JP3316520B2 - Image forming device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an image forming apparatus for forming an image while supplying an appropriate transfer current to an image carrier in accordance with a printing rate of the formed image.

[0002]

2. Description of the Related Art Conventionally, there is an electrophotographic image forming apparatus. The image forming apparatus forms a latent image on an image carrier by optical writing, converts the latent image into a toner image, and transfers and fixes the toner image on paper. When the above toner image is transferred to paper, a corona discharger is used as a transfer unit so that sufficient transferability can be obtained even when the system load fluctuates due to environmental fluctuations. The method requires a relatively large current, which is not only uneconomical in cost, but also increases the amount of ozone generated by corona discharge in order to increase the processing speed, thereby polluting the working environment.
In order to remove the ozone, a processing apparatus is required, which causes a problem that the apparatus is increased in size and cost is increased. For this reason, in recent years, without relying on corona discharge,
Various image forming apparatuses using a contact-type transfer device that directly applies a charge to a transfer unit are being put into practical use. Contact-type transfer units include a transfer roller and a transfer brush, but a system using a transfer brush is simpler in configuration and is suitable for miniaturization.

FIG. 10 is a diagram showing a basic configuration of a transfer section in such an image forming apparatus. As shown in FIG. 1, the transfer unit includes a photosensitive drum 1 and the photosensitive drum 1.
And a transfer brush 2 which is pressed in opposition to the transfer brush. The paper P is carried into the transfer section by the transport roll pair 3.
The photoconductor drum 1 is formed by uniformly applying a photoconductor to a surface of a conductive metal roller, and the metal roller portion is grounded to the apparatus main body. The transfer brush 2 is formed of a conductive brush-like member. The transfer brush 2 is connected to a constant-current power supply 4 having a positive polarity so as to supply a transfer current that is not affected by a load change or the like due to an environmental change.

[0006] In the vicinity of the photosensitive drum 1, although not particularly shown, a cleaner, an initialization charger, a write head,
And a developing device. The initialization charger uniformly charges the peripheral surface of the photosensitive drum 1 to a high potential, and a writing head forms an electrostatic latent image on the peripheral surface by optical writing, and the developing device stores the electrostatic latent image in the developing device. The toner image is formed into a toner image by the toner accommodated in the drum, and the toner image is conveyed to the transfer unit while the photosensitive drum 1 rotates. The transfer brush 2 applies a positive potential transfer current to the sheet P, and the negative polarity toner image on the photosensitive drum 1 is transferred to the sheet P that has a positive potential by this application.

[0005]

By the way, the toner image transferred (printed) on the paper P at the transfer portion has various forms, for example, a portion having a large amount of toner (a portion having a high printing rate) and a toner portion having a high printing rate. A portion having a small amount (a portion having a low printing rate) or the like is conveyed on the photosensitive drum 1 at an indefinite position or an indefinite period. Generally, the printing rate is low in character printing, and the printing rate is high in graphics printing.

[0006] Between the transfer brush 2 and the photosensitive drum 1, the paper P becomes more conductive as the humidity increases, but it generally has a capacitance and is non-conductive.
The toner is non-magnetic and has an extremely high electric resistance. The photoconductor on the surface of the photosensitive drum 1 has good conductivity in a negative high potential portion (a portion where toner does not transfer, that is, a portion having a low printing rate), and a negative low potential portion (a portion in which toner transfers, that is, a high printing ratio portion). Low conductivity. Normally, the photoconductive portion, the paper P, the toner, and the like of the photosensitive drum 1 are generally non-conductive, but the capacitance portion is conducted by an AC component or by discharge, and the resistance portion is converted to a high voltage component. It conducts on the other hand. This current flowing through the transfer section is generally called injection of transfer charge into the transfer section.

[0007] The magnitude of the appropriate current to flow through the transfer section differs between the high printing rate portion and the low printing rate portion. However, as described above, a constant current source is used for the transfer brush 2 for the purpose of always maintaining stable transferability with respect to a load variation of the transfer unit due to a type of paper or an environmental change. That is, a constant current is always supplied to the transfer unit irrespective of the printing ratio described above, and it is not considered to control the current by the printing ratio.

As described above, since all of the constant current output from the constant current source is used for the transfer, for example, the surface of the photosensitive drum 1 has a high potential, and the image portion where the exposure amount due to the light writing is small is small. In the case of printing (in the case of reversal development, when the printing ratio is low, that is, when the amount of toner is small), even if the current allows proper transfer, printing of the image portion where the surface of the photosensitive drum 1 has a low potential and the exposure amount is large is performed. In the case of (1) (a high printing rate, that is, a large amount of toner), there is a problem that the transfer is too strong, and troubles such as transfer failure, image disorder, etc. occur. In addition, for example, in the case of an image in which a black solid portion of 5 mm square is arranged almost everywhere in a white background only, a large amount of transfer current flows in a white background portion having a large potential difference, and therefore, the black solid portion is Transferability decreases. Further, if the entire surface is solid black, the transfer current hardly flows to the photosensitive drum 1 side, so that electric charges are concentrated on the sheet, and the sheet is charged accordingly. When the paper is charged, if the paper is directly conveyed, it is attracted to a guide plate or the like to cause a jam. In the case of belt conveyance, when the belt is separated from the belt at the fixing unit, a peeling discharge is caused to scatter unfixed toner, thereby causing a defective image.

SUMMARY OF THE INVENTION In view of the above-mentioned conventional circumstances,
An image forming apparatus that appropriately controls an output current of a constant current source according to a state of a transfer unit.

[0010]

First, an image forming apparatus according to a first aspect of the present invention comprises a plurality of image carriers arranged in parallel ,
A plurality of toner <br/> over image forming means for respectively forming a toner image on the plurality of image bearing member, to respectively contact the plurality of image bearing members formed a toner image by the plurality of toner image forming means to high resistance for transporting adsorbed revolved transfer material to the outer peripheral surface
Through a conveyor belt of the anti, the transfer material conveyed by the conveying belt, the conveying belt from the inside of the conveyor belt
To a plurality of contact type transferring device for transferring the toner image on each pressing to the transfer material to the plurality of image carriers, to said plurality of <br/> contact transfer unit each current multiple supplies The constant current supply means and the conductive part at the tip are located near the plurality of contact transfer devices.
Each is arranged, a plurality of rolling a portion of each supplied transfer current is grounded connected to the apparatus body via a respective current control elements to the plurality of contact transfer unit is shunted to ground said respective <br/> And a photocurrent control electrode.

