JPH1159947A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device capable of always properly forming carrying face for a belt at a transfer part without applying great tension to the belt. SOLUTION: Transfer rollers 28 (28a', 28b', 28c', 28d') opposed to photosensitive drums 21 for image forming units 20a, 20b, 20c, 20d are formed to hold a relationship, the diameter of a driving roller 18 >= the diameter of the transfer roller 28d' > the diameter of the transfer roller 28c' > the diameter of the transfer roller 28b' > the diameter of the transfer roller 28a', and rotationally driven at the same rotating angular velocity, the frictional coefficient of the peripheral surface being set smaller than the frictional coefficient of the driving roller 18. The transfer rollers 28 which hold the belt 17 together with the photosensitive drum 21 is rotated while being slid along the belt 17 to carry the belt 17 to be extended to the downstream side. The transfer rollers can be formed with a same diameter and different rotating angular speeds.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus which always forms an appropriate transfer belt conveying surface in a transfer section.

[0002]

2. Description of the Related Art Conventionally, there is an electrophotographic image forming apparatus as shown in FIG. The image forming apparatus shown in FIG. 1A is a tandem type printer having two printing (printing) modes, that is, monochrome and full color, and shows only a main part thereof. In this printer, a flat loop-shaped belt 1 (1a, 1b) is stretched between a driving roller 2a and a driven roller 2b with a predetermined degree of tension. The suction roller 3 is pressed against the driven roller 2 b via the belt 1.

[0003] Four photosensitive drums 4 (4a, 4b, 4c, 4d) are pressed against the outer peripheral surface of the upper circulation section 1a of the belt 1,
Four transfer rollers 5 (5a, 5b, 5c, 5d) are provided on the inner peripheral surface of the upper circulating portion 1a so as to face the photosensitive drums 4.
Is pressed. Although not particularly shown, a cleaner, an initializing charger, a recording head, a developing device, and the like are arranged around each of the photosensitive drums 4.
A separation claw, a fixing device, a paper discharge roller, and the like are provided.

The photosensitive drum 4 has a toner image formed on a peripheral surface thereof by an initialization charger, a recording head, and a developing device. Generally, the toner image is formed on the uppermost photosensitive drum 4a.
, Magenta, cyan, yellow, and black toner images are sequentially formed from the photosensitive drum 4d to the most downstream photosensitive drum 4d. The photosensitive drum 4 conveys the toner image to a portion facing the belt 1 while rotating.

A standby roll pair 6 arranged upstream of the belt 1 feeds a sheet fed from a paper feeder (not shown) on the upstream side to the belt 1 at a print timing.
The suction roller 3 causes the belt 1 to suck the sheet. The belt 1 conveys the paper while circulating in a counterclockwise direction indicated by an arrow A in the figure. The transfer roller 5 is connected to a transfer bias power supply (not shown), and applies the transfer bias to the belt 1. The transfer bias is applied to the sheet by the dielectric effect of the belt 1, and the toner image on the photosensitive drum 4 is transferred to the sheet in contact with the photosensitive drum 4 by the electric field.

The paper on which the toner images of center, cyan, yellow, and black are sequentially transferred from the four photosensitive drums 5a to 5d is separated from the belt 1, guided to a fixing device, and heated by a heating roller of the fixing device. And pressing roller
While being conveyed, the toner image is heat-fixed on the paper surface, and is discharged outside the apparatus by a discharge roller.

When performing full-color printing as described above, the rotating arm 8 that rotates about the support shaft 7 rotates to the upper position shown by the solid line in FIG.
The photosensitive drum 4 is pushed up to the position shown in FIG. On the other hand, when performing monochrome printing, the rotating arm 8 is rotated to the lower position shown by the one-dot chain line 8 'in the figure to move the belt 1 downward as shown by the one-dot chain lines 1a' and 1b 'in the figure. The belt 1 (1) between the support shaft 7 and the driven roller 2b.
a ') is separated from the three photosensitive drums 4a to 4c on the upstream side, and the belt 1 is pressed against only the most downstream photosensitive drum 4d. The developing device corresponding to the photosensitive drum 4d contains black toner.

