam position (not shown) is provided, and an image is not formed on the seam.

The recording material 6 is conveyed from the registration rollers 13 onto the transfer belt 8. On the other hand, the transfer belt 8 is charged to the same polarity as the toner by the pre-charging brush 9a.

The toner image on the photosensitive drum 1a is transferred to the recording material 6 under the action of the transfer charging brush 4a. The recording material 6 includes second, third, and fourth image forming units Pb,
Pc and Pd are transferred to the transfer charging brushes 4b to 4
d, the photosensitive drums 1b, 1b
The toner images on c and 1d are superimposed and transferred.

The recording material 6 on which an image has been formed by the fourth image forming unit Pd is separated from the transfer belt 8 and is conveyed to the fixing unit 7.

After the transfer, the photosensitive drums 1a, 1b, 1c, 1
The developer remaining on the photosensitive drum cleaning part 5a
It is removed by 5b, 5c and 5d, and is ready for the subsequent latent image formation. The developer remaining on the transfer belt 8 is neutralized by a belt static eliminator 9 ′ to remove electrostatic attraction, and is then scraped off by a rotating fur brush 16.

FIG. 34 shows the transfer belt 8 of the copying machine shown in FIG.
FIG. 2 is a schematic configuration diagram showing a high-voltage power supply HV and the like related thereto.

When the main body of the copying machine starts operation, the output is started by the high voltage power supply HVg connected to the pre-transfer charging brush 9a. In this embodiment, the high-voltage power supply HVg is a constant-voltage power supply, and supplies a current until the voltage reaches a preset voltage. The set value was -8 KV. Due to the pre-transfer charging, the back surface potential of the transfer belt 8 entering the transfer area is a negative potential having the same polarity as the toner.

When the recording material 6 is placed on the transfer belt 8 and enters the first transfer zone, the first color Y (yellow) toner is transferred by the grounded transfer charging brush 4a. This is because the induced current flows through the transfer charging brush 4a in order to neutralize the negatively charged transfer belt 8, as in the above-described two embodiments.

When the transfer of the second to fourth colors is completed in the same manner, the recording material 6 is separated from the transfer belt 8.

In this embodiment, the transfer charging brushes 4a to 4a
Although d is directly grounded, similar effects can be obtained by grounding via a varistor (constant voltage element) as a resistance element as in the configuration of FIG. However, when the ground is connected via the resistance element, an excessive current is applied to the ground through the transfer charging brushes 4a to 4d from a portion of the recording material carrier such as a transfer sheet or a transfer belt opposite to the side on which the recording material is supported. Can be prevented from flowing, and a better transfer without toner scattering can be performed.

As described above, as described in Examples 4 to 7, the induced current transfer method (TRIC; Transfer by
The image forming apparatus of the present invention using (Induced Current)
Compared with the conventional device configuration, it requires less high-voltage power supply and so on, so the cost is lower. Since the charging potential of a recording material carrier such as a transfer sheet and a transfer belt can be extremely low, good transfer without toner scattering or the like can be performed. A high-resistance sheet or belt such as PET or PC can be used as the recording material carrier, and the range of material selection is expanded. There is such a benefit.

[0124]

As described above, in the image forming apparatus according to the present invention, before the toner image is transferred to the recording material by the transfer means, the side of the recording material carrier opposite to the side on which the recording material is supported is moved. Having charging means for charging to the same polarity as the polarity of the toner image,
When transferring the toner image to the recording material by the transfer means, the transfer means can contact a portion of the recording material carrier charged by the charging means on the side not supporting the recording material, and is grounded via a resistance element. Therefore, the number of high-voltage power supplies and the like can be reduced, and the cost can be reduced. The charge potential of a recording material carrier such as a transfer sheet and a transfer belt can be extremely low, and it can be used stably and repeatedly without being charged up even in continuous image formation. Excessive current can be prevented from flowing from the portion of the recording material carrier opposite to the side on which the recording material is carried to the ground through the transfer means, and good transfer without toner scattering or the like can be performed. As a recording material carrier,
High resistance sheets and belts such as PET and PC can be used, and the range of material selection is expanded. Such an effect can be obtained.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a first or fourth embodiment of an image forming apparatus according to the present invention.

FIG. 2 is a schematic configuration diagram of an apparatus showing a transfer drum, a photosensitive drum, and a high-voltage power supply related thereto in the apparatus of FIG.

FIG. 3 is an operation timing chart of the copying machine according to the first embodiment of the present invention.

FIGS. 4A to 4H are diagrams for explaining a recording material carrier and a charge model of the recording material in a superimposition transfer type copying machine according to a copying process;

FIG. 5 is a schematic configuration diagram of a measuring device for measuring a change in surface potential on the back surface of the transfer belt in the device of FIG. 1;

FIG. 6 is a diagram showing the result of measuring the surface potential of the back surface of the transfer sheet at a position downstream of the suction position in the rotation direction.

FIG. 7 is a diagram showing an embodiment of an adsorption charging unit.

FIG. 8 is a view showing another embodiment of the adsorption charging means.

FIG. 9 is a view showing another embodiment of the adsorption charging means.

FIG. 10 is a view showing another embodiment of the adsorption charging means.

FIG. 11 is a view showing still another embodiment of the adsorption charging means.

FIG. 12 is a graph showing the relationship between the amount of adsorption injection current, the transfer induction current, and the transfer efficiency in the first embodiment.

FIG. 13 is a graph showing a relationship between a suction injection current amount and a suction force between a recording material and a transfer sheet.