[0011] Then, for example, as in claim 2, wherein, above
Each current control element is connected to each of the ground shunt circuits described above.
The generated voltage goes from upstream to downstream in the transfer direction of the transfer material.
It is configured to be arranged in a relationship that becomes higher as the distance increases .

[0012] The current control element is, for example,
Term3It is a resistive element as described.

Further, for example, as according to claim 4, the conveyor belt, in order for the image bearing member of the end of the arrangement direction of the plurality of image bearing members are contacted at the same position transfer material always The transfer material is held at a transferable position, and selectively moved to a transferable position at which the transfer material should be brought into contact with another image carrier and a non-transferable position at which the transfer material is separated from the image carrier so as not to be in contact therewith. It comprises a belt moving means. Then, for example, as in claim 5, wherein the toner image forming means is toner image forming means corresponding to the image bearing member of the juxtaposed direction end portion to form a black toner image, corresponding to the other image bearing member Yes A color toner image is formed. Also, for example, the claims
As described in 6, the transfer current control electrode corresponding to the image carrier at the end portion in the juxtaposition direction includes two types of current control elements of strong and weak, and the transfer belt has the image carrier at the end portion in the juxtaposition direction, and When the transfer belt is moved to a position where it can be transferred to another image carrier, it is connected to the stronger current control element, and only the image carrier at the end in the direction in which the conveyor belt is juxtaposed can transfer other images. When the carrier is moved to a position where transfer cannot be performed, the carrier is provided with switching means for switching to connect to the weaker current control element.

[0014] The image bearing member, for example, a photosensitive drum or the like, the toner image forming means comprises for example a write head, from the developing device or the like, the transfer material, for example, a paper or the like, the upper Ki搬 feed belt Is , for example, a synthetic resin film part
It consists timber, the contact-type transfer device may, for example, a transfer brush or the like, the constant current supply means, for example, a constant current source or the like, and, said belt moving means, for example swing arm, movable support rollers Consists of

Next, an image forming apparatus according to a seventh aspect of the present invention provides a plurality of image carriers, a plurality of toner image forming means for respectively forming toner images on the plurality of image carriers, and a plurality of toner image forming means. A transfer belt having a high resistance for adsorbing the transfer material to the outer peripheral surface thereof and circulating and transferring the transfer material so as to contact the plurality of image carriers on which the toner images have been formed by the toner image forming means; A plurality of contact-type transfer devices for transferring the toner image onto the transfer material by pressing the transfer material from the inside of the transport belt to the plurality of image carriers via the transport belt, respectively; A plurality of constant current supply means for supplying current to the transfer device, and a conductive portion at the tip are respectively disposed in the vicinity of the plurality of contact transfer devices, and each of the conductive portions is grounded to the apparatus main body via a current control element. Supplied to multiple contact transfer units And a plurality of transfer current control electrodes for diverting a part of the transferred transfer current to the respective grounds, and a predetermined current control among the plurality of current control elements corresponding to the plurality of transfer current control electrodes. Assuming that the voltage generated by the element is Y volts and the charging potential of the transport belt immediately before execution of the transfer step in the transfer unit corresponding to the predetermined current control element is X volts, the expression “X−100 <Y <X + 1”
000 ".

The image carrier comprises, for example, a photosensitive drum or the like, the toner image forming means comprises, for example, a writing head, a developing device, or the like. The transfer material comprises, for example, paper or the like. Is composed of, for example, a transfer brush, and the constant current supply means is composed of, for example, a constant current source.

[0017]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1A is a diagram showing a basic configuration of a transfer unit of an image forming apparatus according to the present invention . As shown in FIG. 1, a transfer unit of the image forming apparatus includes a photosensitive drum 11, a transfer brush 12 pressed against the photosensitive drum 11, and a positive polarity constant connected to the transfer brush 12. It comprises a current power supply 13 and a transfer current control electrode 14 whose tip is arranged near the transfer brush 12.

The photosensitive drum 11 is composed of an organic light guide such as OPC or an inorganic light guide such as Se or a-Si, which uniformly covers the surface of a metal roller. It is grounded to the main body (image forming apparatus). The transfer brush 12 is formed by forming a woven fabric woven in a pile from a conductive fiber material such as rayon, nylon, or acrylic into a brush shape and attaching it to a metal or plastic substrate. The transfer current control electrode 14
The conductive brush member is made of a material in which conductive fibers similar to the transfer brush member are linearly arranged and fixed, or a material in which the tip of a conductive sheet-like member is processed in a sawtooth shape. Any material may be used. The paper P is transported to the transfer section
Is carried in by The transfer current control electrode 14 is
It is arranged in an appropriate range so as not to contact the transfer brush 12 and the paper P. For example, transfer brushes 12 to 10
mm or less than 10 mm from the lower surface of the sheet P, good results can be obtained for transfer described later.

FIG. 2B is a cross-sectional view showing the structure of the image forming unit not shown in FIG.
(c) is a perspective view thereof. As shown in FIGS.
The image forming unit 20 includes the photosensitive drum 11 described above as a center, and includes the photosensitive drum 11, a cleaner 21 surrounding the photosensitive drum 11 along a peripheral surface thereof, an initialization charging brush 22, and a developing device 23. Become. The developing device 23 also serves as a toner container, and a developing roller 24
Is rotatably held, and a toner 25 is accommodated therein. Lower side surface of the image forming unit 20 (see FIG. 3C)
The support shaft 24-1 of the developing roller 24 and the support shaft 11-1 of the photosensitive drum 11 protrude from the outside.

A toner agitating member 26 is provided below the inside of the developing device 23. The toner agitating member 26 is provided in FIG.
, The toner 25 is fed to the lower supply roller 27 while stirring the toner 25. The supply roller 27 is made of a sponge member, is pressed against the developing roller 24, and rubs the toner 25 sent from the stirring member 26 so as to rub the developing roller 24.
Supply to the peripheral surface. A blade spring 28 in the form of a leaf spring is in contact with the peripheral surface of the developing roller 24 in the rotation direction, and imparts a triboelectric charge to the toner 25 to promote adhesion to the developing roller 24 and to reduce the adhered toner layer to a certain degree. Reduce to thickness.