By the way, when the belt 1 is moved up and down as described above in monochrome printing and full color printing, the belt 1
Fluctuates. If the tension of the belt 1 fluctuates, the paper cannot be conveyed in the correct posture. Therefore, the tension roller 9 is pressed against the back surface of the lower circulating portion 1b of the belt 1 and an urging force is applied downward as shown by the arrow B in the figure. The belt 1 is always stretched. As a result, when the belt 1 moves downward in monochrome printing, the tension roller 9 is lowered by the above-described urging force to the position shown by the one-dot chain line 9 'in FIG. As a result, the lower circulation portion 1b 'of the belt 1 is pushed down, and the tension of the entire belt 1 is maintained.

[0009]

However, in order to maintain a uniform tension at the transfer position (pressure contact position with the belt 1) of the four photosensitive drums 4a to 4d in such a method, it is extremely high. It is required to apply tension to the belt 1. If the tension applied to the belt is low, the belt 1 conveyed by the pressure contact force between the photosensitive drum 4 and the transfer roller 5 at the entrance to the transfer position is indicated by a circle C in FIG. Rippling occurs as shown. When such waving occurs, the movement of the belt 1 varies, which causes color misregistration in an image formed by applying four colors of toner repeatedly, and significantly lowers the quality of the image. Therefore, the urging force applied to the tension roller 9 is increased to maintain the tension of the belt 1 at a high level. However, since a strong tension is applied to the belt 1 in this manner, a large load is applied to the drive system,
There is a problem that the durability of the entire device is reduced.

[0010] The object of the present invention is to solve the above-mentioned conventional problems.
An object of the present invention is to provide an image forming apparatus which always appropriately forms a belt conveyance surface in a transfer unit without applying a large tension to the belt.

[0011]

The structure of the image forming apparatus of the present invention will be described below. An image forming apparatus according to the present invention includes a plurality of image carriers arranged in parallel, a plurality of toner image forming means for individually forming a predetermined color toner image on the plurality of image carriers, and the plurality of image carriers. The transfer material is attached to the outer peripheral surface by being wrapped around a downstream rotating roller and an upstream rotating roller disposed outside the image bearing members at both ends in the juxtaposing direction of the body, and transferring the transfer material to the plurality of image carriers. A transfer belt that circulates so as to sequentially contact the materials, and is pressed against the image carrier via the transfer belt to form a transfer unit corresponding to the plurality of image carriers inside the transfer belt, and the downstream side A plurality of transfer rotations, which are lower than the belt transfer force of the rotating roller but have a transfer assist function for the transfer belt and are supplied with a transfer bias for transferring a toner image to the transfer material being in contact with the image carrier. Roller and multiple transfer rotations above Each of the belt conveying force over La constituted by a transfer rollers conveying force control means for setting as that of the juxtaposed direction downstream side is larger than that of the upstream side.

Further, for example, as set forth in claim 2, the downstream rotation roller has a larger friction coefficient with respect to the transfer belt than any other roller for driving the transfer belt, and The roller has a smaller coefficient of friction with respect to the transfer belt than the downstream rotating roller, and is configured to assist the transfer of the transfer belt while causing slippage between the roller and the transfer belt. Further, for example, as described in claim 3, when the plurality of transfer rotating rollers are the transfer roller D, the transfer roller C, the transfer roller B, and the transfer roller A from the side closer to the downstream side rotation roller to the upstream side, Downstream rotating roller diameter ≧ transfer roller D diameter> transfer roller C diameter> transfer roller B diameter> transfer roller A
The diameter of each roller is formed such that the relationship of “diameter” is satisfied.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a side sectional view schematically illustrating the configuration of the image forming apparatus according to the first embodiment. The image forming apparatus shown in FIG. 1 is a tandem type full color printer. As shown in FIG. 1, the full-color printer (hereinafter, also referred to as an apparatus main body) 10 has an openable / closable paper feed tray 11 at an upper part on the front surface (right side in the figure), and a paper cassette 12 detachable from the front at a lower part. , And an upper cover 14 on the upper surface. The sheet cassette 12 stores a large number of sheets (transfer materials).

An operation panel 15 is provided on the upper surface of the apparatus main body 10 in front of the upper lid 14, and a power switch, a liquid crystal display, and a plurality of input keys (not shown) are provided on the operation panel 15. I have. The upper cover 14 has a rear portion and an upper discharge tray 16 together with a rear upper surface of the apparatus main body 10.
Is formed.