FIG. 14 is a configuration diagram of a second or fifth embodiment of the image forming apparatus according to the present invention.

FIG. 15 is a perspective view of a transfer drum used in the apparatus of FIG.

FIG. 16 is a schematic configuration diagram of an apparatus showing a transfer drum, a photosensitive drum, and a high-voltage power supply related thereto in the apparatus of FIG. 14;

FIG. 17 is an operation timing chart of the copying machine according to the second embodiment of the present invention.

FIG. 18 is a diagram showing an embodiment of an adsorption charging unit.

FIG. 19 is a view showing another embodiment of the adsorption charging means.

FIG. 20 is a diagram showing another embodiment of the adsorption charging means.

FIG. 21 is a schematic diagram illustrating a configuration of a third embodiment of the image forming apparatus according to the present invention.

FIG. 22 is a schematic configuration diagram of an apparatus showing a transfer belt, a photosensitive drum, and a high-voltage power supply related thereto in the apparatus of FIG. 21;

23 is a schematic diagram of another configuration of the apparatus shown in FIG. 21 showing a transfer belt, a photosensitive drum, a high-voltage power supply related thereto, and the like.

FIG. 24 is a configuration diagram illustrating a conventional image forming apparatus.

FIG. 25 is a configuration diagram illustrating a conventional image forming apparatus.

FIG. 26 is an enlarged view showing an adsorption charging unit of the apparatus of FIG. 25.

FIG. 27 is an operation timing chart of the copying machine according to the fourth embodiment of the present invention.

FIGS. 28A to 28E are diagrams illustrating a recording material carrier and a charge model of the recording material in a superposition transfer type copying machine according to a copying process.

FIG. 29 is a diagram illustrating a change in surface potential on the back surface of the transfer belt.

FIG. 30 is a graph showing the relationship between the amount of adsorption injection current, the transfer induction current, and the transfer efficiency in the fourth embodiment.

FIG. 31 is a graph showing the relationship between the amount of adsorption injection current, the transfer induction current, and the transfer efficiency in the fifth embodiment.

FIG. 32 is an operation timing chart of the copying machine according to the sixth embodiment of the present invention.

FIG. 33 is a configuration diagram of a seventh embodiment of the image forming apparatus according to the present invention.

FIG. 34 is a schematic configuration diagram of an apparatus showing a transfer drum, a photosensitive drum, and a high-voltage power supply related thereto in the apparatus of FIG.

FIG. 35 is a schematic diagram of another configuration of the apparatus shown in FIG. 33 showing a transfer drum, a photosensitive drum, and a high-voltage power supply related thereto;

[Explanation of symbols]

REFERENCE SIGNS LIST 1 image carrier (photosensitive drum) 2 charger 3 developing device 4 transfer charging means 8, 81 recording material carrier (transfer sheet, transfer belt) 10a, 10b, 14, 15 recording material carrier charging means (adsorption charging means)

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Nobuhiko Takekoshi 3-30-2 Shimomaruko, Ota-ku, Tokyo Inside Canon Inc. (56) References JP-A 2-148075 (JP, A) (58) Survey Field (Int.Cl. ⁶ , DB name) G03G 15/16 G03G 15/01 114

Claims (11)

(57) [Claims] An image carrier that carries a toner image, a movable recording material carrier that carries a recording material, and a toner image on the image carrier that is carried on the recording material carried by the recording material carrier. Transfer means for electrostatically transferring, wherein the recording material carrier can hold different potentials at different positions in the moving direction, wherein the transfer means transfers a toner image to a recording material. before,
A charging unit configured to charge a side of the recording material carrier opposite to a side supporting the recording material to have the same polarity as the polarity of the toner image, wherein the transfer unit transfers the toner image to the recording material by the transfer unit; Is capable of contacting a portion of the recording material carrier charged by the charging means on a side opposite to a side on which the recording material is carried, and is grounded via a resistance element. apparatus. 2. An image forming apparatus according to claim 1, wherein said charging means adsorbs a recording material on said recording material carrier. 3. The image forming apparatus according to claim 1, wherein the charging unit includes an inner charging unit and an outer charging unit that are provided to face the inside and the outside via the recording material carrier. . 4. The recording medium carrier according to claim 1, wherein the inner charging unit charges the opposite side of the recording material carrier with the recording material to the same polarity as the polarity of the toner image. 4. The image forming apparatus according to claim 3, wherein the side carrying the recording material is charged to a polarity opposite to the polarity of the toner image. 5. The image according to claim 3, wherein a DC voltage or a voltage obtained by superimposing a DC voltage and an AC voltage is applied to at least one of the inner charging unit and the outer charging unit. Forming equipment. 6. A multi-color toner image can be formed on a recording material by repeating a step of transferring a toner image on the image carrier onto a recording material carried on the recording material carrier. The image forming apparatus according to claim 1, wherein: 7. A toner image of a second color subsequent to the first color on the image carrier after the first color toner image on the image carrier is transferred to a recording material. 7. The image forming apparatus according to claim 6, wherein before charging the recording material onto the recording material, the charging unit charges the opposite side of the recording material carrier from the side on which the recording material is carried to the same polarity as the toner image. apparatus. 8. The charge amount per unit weight of the toner of the first color is larger than the charge amount per unit weight of the toner of the second color.
Image forming apparatus. 9. The image forming apparatus according to claim 6, wherein a plurality of the image carriers are provided to carry the toner images of the plurality of colors, respectively. 10. The image forming apparatus according to claim 1, wherein the transfer unit includes a conductive brush. 11. The image forming apparatus according to claim 1, wherein the resistance element is a constant voltage element.