An initializing charging brush 22 which is located just above the photosensitive drum 11 of the image forming unit 20 (see FIG. 1C).
The photosensitive drum 11 is provided on the upper surface between the photosensitive drum 11 and the developing roller 24.
A slot 29 is provided in parallel with the axial direction. The write head 31 (see FIG. 3 (b)) is fixed to the back of the upper cover (not shown), and draws an arc as shown by the arrow A in FIG. After descending and fitting the leading end into the above-mentioned slot 29, the position is fixed at the image forming position by a predetermined positioning mechanism. When the write head 31 is pulled up with the top cover opened, the image forming unit 20 can be taken out of the apparatus main body independently.
At this time, the protective lid 32 slides counterclockwise to cover and protect the exposed lower surface of the photosensitive drum 11. A transfer unit is formed by the photosensitive drum 11 of the image forming unit 20, the transfer brush 12, the constant current source 13, and the transfer current control electrode 14 on the apparatus main body side.

In this configuration, the operation of image formation will be described. The initialization charging brush 22 is connected to a power supply (not shown) and applies a negative high voltage to the photosensitive drum 11. By this application, the photosensitive layer on the peripheral surface of the photosensitive drum 11 is uniformly charged to a negative high potential of about "-650 V".

The writing head 31 is composed of a laser head or an LED head, and selectively exposes the peripheral surface of the photosensitive drum 11 charged to a negative high potential in accordance with image information. By this exposure, a negative low potential portion attenuated to approximately “−50 V” is formed, and an electrostatic latent image is formed by the low potential portion of “−50 V” and the high potential portion of “−650 V”. .

A developing bias of about -250 V is applied to the developing roller 24 of the developing device 23 from a power source (not shown), and the toner 25, which is regulated to a constant layer thickness by a doctor blade 28 and adheres, is applied to the photosensitive member. The sheet is conveyed to a portion facing the drum 11. Developing roller 24 and photosensitive drum 11
A potential difference of “−200 V” is formed between the low potential portion of “−50 V” of the electrostatic latent image and the developing roller 24, and the low potential portion of the electrostatic latent image is On the other hand, a potential having a relatively positive polarity is formed. The non-magnetic toner 25 charged to the negative polarity is transferred to the low potential portion of the electrostatic latent image of the photoconductor drum 11 by the electric field due to the potential difference to form (develop) a toner image. The developed toner image is transferred to the photosensitive drum 1 by rotation of the photosensitive drum 11.
The transfer brush 12 is conveyed to an opposing portion of the transfer brush 12.

A sheet P fed from a sheet feeding unit (not shown) is provided at an opposing portion between the photosensitive drum 11 and the transfer brush 12.
Is carried in while being pinched by the pair of transport rolls 15, and is pressed against the photosensitive drum 11 by the transfer brush 12.

The transfer brush 12 applies a constant current of positive polarity, which is output from the constant current power supply 13, to the sheet P.
The negative toner image on the photosensitive drum 11 is transferred.

The sheet P to which the toner image has been transferred is continuously conveyed to the left in the figure, where the toner image is heat-fixed in a fixing unit (not shown), and discharged outside the apparatus body. In the transfer of a toner image, a transfer current is small in an image portion having a high printing rate with a large amount of toner, and a large transfer current flows in an image portion having a low printing rate. In the present embodiment, the current value of the constant current source is set to correspond to image formation with a low printing rate. Then, when forming an image with a high printing rate, so-called creeping discharge is performed from the back surface of the paper P to the transfer current control electrode 14 by an amount corresponding to the small transfer current and the difficulty in flowing, and the excess current flows to the ground side. .

By the way, the transport roll pair 1 shown in FIG.
In the method in which the sheet P is directly conveyed to the transfer unit by the method 5, an unstable state such as vertical fluttering is likely to occur in the conveyance of the sheet P, and thus image formation is often unstable.

FIG. 2A is a view showing the structure of a belt-transfer type transfer section for conveying a sheet in a stable posture. In this case, the configuration of the transfer unit is the same as that of the above-described direct paper transport system of FIG. Hereinafter, a description will be given based on the belt conveyance system shown in FIG. As shown in FIG. 3A, in the belt transport system, a transport belt 34 and a driving roller for driving the transport belt 34 are used instead of the transport roll pair 15 of FIG. 35 are provided.

The transport belt 34 is arranged in a loop in the horizontal direction, and is stretched between the driving roller 35 and a driven roller on the left side in the figure. The upper surface of the belt is in sliding contact with the photosensitive drum 11, and the arrow B in the figure is used. Circulates counterclockwise as indicated by. The transport belt 34 is made of a synthetic resin such as polyester, polyimide, polycarbonate, or polyamide, or a fluororesin such as polyvinylidene fluoride (PVDF) or ethylene tetrafluoroethylene copolymer (ETFE), and has a thickness of 30 μm.
m (millimicron) to 300 μm, preferably 50 μm
What is formed into a film having a thickness of 200 μm is used. That is, this film member is formed into a belt shape by joining, or is formed into a cylindrical shape, and cut into a belt shape to be used in a seamless belt state. Also, by adding a conductive agent such as carbon black to these materials and forming them,
The resistance of the belt can be adjusted in advance. The transport belt 34 is made of such a dielectric material, and electrostatically attracts the paper P to the outer peripheral surface of the upper circulating portion and transports the paper P to the sliding contact portion with the photosensitive drum 11. The transfer brush 12 includes a transfer belt 34.
Is pressed against the back surface of the sliding portion with the photosensitive drum 11 to apply a charge from the constant current source 13 to the paper P via the transport belt 34 to charge the paper P to a positive potential. The negative toner image on the drum 11 is transferred. In this case, the transfer current control electrode 14 is
mm, and within 10 mm from the lower surface of the conveyor belt 34. Since the transport belt 34 is made of a dielectric material, when the toner image on the photosensitive drum 11 is formed at a high printing rate, for example, the transport belt 34 is charged with a large amount of charges that cannot flow to the photosensitive drum 11 side. . And
In this case as well, the transfer current control electrode 14 receives the creeping discharge from the transport belt 34 at its tip, and lowers the potential of the transport belt 34, thereby controlling the transfer current to the paper P so as not to be unduly large.

FIG. 3B shows the control current flowing through the transfer current control electrode 14 in the image portion having a low printing rate, such as a text image, and the image portion having a high printing rate, which is almost black, due to the above configuration. FIG. Same figure
(b) shows the control current on the vertical axis and the processing time on the horizontal axis.
The period shown in FIG. 7A shows the processing time corresponding to the image portion having a low printing rate, and the period shown in FIG. 7B shows the processing time corresponding to the image portion having a high printing ratio. In this example, the output current of the constant current source 13 is set to 6 μA, A4 size paper is transported (6 PPM) in the vertical direction, and the toner image on the photosensitive drum 11 is changed from “text image” to “black solid”. When the “image” and “black solid image” are changed to the “text image” again, it has been found by measurement that the period A is approximately 4 μA and the period B is approximately 4.8 μA.