In the inside of the apparatus main body 10, a transfer belt 17 formed in a flat loop shape in the front and rear direction is disposed substantially at the center. Transfer belt (hereinafter simply referred to as belt) 17
Is provided between the driving roller 18 (downstream rotating roller) and the driven roller 19 (upstream rotating roller).
8 to circulate in the counterclockwise direction in the figure.
Above the belt 17, four image forming units 20
(20a, 20b, 20c, 20d) are arranged in a multi-stage manner along the paper transport direction of the belt 17 (from right to left in the figure).

Each of the image forming units 20 includes a photosensitive drum (image carrier) 21 and an initializing charger 22, a developing device 23, a developing roller 24, a cleaner surrounding the peripheral surface of the photosensitive drum 21. 25, etc. (only the devices of the image forming unit 20a are numbered in the figure)
Is assembled.

The four photosensitive drums 21 are arranged on the belt 1
7 are arranged in contact with the upper circulation portion (sheet transport surface).
In the developing units 23 of the image forming units 20a to 20d, respective colors of M (magenta: red dye) toner, C (cyan: greenish blue) toner, and Y (yellow: yellow) toner, which are three subtractive primary colors, are provided. The toner and K (black: black) toner dedicated to printing of a black portion of a character or an image are stored, respectively.

A recording head 27 mounted on the back surface of the upper lid 14 via a support member 26 is disposed between the charger 22 and the developing device 23. Recording head 27
Descends with the closing of the upper cover 14 and faces the photosensitive drum 21 to form an image recording section. The developing roller 24 is rotatably supported by the lower opening of the housing of the developing device 23 and presses against the photosensitive drum 21 to form an image developing section there. Then, the transfer rotating roller 28
(28a, 28b, 28c, 28d) are disposed in contact with the back surface of the upper circulation portion of the belt 17. A transfer rotating roller (hereinafter, referred to as a transfer roller) 28 faces the photosensitive drum 21 via the belt 17 and forms an image transfer portion there.

An auxiliary roller 29 is pressed against the upstream end of the belt 17 supported by the driven roller 19 in the paper transport direction to form a paper carry-in portion, and a paper thickness sensor is interlocked with the auxiliary roller 29. 31 are provided. A standby roll pair 32 is disposed on the upstream side in the sheet transport direction from the sheet carry-in section, and the upstream thereof is branched laterally and downward, and in the lateral direction, a sheet feed roller 33, a separating member 34, and the above-described opening / closing member. A paper feed tray 11 is provided. A feed path 35 composed of two guide plates is formed below, and a pair of transport rolls 36 is disposed at the upstream (lower) end thereof. Below the feed path, the feed end of the paper cassette 12 described above is provided. positioned. Above the paper feed end of the paper feed cassette 12, a paper feed roller 37 having a half-moon cross section is disposed.

On the other hand, downstream of the belt 17 in the paper transport direction,
At the position opposing the drive roller 18, a conveyance section separation claw 38 is disposed in pressure contact with the belt 17, and at a position slightly away from the drive roller 18, a tension roller 39 is disposed in pressure contact with the lower circulation section back surface of the belt 17. Is done. The transport section separation claw 38 separates the sheet adsorbed on the belt 17 and carried out from the belt 17 and correctly guides the sheet downstream. Tension roller 39
The belt 17 is urged downward so that a constant tension is applied to the belt 17 so that the paper transport surface (upper circulating portion) of the belt 17 always keeps a correct plane.

A fixing unit 40 is disposed downstream of the transport separation claw 38. The fixing unit 40 has a heat-insulating roller 41, a pressure roller 42, an oil application roller 43, a fixing unit separating claw 45, and the like assembled in a heat insulating housing. A carry-out roll pair 46 and a switching lever 47 are disposed behind the fixing unit 40 (downstream in the sheet conveyance direction). When the switching lever 47 is at a position rotated downward as shown in the figure, the sheet conveyed from the fixing unit 40 by the discharge roll pair 46 is moved to the upper discharge path 48 and the discharge roller 49 at the end thereof. The paper is guided to the upper paper discharge tray 16 through the upper paper discharge tray 16, while the paper is guided to the rear paper discharge port 51 when the paper is rotated upward.