As shown in FIG. 2B, comparing the low printing rate in the period A with the high printing rate in the period B, the control current flows more at the high printing rate as described above. . That is, it can be seen that the transfer current flowing to the transfer section is reduced accordingly. That is, it can be seen that the transfer current is small and appropriate in accordance with the high printing ratio. Further, the fact that the control current is small when the printing rate is low indicates that a larger amount of current is flowing in the transfer unit, and that the appropriate large current corresponding to the low printing rate is flowing in the transfer unit. I have.

That is, in the configuration of the transfer section shown in FIGS. 1A and 2A, an excessive current exceeding the transfer current necessary for transferring the image of the printing rate at that time is transferred to the transfer current control electrode 14. As a result, an appropriate amount of current is supplied as a transfer current to the photosensitive drum 11 side, whereby a stable transfer result can be obtained regardless of the printing ratio. As described above, it is not necessary to perform complicated and troublesome processing such as recognizing the printing rate in advance and electrically controlling at the time of writing, and the transfer is performed with a relatively simple configuration in which the transfer current control electrode is arranged at an appropriate position. The current can be controlled more appropriately.

When a fibrous flexible member is used for the transfer current control electrode 14, the fibrous portion is electrically attracted to the transfer brush side to be in contact with or the depilated fiber bridges both. There is a risk of forming a short circuit. In such a case, if an appropriate insulating member such as a sheet or a resin plate is interposed between the transfer current control electrode 14 and the transfer brush 13, the occurrence of the trouble can be prevented. In this case, the discharge performance is somewhat reduced due to the interposition of the insulating member. Therefore, if the distance between the transfer current control electrode 14 and the transfer brush 13 is corrected to be slightly closer, the above description is made without any trouble. A similar appropriate control current is obtained.

When the constant current is controlled to flow directly to the ground side as described above, on the other hand, when the processing speed of the apparatus main body is relatively high, when the paper is thick paper, or when the paper is OHP paper, In either case, the transfer current supplied per unit area of the image may be slightly insufficient due to a short processing time or a large resistance of the paper.

FIGS. 3A and 3B show that even in such a case,
While maintaining the structure of the transfer current control electrode 14 is a diagram to indicate that increasing the transfer current amount to the photoreceptor drum 11 properly. FIG (a) shows the case of the paper directly conveyance method, FIG. (B) shows the case of a belt conveyance method.
In each case, a current control element 36 is arranged in series between the transfer current control electrode 14 and the ground. In the current control element 36,
An element that generates a voltage by a current flow, such as a resistor, a varistor, or a zener diode, is used. The other components are
Since the configuration is the same as that shown in FIGS. 1 (a), (b), (c) and FIG. 2 (a), the same reference numerals as those in FIGS. 1 and 2 are used. The configurations shown in FIGS. 3 (a) and 3 (b) are basically configurations that exhibit basically the same functions.
Will be described. FIG.
FIG. 4 is an equivalent circuit diagram of the belt conveyance system shown in FIG. FIG. 11C shows the numbers assigned to the respective components in FIG. 10B, attached to the equivalent circuits corresponding to the components.

As shown in the equivalent circuit of FIG. 3C, the current flowing from the transfer portion brush 12 is branched into a current flowing to the transport belt 34 and a current flowing to the current limiting element 36. The current flowing to the conveyor belt 34 side is
The transport belt 34 and the paper P are charged in accordance with the capacitance of the paper P and the capacitance of the paper P.
An electric field is formed between the photosensitive drum 11 and the photosensitive drum 11 via a space capacitance 37 between the photosensitive drum 11 and the photosensitive drum 11. The toner image on the photosensitive drum 11 is transferred onto the paper P by this electric field.

The current branched to the other current limiting element 36 side includes the spatial capacitance between the transfer current control electrode 14 and the transport belt 34, the internal resistance of the transfer current control electrode 14, and the voltage generated by the current limiting element 36. Is determined. The capacitance of the transport belt 34, the capacitance of the paper P, the capacitance of the photosensitive drum 11, and the like all increase as the processing speed of the apparatus main body increases. Further, the capacitance of the paper P increases depending on the type of the paper P, for example, for thick paper or OHP paper. At this time, an appropriate voltage is generated in the current limiting element 36 to limit the current flowing to the ground side via the transfer current control electrode 14, thereby increasing the current flowing to the transfer side, thereby increasing the processing speed. In addition, it is possible to secure an appropriate transfer current corresponding to a change in the type of paper.

In the above-described embodiment, an image forming apparatus dedicated to black printing, which has one transfer unit, that is, has a single photosensitive drum 11 and forms a black toner image, will be described. and that although the image forming apparatus is not necessarily limited to the image forming apparatus of the black printing only, the present invention
Are those that can be expected further effect of the photosensitive drum is applied to a full-color image forming apparatus having a multi-stage.

[0040] Figures 4 and 5 shows an embodiment of the present invention, an example of applying the present invention to a full-color image forming apparatus in side sectional view. FIG. 4 is a side sectional view of the entire apparatus main body, and FIG. 5 is an enlarged view showing only a transfer portion serving as a main portion. As shown in FIGS. 4 and 5, the full-color image forming apparatus (apparatus main body) 40 includes an opening / closing tray 41 on the front surface (right side in the figure) of the apparatus main body, and a detachable paper cassette 42 at the lower part. I have. A paper discharge tray 44 is formed on the upper cover 43 at the rear of the upper surface thereof.
The sheets on which images have been formed and discharged from the upper discharge port 45 are stacked there. A power switch, a liquid crystal display, a plurality of input keys, and the like are arranged on the front side of the upper lid 43, which are not visible in the drawing.

In the apparatus main body, a transport belt 48 circulating and held by a driving rotary roller 46 and a driven rotary roller 47 is disposed substantially at the center. The four photosensitive drums 49 (49a, 49b, 49c, and 4c) which are in contact with the upper circulation portion of the transport belt 48 and are arranged in a multistage manner in the paper transport direction.
9d) and transfer brushes 51 (51a, 51b, 51c, 51d) arranged by pressing the contact portion between the transport belt 48 and the respective photosensitive drums 49 from the rear surface of the transport belt 48, and near the transfer brushes 51. An image transfer section including the transfer current control electrodes 52 (52a, 52b, 52c, 52d) respectively disposed is disposed.

The four photoreceptor drums 49 are provided in image forming units 53 (53a, 53b, 53c, 53d), respectively.
It is assembled and assembled. The structure of the image forming unit 53 is the same as that of the image forming unit 20 shown in FIG. 1B, and here, the colors of the toners contained in the developing units are different. In the examples of FIGS. 4 and 5, Y (yellow), M (magenta), C
(Cyan) and Bk (black) toners.

Also in this case, the write head 54 (54a, 54b, 54) fixed to the back surface of the upper lid 43 is provided on the upper surface of the image forming unit 53 directly above the photosensitive drum 49.
c, 54d) are inserted and arranged by closing the upper lid 43. The upper lid 43 opens and closes about a support shaft 55 behind the apparatus main body 40. Open the upper cover 43 and write the write head 5
By pulling up 4, the image forming units 53 can be individually taken out of the machine.

A constant current source 56 is connected to each of the transfer brushes 51 (see FIG. 5) which press the photosensitive drums 49 in opposition to each other via the transport belt 48.
Current limiting elements 57 (57a, 57b, 57c, 57d) are connected in series between the transfer current control electrode 52 and the ground. The individual configuration and function in each transfer section are the same as in the case of FIG.

A paper pressing roller 50 that is in pressure contact with the driven roller 47 via the transport belt 48 is provided at the paper transport entrance at the upstream end of the transport belt 48 in the paper transport direction. A transport roll pair 61 and a paper detection sensor 62 are disposed upstream of the paper carry-in port. The upstream side is branched laterally (front of the apparatus) and downward, and a paper feed roller 63 and a separating member are disposed laterally. 6
4 and the above-mentioned opening / closing tray 41 are provided. A transport path 65 formed by two guide plates is provided below.
The paper cassette 42 described above is located upstream (below the figure) to accommodate a large number of sheets P. A paper feed roller 66 is provided above the paper feed end of the paper cassette 42. Then, downstream of the belt 48 in the paper transport direction,
A separation claw 67, a fixing device 68, a paper discharge roller 69, and a switching lever 71 are provided. The fixing device 68 includes a pressure roller, a heat roller, a peripheral surface cleaner,
It is composed of an oil application roller, a thermistor, etc.
The toner image transferred thereon is heat-fixed to the paper surface. The switching lever 71 guides the sheet P to an upper discharge path 72 when it is at a lower position as shown in the figure, and a discharge port that opens the sheet P to the rear surface of the apparatus when it is rotated upward. Guide to 73. The downstream side of the discharge path 72 communicates with the upper discharge port 45 via a discharge roll pair 74.

Further, a cleaner bottle 75 is detachably provided between the belt 48 and the paper cassette 42. this,
A blade scraper 76 is provided above the cleaner bottle 75.
Is attached, and its tip is in contact with the surface of the lower circulation portion of the transfer belt 48. Blade scraper 76
Removes the toner remaining on the surface of the transfer belt 48 to clean the transfer belt 48, and stores the scraped unnecessary toner in the cleaner bottle 75.

In this configuration, when the full-color image forming apparatus 40 is turned on, and the number of sheets, full-color printing (image formation), and other designations are input by keys, the sheet feeding roller 66 is placed on the sheet cassette 42. The accommodated sheet P is fed to a pair of transport rolls 61 via a transport path 65.
Alternatively, the paper feed roller 63 feeds the paper placed on the open / close tray 41 to the transport roll pair 61. The fed sheet P is detected by the sheet detection sensor 62. The rotation of the transport roll pair 61 is stopped, and the leading end of the paper P is brought into contact with the holding portion to be on standby.

The driving rotary roller 46 starts rotating in the counterclockwise direction, and the driven rotary roller 47 is driven to start rotating in the same counterclockwise direction. As a result, the belt 48
The upper circulating portion abuts on the four photosensitive drums 49 and circulates counterclockwise as a whole.

At the same time, the image forming unit 53 is sequentially driven in synchronization with the print timing, and the photosensitive drum 49 is sequentially driven to rotate clockwise in accordance with the drive. Then, the write heads 54 corresponding to the respective image forming units 53 are sequentially driven. First, the initialization charging brush (see the initialization charging brush 22 in FIG. 1B) applies a uniform charge to the peripheral surface of the photosensitive drum 49, and the write head 54 causes the photosensitive drum to rotate around the photosensitive drum. The surface is exposed according to the image signal to form an electrostatic latent image on the peripheral surface of the photosensitive drum. Developing roller (the developing roller 24 in FIG. 1B)
2) transfers each toner to the low potential portion of the electrostatic latent image to form (develop) a toner image of each color on the peripheral surface of the photosensitive drum.

Y on the peripheral surface of the photosensitive drum 49a at the uppermost stream
At the timing when the leading end of the (yellow) toner image is rotated and conveyed to a point facing the belt 48, the conveying roll pair 6 is moved so that the printing start position of the paper P coincides with the point.
1 starts rotating, and feeds the sheet P to the sheet entrance of the image forming unit. The driven rotary roller 47 and the paper pressing roller 50
Transports the paper P while nipping the fed paper P together with the transport belt 48. Even after the sheet P is released from being nipped by the driven rotation roller 47 and the sheet pressing roller 50, the sheet P is adsorbed to the transport belt 48 and is transported as it is. The toner images are sequentially transferred onto the paper by the current applied from each transfer brush 51. The paper P on which the toner images of four colors of Y (yellow), M (magenta), C (cyan) and Bk (black) are transferred is separated from the conveyor belt 48 by the separation claws 67 and is carried into the fixing device 68. You. After the toner image is thermally fixed on the sheet P by the fixing device 68, the toner image is directed upward from the rear discharge port 73 or the toner image is directed downward from the upper discharge port 45 by the discharge roller 69. Emitted outside the machine.

In this transfer process, the transport belt 4
8, the charge is applied in order from the uppermost stream Y (yellow) transfer section to the M (magenta) transfer section, the C (cyan) transfer section, and the Bk (black) transfer section. Accordingly, the charge amount of the transport belt 48 gradually increases. Therefore, the transfer current from the transfer brush 51 is repelled by the charge of the conveyor belt 48 and becomes difficult to flow.
In this embodiment, the current limiting elements 57a, 57b, 57c and 5
The current limit value of 7d is configured to be sequentially increased.
This makes it difficult for the limiting current to flow from upstream to downstream, so that the transfer current increases in reverse from upstream to downstream, maintaining a more appropriate potential beyond the potential due to charging of the transport belt 48. This allows an appropriate transfer current to flow through the equivalent circuit in each transfer section.