Further, between the belt 17 and the paper cassette 12, there is provided an electrical unit 52 on which an appropriate circuit board can be mounted. A control device including a plurality of electronic components is mounted on a circuit board mounted on the electrical unit 52. The control device includes a controller and an engine. The controller includes a CPU (central processing unit), a ROM (read only memory), an EEPROM (rewritable read memory), a frame memory, an image data transfer circuit, and the like. Then, print data input from a host computer or the like is analyzed, print data is created, and transferred to the engine unit.

The engine unit includes a CPU, a ROM, and the like.
On the input side, data and command signals from the controller unit, outputs of various sensors such as a paper thickness sensor are input, and on the output side, a motor driver for driving a plurality of motors as power sources,
A clutch driver for switching between drive and cutoff of a drive transmission system for transmitting the drive of the motor to each section, a print driver for driving the recording head 27 based on the print data,
A bias power supply driver that supplies a predetermined bias voltage (or current) to the initialization charger 22, the developing roller 24, the transfer roller 28, and the like is connected. The engine unit controls the driving of each unit based on data and command signals from the controller unit, the output of each sensor, and the like.

In the first embodiment, each transfer roller 28a, 28b, 28c in the drive transmission system is used.
And 28d, respectively, so that the driving rotation speeds are different.
That is, the gear ratio of the drive transmission system (not shown) is set such that the rotation speed of the downstream roller is higher than that of the upstream roller from the most upstream transfer roller 28a to the most downstream transfer roller 28d in the paper transport direction. Is set. However, as a characteristic of the member forming the peripheral surface of each transfer roller 28, the coefficient of friction with the belt 17
Is set so as to be smaller than the friction coefficient.
Thus, the transfer rollers 28 (28a, 28
b, 28c, 28d) sandwich the upper circulating portion of the belt 17 with the photosensitive drum 21 of the image forming unit 20 (20a, 20b, 20c, 20d) opposed thereto.
It rotates with a slight slip.

FIG. 2 is a side sectional view showing the state of conveyance of the belt in the transfer section having the above configuration. As shown in the figure,
At the entrance of the belt 17 passing through the transfer section formed at the opposing portion of the photosensitive drum 21 and the transfer roller 28 as indicated by the circle D in the figure, the transfer roller 28 of this transfer section is located at the previous stage (immediately). Since the belt 17 is rotating at a slightly higher rotational speed than the transfer roller 28 (upstream), and the belt 17 is conveyed so as to be stretched downstream, the belt 17 flattens the paper conveying surface without causing a waving phenomenon. Enters the transfer section while maintaining the proper state, and passes. Further, since the transfer roller 28 is conveyed while sliding with respect to the belt 17 as described above, the belt 17 is not pulled out unnecessarily downstream, and the belt 17 reduces the conveyance force of the drive roller 18. It is conveyed at the appropriate speed. Therefore,
Excessive stress is not generated on the belt 17 other than the force for maintaining the posture correctly (flat). In this manner, the toner images of four colors are transferred onto the paper conveyed to the belt 17 circulating in the correct posture at the correct position without any positional deviation due to the waving. As a result, a high-quality image is formed.

In the above-mentioned embodiment, the gear ratio of the drive transmission system of the transfer roller is changed for each transfer roller in order to make the rotation speed of the transfer roller 28 different. In order to make the transfer roller different, the diameter of each transfer roller may be changed without being limited to this. This will be described as a second embodiment.

FIG. 3 is a side sectional view showing a configuration of a main part of an image forming apparatus according to the second embodiment. In the figure, the same components as those shown in FIG. 1 are assigned the same reference numerals as those in FIG. As shown in FIG. 3, each of the image forming units 20a, 20b, 20c and 2
Transfer roller 28 opposing photosensitive drum 21 of 0d
a ', 28b', 28c 'and 28d' are formed such that their diameters gradually increase from upstream to downstream, and that the diameter of the most downstream transfer roller 28d 'does not become larger than the diameter of the drive roller 18. Have been.