For example, as an example, an OPC made of an organic light guide is used for the photosensitive drum 49, and the transport belt 48 has a thickness of 10 mm.
The transfer brush 51 is made of a 0 μm polyester film.
Is made conductive with carbon black and has a specific resistance of 10 ⁷ Ω · cm
The pile brush of rayon fiber is fixed to an iron plate with a conductive adhesive, and the output of the constant current source 56 is 6
μA, and a brush member formed by sandwiching a carbon fiber between aluminum plates is used for the transfer current control electrode 52. The current limiting elements 57a, 57b, 57c, and 57d have 0 MΩ, 300 MΩ, 500 MΩ, and 800 M
When each of the resistances of MΩ is used, a good image can be obtained by sequentially transferring images formed with toners of different colors sequentially from the first transfer portion at the most upstream to the fourth transfer portion at the most downstream.

As described above, the voltage generated by the current limiting element 57 connected to the transfer current limiting electrode 52 needs to be increased toward the downstream side. However, it is only necessary to increase the upstream side indiscriminately. However, if the voltage generated by the current limiting element 57 is too high, the constant current source 56
All of the output flows to the transfer section, resulting in an excessive current, which causes an obstacle when the sheet is peeled off before fixing.

FIG. 6 shows the measured values of the potential of the transport belt 48 in each transfer section, the voltage (current limit voltage) formed by the current limiting element 57, and the quality of the transferability in those states (conditions). It is a chart showing a judgment. The chart shows the first transfer of the Y (yellow) transfer portion, the second transfer of the M (magenta) transfer portion, and C (cyan) at the left end.
And the fourth transfer of the Bk (black) transfer portion. The next column frame shows the charged potential of the transfer belt 48 immediately before transfer in each transfer step. Further, the next column frame shows the voltages generated by the current limiting elements 57 corresponding to the respective charged potentials, and the rightmost column frame shows the determination of the quality of the transferability by the symbol “○”, This is indicated by “△” or “×”.

As shown in the drawing, the potential immediately before the transfer of the transport belt 48 in the first transfer section is naturally 0 V, but the potential immediately before the transfer of the transport belt 48 in the second to fourth transfer sections is respectively. In addition, each of the transfer sections varies within a certain range depending on the transfer state of the previous transfer section. That is, 700 V to 900 V in the second transfer portion, and 14 V in the third transfer portion.
00V to 1800V, 2100V to 27 in the fourth transfer section
It is in the range of 00V. On the other hand, when the configuration of the current limiting element 57 is variously changed and the voltage applied from the current limiting element 57 to the transfer current control electrode 52 is changed to determine whether the transferability is good or not, as shown in FIG. It is good in the first transfer portion. In the second transfer portion, the current limiting voltage is 950
It can be seen that V and 1080 V are good, that is, the current limit voltage is at least in the range of 950 V to 1080 V. In addition, in the third transfer portion, it is found that the current limiting voltage is good at 1830 V, 1900 V, and 2100 V, that is, the current limiting voltage is good at least in the range of 1830 V to 2100 V. Then, in the fourth transfer section, the current limiting voltages are 2800V, 3040V, 3200V and 3200V.
It is good when it is 400V. Ie at least 2
It can be seen that the range of 800 V to 3400 V is good.

In order to obtain at least a judgment result that the evaluation is “Δ” from the above experimental results, the current limiting voltage needs to be a potential close to the potential of the conveyor belt 48 immediately before transfer. . Preferably, the evaluation is “「 ”. In this case, when the current limiting voltage is Y volts and the accumulated charging potential of the transport belt 48 is X volts,“ X ”
It can be seen that when the generated voltage is applied to the transfer current control electrode 52 such that -100 <Y <X + 1000 ", appropriate transfer properties can be obtained.

Next, another embodiment of the multistage full-color image forming apparatus will be described. Described below full-color image forming apparatus has two printing modes of monochrome printing mode for printing only the color printing mode and black performing Furka La over printing. Then, an appropriate transfer current is obtained corresponding to any print mode .

FIG. 7 shows the structure of each transfer section, which is the main part of the full-color image forming apparatus, and shows the state of each transfer section in the color print mode. FIG. 8 shows a state change of each transfer unit in the monochrome print mode in the same configuration of the main unit. In both figures, the same components as those in FIGS. 4 and 5 are denoted by the same reference numerals as those in FIGS. 4 and 5.

Referring to FIG. 7, in the full-color image forming apparatus, a tension roller 77 is arranged near the drive rotation roller 46 in contact with the inner peripheral side of the lower circulation portion of the conveyor belt 48. The tension roller 77 is rotatably supported at the tip of a rotatable arm lever (not shown), and is urged downward by an urging member (not shown) to convey the conveyance belt 48 regardless of a later-described state change of the conveyance belt 48. The transport belt 48 is entirely stretched with a predetermined tension between the driving rotary roller 46 and the driven rotary roller 47 by absorbing the bending of the belt 48.

A swing arm 78 is disposed between the upper circulation part and the lower circulation part of the conveyor belt 48 and extends parallel to the conveyance direction. The swing arm 78 has a fixed support roller 79 at one end and a movable support roller 81 at the other end. The swing arm 78 is fixedly supported by a fixed roller 79 by a cam (not shown) which slides in contact with the lower portion.
The other end side rotates up and down about the support shaft of.

The fixed support roller 79 is provided with a photosensitive drum 49.
d and a transfer brush 51d, and a transfer unit consisting of a photosensitive drum 49c and a transfer brush 51c, which are located immediately upstream and slightly downstream, and are rotatably mounted on a frame of the apparatus main body. It is fixed and abuts against the inner peripheral surface of the transport belt 48 to support it from below, and the transport belt 48 is always in contact with the most downstream photosensitive drum 49d.

The movable support roller 81 is supported by the rotating free end of the swing arm 78, and the end of the transport belt 48 supported by the driven rotary roller 47, the most upstream photosensitive drum 49a and the transfer brush It is located in the middle of the transfer section 51a. When the swing arm 78 is rotating upward, the movable support roller 81 contacts the inner peripheral surface of the transport belt 48 to support the transport belt 48 from below, as shown in FIG. Each photosensitive drum 49a, 49b,
The transport belt 48 is brought into contact with all of 49c and 49d.

The lowermost transfer brush 51 d is fixed to the frame of the apparatus main body and is in contact with the transport belt 48.
On the other hand, the other transfer brushes 51a to 51c
8, and comes into contact with the transport belt 48 when the swing arm 78 is rotating upward as shown in FIG.
Thereby, four photosensitive drums 4 from upstream to downstream are provided.
A transfer portion is formed corresponding to No. 9, and a four-color printing mode is set.

As in the case of FIG. 5, a constant current source 56 is connected to the transfer brush 51, and transfer current control electrodes 52 (52a, 52a,
52b, 52c, and 52d) are provided respectively. However, in this embodiment, another adjustment current limiting element 82 for further adjustment is provided in parallel with the current limiting element 57d on the transfer current control electrode 52d of the lowermost transfer section for forming a black toner image. Connected via Switch 8
3 is switched so as to be connected to the current limiting element 57d when the color print mode shown in FIG. 7 is set, so that the transfer current of each transfer section is the same as the transfer current described in FIG. Works as if.

Next, the monochrome print mode will be described. In the monochrome printing mode, the swing arm 78 rotates downward as shown in FIG. As a result, the movable support roller 81 moves downward and separates from the transport belt 48. The transport belt 48 is supported between the driving rotary roller 46 and the fixed support roller 79 as before, and the photosensitive drum 49 is supported.
The contact state with respect to d is maintained. On the other hand, between the fixed support roller 79 and the driven rotary roller 47, the movable support roller 81 is separated from the movable support roller 81 so as to be inclined obliquely downward toward the upstream side. Therefore, the three photosensitive drums 49a for forming a color toner image are formed. , 49b and 49c together with the transfer brushes 51a, 51b and 51c.

By the rotation of the swing arm 78, a monochrome printing mode in which only a black toner image is formed by the transfer portion including the photosensitive drum 49d and the transfer brush 51d at the most downstream is set. At this time, the switch 83 connected to the transfer current control electrode 52d switches from the connection with the current limiting element 57d to the connection with the current limiting element 82 for adjustment. In monochrome printing mode, the transfer unit is 1
Since the transfer belt 48 is not charged at other transfer portions, there is no need to increase the transfer current as described above. Therefore, the connection of the current limiting element 57d that generates a strong voltage is released, and the current control according to the printing rate is performed by the adjusting current limiting element 82 that appropriately generates a weaker voltage. Is controlled so that the transfer current flowing through the printer becomes an appropriate current.

Incidentally, even in such a multi-stage configuration,
It is considered that a mode in which the processing speed of the apparatus body is high, a mode in which printing is performed on thick paper, and a mode in which OHP paper is used are required. In this case, the transfer current is reduced as described above, and there is a possibility that a transfer failure occurs.

FIG. 9 is a diagram showing an example of a color image forming apparatus capable of coping with both a print mode requiring such a large amount of transfer current and a print mode requiring only a small amount of normal transfer current. 7 shows only the transfer portion, and FIGS.
The same components as those shown in FIGS. 7 and 8 are given the same reference numerals as in FIGS. The color image forming apparatus shown in FIG. 9 differs in the connection configuration of the current limiting element to the transfer current control electrode 52 in each transfer section. As shown in the figure, each transfer current control electrode 52 has a current limiting element 57 (57a, 57b, 57c, 57c,
7d) are connectably disposed and generate a higher voltage than the current limiting elements 84 (84a, 84b, 84b).
84c, 84d) are provided so as to be connectable. A switch 85 (85a, 85b, 85
c, 85d) are connected, and the connection of the switch 85 is switched to the current limiting element 57 or 84 according to the print mode. FIG. 14 shows a state in which the switch 85 has been switched to the current limiting elements 84 (84a, 84b, 84c, 84d) that generate higher voltages in response to the print mode requiring a large amount of transfer current. I have. This color image forming apparatus also has a full-color and monochrome print mode in addition to the above-described print mode.
Is rotated downward about the fixed support roller 79 in the figure to enter the monochrome print mode, and the transfer current control electrode 52 of the lowermost transfer portion for transferring a black image corresponding to this is set.
In addition to the current limiting elements 57d and 84d, an adjusting current limiting element 82 similar to that shown in FIG.
It is possible to switch the connection.

In the above-described embodiments, the case where the transfer brush is used has been described.
The configuration of the transfer unit is not limited to this, and may be any type of contact-type transfer device, such as a transfer roll, which directly contacts a paper or a conveyor belt and presses it against the toner image carrier.

As described above, according to the present invention, the transfer current control electrode is arranged in the vicinity of the transfer unit, and the output of the constant current source for supplying the transfer current to the transfer unit is changed according to the printing rate. Since the transfer current is adjusted by flowing out to the ground side, the output of the constant current source that maintains the reference current regardless of environmental changes can be appropriately changed according to the printing rate, and therefore, the printing rate often changes. Even in the case of an image, it is possible to print a high-quality image by appropriately transferring any gradation portion. Further, since the ground via a current control element transfer current control electrode, the increase or decrease of the transfer current can be properly changed in response to changes in the print mode such as plain paper or special paper, therefore, diverse printing High quality images can always be used for forms, and applications are expanded. Moreover, since a higher voltage generated current control element toward the downstream from the upstream in the transfer portion of the multi-stage, to obtain a proper transfer current obtained by correcting the charging of the conveyor belt in any of the transfer portion, therefore, the It is possible to print a high-quality color image by proper transfer for each transfer unit. In addition, since two types of current control elements, strong and weak, are provided in each of the multi-stage transfer units and are switched by switches, it is possible to appropriately change the increase and decrease of the transfer current according to the change of the print mode even in color image formation.

[Brief description of the drawings]

1A is a diagram showing a basic configuration of a transfer unit of an image forming apparatus according to the present invention, FIG. 1B is a cross-sectional view showing a configuration of an image forming unit not shown in FIG. 1A, and FIG. Is a perspective view thereof.

FIG. 2A is a diagram illustrating a configuration of a belt-conveying transfer unit;
(b) is a diagram showing the state of the control current flowing through the transfer current control electrode in the image portion having a low printing rate and the image portion having a high printing rate in the configuration of (a).

3 (a) is a diagram showing a case where the sheet directly conveyance method in transcription current appropriate to the configuration of the transfer section to increase the transfer current amount of a control electrode to the photosensitive drum, (b) is a belt conveyor FIG. 3C is a diagram illustrating a case of a transfer method, and FIG. 3C is a diagram illustrating an equivalent circuit of a transfer unit in a configuration of a belt conveyance method.

FIG. 4 is a side sectional view of a full-color image forming apparatus according to an embodiment of the present invention .

FIG. 5 is an enlarged view showing only a transfer portion of the full-color image forming apparatus.

FIG. 6 is a table showing measured values of a potential of a conveyor belt and a current limiting voltage formed by a current limiting element in each transfer unit, and determination of whether transferability is good or not in those states (conditions).