That is, it is set so that the relationship of “diameter of drive roller 18 ≧ diameter of transfer roller 28d ′> diameter of transfer roller 28c ′> diameter of transfer roller 29b ′> diameter of transfer roller 28a ′” is satisfied. . The gear ratio of the drive transmission diameter for rotationally driving these transfer rollers 28a ', 28
b ', 28c', and 28d 'are formed to rotate at the same rotational angular velocity. In this case as well, each of the transfer rollers 28a ', 28b', 28c 'and 28d'
Is a characteristic of a member forming the peripheral surface thereof.
7 is set to be smaller than the friction coefficient between the drive roller 18 and the belt 17. Therefore, also in this case, these transfer rollers 28a 'and 28
b ', 28c' and 28d 'sandwich the upper circulation portion of the belt 17 with the photosensitive drum 21 of the image forming units 20a, 20b, 20c and 20d opposed thereto, respectively.
It rotates with a slight slip.

The downstream transfer rollers 28b ', 2b
As described above, since the diameter of the transfer roller 8c 'or 28d' is larger than that of the upstream transfer roller 28a ', 28b' or 28c ', it rotates at a rotational speed slightly higher than that of the upstream transfer roller. Then, the belt 17 is conveyed so as to be stretched downstream. Thus, also in this case, the belt 17 enters and passes the transfer section while maintaining the flat state of the sheet conveying surface without causing the waving phenomenon. Also in this case, the transfer rollers 28a 'and 28
Since the belts b ', 28c' and 28d 'are conveyed while sliding on the belt 17, the belt 17 is not pulled out unnecessarily downstream.
7 is conveyed at a speed corresponding to the conveying force of the driving roller 18. Therefore, no unreasonable stress is generated on the belt 17 other than the force for maintaining the posture correctly (flat). As a result, on the paper conveyed to the belt 17, the four color toner images are superimposed and transferred to a correct position without positional deviation due to waving, and a high quality image is formed.

As described above, even if the bending of the belt 17 and the waving of the belt 17 in the transfer section are eliminated, if the paper is thicker than a normal office paper, for example, as a postcard, the transfer is usually performed. Since the holding force of the photoconductor drum and the transfer roller on the belt in the section is set so as to form a pressing force corresponding to the thickness of ordinary office paper, a thick sheet such as the postcard described above is transferred to the transfer section. When the sheet enters the sheet, the thickness of the sheet becomes an obstacle, and a resistance force greater than that in the case of normal paper is generated in the holding portion, and there is a possibility that the conveyance force becomes uneven.

FIGS. 4 (a) and 4 (b) are diagrams showing the configuration of the main part of the image forming apparatus for solving the problem caused by the difference in the thickness of the paper in the third embodiment, and the operating state thereof. It is. In FIG. 2A, the same components as those shown in FIG. 1 are denoted by the same reference numerals as those in FIG. FIG. 6B shows the difference in the position of the operation result, and only the components required for the explanation and having a change in the operation are numbered.

As shown in FIG. 1A, the image forming units 20a to 20d fixed to the image forming unit receiving portions (not shown) of the apparatus main body 10 shown in FIG.
The belt 17 which comes into contact with the photosensitive drums 21 and the transfer roller 2 which presses and supports the belt 17 from behind.
8, a driving roller 18 for driving the belt 17, a driven roller 19 for supporting the belt 17 following the driving roller 18, and a belt 17
The tension roller 39 and the like for maintaining the tension of the lens at a constant level are supported by the moving frame 55. This moving frame 5
5, two moving rollers 56a and 56
b is disposed so as to be rotatable clockwise and counterclockwise at appropriate intervals as indicated by double-headed arrows E and E 'in the figure. The transfer rollers 56 (56a, 56b) include:
A gentle convex portion 57 is formed on a part of the circumference.

The state shown in FIG. 4A shows the positions of the belt 17 and the transfer roller 28 when printing normal office paper, that is, thin paper.
The protruding end of the convex portion 57 of 6 is in contact with the lower surface of the moving frame 55. For example, when printing on a postcard, FIG.
When the postcard is fed from the standby roll pair 32 to the paper carry-in section, the paper thickness sensor 31 detects the thickness of the postcard. The engine unit of the control device of the electrical unit 52 controls the movement roller 56 based on the thickness information detected by the sheet thickness sensor 31.
Is rotated 90 degrees, for example, as shown in FIG.
The convex portion 57 is released from contact with the moving frame 55, thereby pulling down the belt 17 and the transfer roller 28 downward, that is, in a direction away from the photosensitive drum 21, and circulating the photosensitive drum 21 and the belt 28 upward. Gap F between
To form As a result, paper, such as a postcard, which is thicker than normal paper, can enter the transfer section without receiving resistance due to the thickness, and stable image formation can be performed.

The gap F shown in FIG. 4B is shown as a gap formed between the photosensitive drum 21 and the belt 17 for convenience of explanation, but the belt 17 is actually a photosensitive body in this case as well. It is in contact with the drum 21 and the difference between the gap F in FIG. 3A and the gap F in FIG. 3B is actually a visual indication of the strength of the pressing force. In addition, the rotation drive of the moving roller 56 is not limited to 90 degrees. For example, in the case of OHP paper that is thicker than ordinary office paper and thinner than postcards,
The gap F is rotated by 45 degrees in the middle between FIGS.
May be set narrower than FIG.

In the first to third embodiments,
In each case, a printer that performs only full-color printing is described. However, a printer having two types of printing modes, that is, monochrome and full-color printing, can be configured in the same manner as described above. That is, the configuration of the first embodiment relates to the drive system of the transfer roller. In the case of a printer having two types of printing modes, that is, monochrome and full color, the drive system of the transfer roller is sequentially increased in the downstream direction. May be configured. The configuration in the second embodiment relates to the diameter of the transfer roller, and can be applied to a transfer roller of a printer having two printing modes, monochrome and full color.

FIGS. 5A and 5B are diagrams showing the configuration of the main part of an image forming apparatus according to the fourth embodiment, which is applied to a printer having two printing modes, monochrome and full-color. FIGS. 4A and 4B are diagrams illustrating a configuration of a printer having a mechanism for forming a gap between a photosensitive drum and a belt according to the thickness of a sheet, as in the cases of FIGS. 5 (a) and 5 (b)
1 is almost the same as that of FIG. 1 except for a part, and FIGS. 5A and 5B show the same components as those of FIG. Are given.

As shown in FIG. 5 (a), the transfer which opposes the photosensitive drums 21 (21a, 21b, 21c, 21d) of the image forming units 20a, 20b, 20c and 20d, not shown, via the belt 17 is shown. Roller 28 (28a,
28b, 28c, 28d) are arranged on the moving frame 58 and the bracket 59 connected to the moving frame 58. The moving frame 58 has two points near the front and rear ends connected to the upper ends of the two connecting rods 61 (61a, 61b). The lower ends of the connecting rods 61 are
The ends of (62a, 62b) are connected to the tips of rotating arms 63 (63a, 63b) projecting perpendicular to the shaft 62, respectively. The shaft 62 is connected to an electromagnetic clutch 64 (64a, 6
4b) are engaged with the clutch gears 65 (65a, 65b), respectively. These clutch gears 65 drive gears 68 of motors 67 (67a, 67b) via one-way rotating gears (one-way gears) 66 (66a, 66b).
(68a, 68b).

Also in this case, the standby roll pair 32 shown in FIG.
When a postcard is fed from the printer to the paper carrying unit, the paper thickness sensor 31 detects the thickness of the postcard. Based on the thickness information detected by the sheet thickness sensor 31, the engine unit of the control device of the electrical unit 52 controls the motor 6 by a rotation amount corresponding to the thickness.
7 is driven clockwise in the figure.

As a result, the clutch gear 65 is driven via the one-way gear 66 to rotate the shaft 62 in the clockwise direction, and the rotating arm 63 fixed to this rotates the arrow H in FIG.
By rotating clockwise as indicated by H ', the moving frame 58 and the bracket 59 are moved downward by the distance F' according to the thickness information via the connecting rod 61. Also in this case, by moving the distance F ', a postcard thicker than a normal sheet can enter the transfer unit without receiving resistance due to the thickness.
Stable image formation can be performed.