FIG. 7 is a diagram illustrating a configuration of each transfer unit and a state of a color print mode of a full-color image forming apparatus according to another embodiment of the present invention .

FIG. 8 is a diagram illustrating a state of each transfer unit of the full-color image forming apparatus in FIG . 7 in a monochrome print mode.

FIG. 9 is a diagram illustrating an example of a color image forming apparatus capable of coping with a print mode that requires a small transfer current and a print mode that requires a large transfer current.

FIG. 10 is a diagram illustrating a basic configuration of a transfer unit using a transfer brush in a conventional image forming apparatus.

[Explanation of symbols]

Reference Signs List 1 photoconductor drum 2 transfer brush 3 transport roll pair 4 constant current power supply P paper 11 photoconductor drum 11-1 support shaft 12 transfer brush 13 constant current power supply 14 transfer current control electrode 20 image forming unit 21 cleaner 22 initialization charging brush 23 Developing device 24 Developing roller 24-1 Support shaft 25 Toner 26 Toner stirring member 27 Supply roller 28 Doctor blade 29 Slot 31 Writing head 32 Protective lid 34 Transport belt 35 Driving roller 36 Current limiting element 40 Full color image forming apparatus (apparatus main body) 41 Opening / closing tray 42 Paper cassette 43 Top lid 44 Paper discharge tray 45 Upper paper discharge port 46 Drive rotation roller 47 Follower rotation roller 48 Conveyor belt 49 (49a, 49b, 49c, 49d) Photoconductor drum 50 Paper press roller 51 (51a, 51b) , 51c 51d) Transfer brush 52 (52a, 52b, 52c, 52d) Transfer current control electrode 53 (53a, 53b, 53c, 53d) Image forming unit 54 (54a, 54b, 54c, 54d) Write head 55 Support shaft 56 Constant current source 57 (57a, 57b, 57c, 57d) Current limiting element 61 Conveying roll pair 62 Paper detection sensor 63 Feeding roller 64 Separation member 65 Conveying path 66 Feeding roller 67 Separating claw 68 Fixing device 69 Discharge roller 71 Switching lever 72 Discharge Path 73 Discharge port 74 Discharge roll pair 75 Cleaner bottle 76 Blade scraper 77 Tension roller 78 Swing arm 79 Fixed support roller 81 Movable support roller 82 Adjusting current limiting element 83 Switch 84 (84a, 84b, 84c, 84d) ) Current limiting element 85 (85a, 85 , 85c, 85d) switch

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Masato Sakurai 2-229 Sakuragaoka, Higashiyamato-shi, Tokyo Casio Electronics Co., Ltd. (56) References JP-A-8-160777 (JP, A) JP-A Heihei 6-124004 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) G03G 15/16 G03G 15/01 114

Claims (7)

(57) [Claims] (1)Multiple juxtaposedAn image carrier;pluralToner image on image carrierRespectivelyFormpluralG
Toner image forming means;pluralThe toner image is formed by the toner image forming means.
SaidpluralFor image carrierRespectivelyTransfer material to contactPerimeter
Adsorb to the surface and circulate and moveConveyHigh resistance transport belt
And theConveyor beltThe transfer material conveyed byThe transport
From the inside of the belt via the conveyor beltImage bearer
Holding bodyRespectivelyPress to transfer the toner image to the transfer material
TopluralA contact-type transfer device;pluralFor contact type transfer machineRespectivelySupply currentpluralConstant power
Flow supply means,pluralNear the contact type transfer unitRespectivelyArrangement
PutVia the current control element respectivelyGround connection to the main unit
Been saidpluralTransfer power supplied to the contact type transfer unit
Part of the flowEachShunt to groundpluralTransfer current
An image forming apparatus, comprising: a control electrode. Wherein said each of the current control element, a voltage generated in the respective <br/> ground shunt circuit is arranged to be higher relationship toward downstream from upstream in the transport direction of the transfer material < The image forming apparatus according to claim 1, wherein: 3. An image forming apparatus according to claim 1 or 2, wherein said current control element is a resistive element. The 4. Before SL conveyor belt, held in order to contact the same posture transfer material always transferable position relative to the image bearing member of the end of the arrangement direction of the plurality of image bearing members,
A belt moving means for selectively moving a transfer material to another image carrier to a transferable position at which the transfer material should be brought into contact and a non-transferable position at which the transfer material is separated from the image carrier so as not to contact the transfer material. The image forming apparatus according to claim 1, 2 or 3, wherein: 5. A toner image forming means corresponding to the image carrier at the end in the juxtaposition direction forms a black toner image, and a toner image forming means corresponding to another image carrier forms a chromatic toner image. The image forming apparatus according to claim 4, wherein: 6. The transfer current control electrode corresponding to the image carrier at the end in the juxtaposition direction includes two types of current control elements of strong and weak, and the transfer belt has an image carrier at the end in the juxtaposition direction and other components. When it is moved to a position where transfer is possible with the image bearing member, it is connected to the stronger current control element, and only the image bearing member at the end in the direction in which the conveyor belt is juxtaposed can transfer and other image bearing members can be transferred. the image forming apparatus according to claim 4 or 5, wherein further comprising a switching means for switching to connect to the weaker of the current control element when the body is moved to a position which is impossible transfer. 7. A plurality of image carriers, a plurality of toner image forming units for respectively forming toner images on the plurality of image carriers, and the plurality of toner images formed by the plurality of toner image forming units A high-resistance transport belt that adsorbs and circulates and transports the transfer material to the outer peripheral surface thereof so as to make contact with the image carrier, and transports the transfer material transported by the transport belt from the inside of the transport belt to the transport belt. A plurality of contact-type transfer devices for transferring the toner image onto the transfer material by pressing each of the plurality of image carriers via the plurality of image carriers; and a plurality of constant-current supply devices for respectively supplying current to the plurality of contact-type transfer devices. Means, and a conductive portion at the tip is respectively arranged in the vicinity of the plurality of contact type transfer devices, and the transfer currents respectively supplied to the plurality of contact type transfer devices are connected to the apparatus main body via current control elements. Part of the ground to each of the above A plurality of transfer current control electrodes to be passed; and a voltage generated by a predetermined current control element among the plurality of current control elements corresponding to the plurality of transfer current control electrodes is set to Y volt, and the predetermined current control is performed. The charging potential of the transport belt immediately before execution of the transfer process in the transfer unit corresponding to the element is X
In terms of volts, the formula "X-100 <Y <X + 100
An image forming apparatus configured to satisfy “0”.