When printing normal paper from the above state, the power supply to the electromagnetic clutch 64 is cut off and the shaft 62 is turned off.
And the clutch gear 65 is disconnected, whereby
The rotating arm 63 is rotated upward by the urging force of the urging member (not shown), and also comes into contact with a stopper (not shown) to stop at the reference position, that is, the position shown in FIG.

The shaft 6 connecting the moving frame 58 and the bracket 59 at the position shown in FIG.
9 is fixed to the frame of the apparatus main body 10, and the motor 68a
Is further driven in the clockwise direction by a predetermined amount, so that the moving frame 58 is rotated in the clockwise direction with the shaft 69 as a fulcrum, so that the three upstream transfer rollers 28a,
28b and 28c are released from the pressure contact with the belt 17,
As a result, only the black toner transfer portion composed of the photosensitive drum 21d and the transfer roller 28d functions effectively,
An arrangement corresponding to the monochrome print mode can be formed.

In each of the above-described embodiments, the transfer roller (or the transfer roller and the belt) is moved downward according to the thickness of the sheet to form the gap F (F '). The formation of the gap F (F ′) is not limited to this, and the gap may be formed by moving the photosensitive drum, that is, the image forming unit side, upward. Hereinafter, this will be described as a fifth embodiment.

FIGS. 6A and 6B are views showing the structure of the main part of an image forming apparatus according to a fifth embodiment for appropriately forming a gap between transfer portions in accordance with the thickness of a sheet. . This figure 6
The overall configuration of the image forming apparatus shown in FIGS. 5A and 5B is substantially the same as the configuration described in FIGS. 5A and 5B except for a part.
5 (a) and 5 (b) show the same components as in FIGS. 5 (a) and 5 (b).
The same numbers as in (a) and (b) are given.

As shown in FIG. 6A, in this embodiment, the transfer roller 28 is fixed to the frame of the apparatus main body 10 together with the belt 17. The upper end of the connecting rod 61 is connected to the front and rear two points of the movable positioning member 71, and the bottom surfaces of the four image forming units 20 are mounted on the upper surfaces of the abutting portions 72 erected at various positions of the movable positioning member 71. The parts are in contact with each other and positioned. When the thick paper is fed, the motor 67 is driven in a counterclockwise direction in this case by a rotation amount corresponding to the thickness of the paper as in the case of FIGS. 5 (a) and 5 (b). As a result, the rotating arm 63 rotates counterclockwise to move the movement positioning member 71 upward. As a result, the image forming unit 20 is pushed upward to form a predetermined gap in the transfer section.

The support shaft 75 of the photosensitive drum 21 of each image forming unit 20 is provided with a notch groove 74 formed in the frame 73 of the apparatus main body 10 as shown in FIG.
The photosensitive drum 21 is vertically slidably fitted to the photosensitive drum 21 so that the photosensitive drum 21 is freely positioned vertically by the movable positioning member 71 via the image forming unit 20, and is fixed by the cut groove 74 in the sheet transport direction. Positioned.

[0046]

As described above in detail, according to the present invention, the transfer roller is provided with an auxiliary function of conveying the belt with a smaller frictional force than the drive roller, and the surface speed at which the transfer roller rotates is reduced downstream. Since the side is set to be larger, no waving occurs in the transfer section even if the belt is not strongly stretched, thereby reducing the load on the drive system by the reduced tension of the belt. Therefore, the durability of the entire apparatus is improved, and a high-quality image with no displacement can be formed.

[Brief description of the drawings]

FIG. 1 is a side sectional view schematically illustrating a configuration of an image forming apparatus according to a first embodiment.

FIG. 2 is a side cross-sectional view illustrating a favorable belt conveyance state in a transfer unit according to the first embodiment.

FIG. 3 is a side sectional view illustrating a configuration of a main part of an image forming apparatus according to a second embodiment.

FIGS. 4A and 4B are diagrams illustrating a configuration of a main part of an image forming apparatus and an operation state thereof according to a third embodiment for solving a problem caused by a difference in sheet thickness;

FIGS. 5 (a) and 5 (b) show main components of an image forming apparatus which eliminates an obstacle caused by a difference in sheet thickness in a fourth embodiment and has two types of printing modes, monochrome and full color. FIG. 3 is a diagram illustrating a configuration.

FIGS. 6A and 6B are diagrams illustrating a configuration of a main part of an image forming apparatus that appropriately forms a gap between transfer portions according to a sheet thickness according to a fifth embodiment.

7A is a diagram illustrating only a main part of a conventional electrophotographic tandem image forming apparatus (printer), and FIG. 7B is a diagram illustrating a problem of waving occurring on a belt in a transfer unit.

[Explanation of symbols]

Reference Signs List 1 (1a, 1b) Belt 2a Drive roller 2b Followed roller 3 Suction roller 4 (4a, 4b, 4c, 4d) Photoconductor drum 5 (5a, 5b, 5c, 5d) Transfer roller 6 Standby roll pair 7 Support shaft 8 times Moving arm 9 Tension roller 10 Full-color printer (apparatus main body) 11 Open / close paper feed tray 12 Paper cassette 14 Upper lid 15 Operation panel 16 Upper paper discharge tray 17 Transfer belt (belt) 18 Drive roller 19 Follower roller 20 (20a, 20b, 20c, 20d) Image forming unit 21 (21a, 21b, 21c, 21d) Photoconductor drum 22 Charger 23 Developing device 24 Developing roller 25 Cleaner 26 Support member 27 Recording head 28 (28a, 28b, 28c, 28d) Transfer roller 29 Auxiliary roller 31 Paper thickness sensor 32 Standby roll pair REFERENCE SIGNS LIST 3 paper feed roller 34 separating member 35 feed path 36 transport roll pair 37 paper feed roller 38 transport section separation claw 39 tension roller 40 fixing section 41 heating roller 42 press contact roller 43 oil application roller 45 fixing section separation claw 46 discharge roller pair 47 Switching lever 48 Discharge path 49 Discharge roller 51 Rear discharge port 52 Electrical unit 55 Moving frame 56 (56a, 56b) Moving roller 57 Convex part 58 Moving frame 59 Bracket 61 (61a, 61b) Connecting rod 62 (62a) , 62b) Shaft 63 (63a, 63b) Rotating arm 64 (64a, 64b) Electromagnetic clutch 65 (65a, 65b) Clutch gear 66 (66a, 66b) One-way rotating gear (one-way gear) 67 (67a, 67b) Motor 68 (68a, 68b) Drive gear 69 Axis 71 Movement positioning Wood

Claims (3)

[Claims] A plurality of image carriers arranged side by side; a plurality of toner image forming means for individually forming a predetermined color toner image on the plurality of image carriers; The transfer material is stretched over a downstream rotation roller and an upstream rotation roller disposed outside the image carriers at both ends in the installation direction, adsorbs the transfer material to the outer peripheral surface, and sequentially transfers the transfer material to the plurality of image carriers. A transfer belt that circulates and moves so as to be in contact with the image bearing member via the transfer belt to form a transfer unit corresponding to the plurality of image carriers inside the transfer belt; With a transfer assist function for the transfer belt, which is lower than the belt transfer force,
A plurality of transfer rotating rollers to which a transfer bias for transferring a toner image to the transfer material being in contact with the image carrier is provided; and a belt conveying force of each of the plurality of transfer rotating rollers, which is set in the parallel direction. An image forming apparatus comprising: a transfer rotating roller conveyance force control unit that sets a downstream roller to be larger than an upstream roller. 2. The transfer belt according to claim 1, wherein the downstream rotation roller has a larger coefficient of friction with respect to the transfer belt than any other roller that drives the transfer belt, and the transfer rotation roller has a lower friction coefficient with respect to the transfer belt. 2. The image forming apparatus according to claim 1, wherein the image forming apparatus has a smaller coefficient of friction than the side rotation roller and assists the transfer of the transfer belt while causing slippage between the transfer belt and the transfer belt. 3. When the plurality of transfer rotating rollers are a transfer roller D, a transfer roller C, a transfer roller B, and a transfer roller A from a side closer to the downstream side rotation roller to an upstream side,
“Diameter of the downstream rotating roller ≧ diameter of the transfer roller D>
The diameter of each of the rollers is formed such that a relationship of "diameter of the transfer roller C> diameter of the transfer roller B> diameter of the transfer roller A" is satisfied.
The image forming apparatus as described in the above.