JP2005181698A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus in which shock jitters (horizontal band-like density irregularity) occurring especially at the rush of transfer paper to a fixing part can be reduced, the void of superposed characters in a color mode can be prevented, and consequently images of high picture quality can be formed. <P>SOLUTION: The image forming apparatus having speed differences among photoreceptor drums 11BK, 11Y, 11C, 11M, and a transfer belt 60 and having a plurality of paper feeding modes has a paper feeding mode set to (surface speed of photoreceptor drum)>(surface speed of transfer belt) and a paper feeding mode set to (surface speed of photoreceptor drum)<(surface speed of transfer belt). <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a quadruple tandem electrophotographic image forming apparatus, and more particularly to an image forming apparatus that has a contact / separation mechanism of a transfer portion and has a color mode and a monochrome (monochrome) mode.

Conventionally, a transfer sheet is conveyed by a transfer belt in a quadruple tandem configuration, and a bias applying member is disposed at a position facing the photosensitive drum via the transfer belt, and a bias is applied to the bias applying member to thereby provide a photosensitive member. A color image forming apparatus configured to sequentially transfer toner images on a drum onto transfer paper is known (see, for example, Patent Documents 1 to 4).
In this quadruple tandem image forming apparatus, the transfer sheet is generally conveyed at the same speed as the transfer belt surface because the transfer sheet is conveyed by being attracted to the transfer belt.
With regard to the speed of the transfer belt and the surface of the photosensitive drum, transfer is performed only by electrostatic force when the speed is the same, but when the speed difference is added, mechanical stripping force is added in addition to electrostatic force. In addition, image quality deterioration due to poor transfer can be prevented.
More specifically, since the surface speed of the transfer belt and the photosensitive drum is set to (surface speed of the photosensitive drum)> (surface speed of the transfer belt), the downstream side of the transfer nip becomes the loose side. In addition, since the transfer nip pressure is microscopically reduced, it is possible to improve voids generated in a character portion in which two or more colors are superimposed.
When there is no color superimposition such as a single color, the toner layer is relatively easily held on the paper surface, and it is easy to ensure transferability of a certain level or more. However, the part where two or more colors are superimposed is already on the transfer paper. Since there is a transferred toner layer, such a problem occurs when the transferability of new toner is poor.
Therefore, a method is effective in which there is a speed difference between the surface speed of the transfer belt and the photosensitive drum so that (surface speed of the photosensitive drum)> (surface speed of the transfer belt).
Thus, an apparatus aimed at improving the transferability of two or more colors is such that (surface speed of the photosensitive drum)> (surface speed of the transfer belt) on the surface of the transfer belt and the surface of the photosensitive drum. The system set in this way is designed to maintain the same speed difference (same speed ratio) regardless of the single color or color mode, and at least it is designed not to reverse positively or negatively depending on the mode. It is common.
For example, Japanese Patent Application Laid-Open No. H10-228707 discloses a conventional technique in which a transfer belt and a photosensitive drum have a speed difference, and the photosensitive drum speed> the transfer belt speed.
On the other hand, it has been experimentally known that the life of the photosensitive drum is substantially proportional to the photosensitive drum driving time, and in order to achieve a longer life of the photosensitive drum, the photosensitive drum is not used at the time of use. A method of stopping is proposed.
For example, in Patent Document 2, a color (CMY) photosensitive drum other than a BK photosensitive drum that is frequently used is configured to be separated from a transfer belt. The drum is stopped to achieve a long life of the color (CMY) photosensitive drum.
JP 2001-201902 A JP-A-9-146383 JP 2001-005363 A JP 2002-351177 A

Conventionally, as described above, the color (CMY) photosensitive drum is separated from the transfer belt, only the BK photosensitive drum is brought into contact with the transfer belt, and further between the photosensitive drum and the transfer belt (photosensitive drum). Speed difference)> (transfer belt surface speed).
In this case, the transfer belt and the transfer paper on the downstream side of the transfer nip are on the loose side, and the transfer portion that transfers only one color of the transfer paper is in contact with the photosensitive drum, so that the leading edge of the transfer paper is fixed. When entering the part, speed fluctuation occurs.
In the color mode, since the transfer paper is held by the four photosensitive drums, it is difficult for the speed of the transfer paper to vary. When the speed variation occurs on the transfer paper, an abnormal image having a horizontal band-like density unevenness is generated in a halftone image or the like (this horizontal band-shaped abnormal image is referred to as a shock jitter).
In order to solve this problem, in the BK mode, there is no missing character in a single color (the margin is high), and therefore, the missing character in the character is defined as (photosensitive drum surface speed)> (transfer belt surface speed). It is not necessary to secure a margin.
However, on the contrary, by setting (photosensitive drum surface speed) <(transfer belt surface speed) and eliminating the looseness on the downstream side of the transfer belt, the shock jitter (paper speed fluctuation) at the time of fixing entry is reduced. Can be reduced.
In Patent Document 1, the speed difference between the photosensitive drum peripheral speed and the (intermediate) transfer belt peripheral speed is made different between the full color mode and the monochrome mode. For example, in the full color mode, when the peripheral speed of the photosensitive drum is larger than the peripheral speed of the transfer belt, it may be small.
Even in monochrome mode, there is a difference in peripheral speed, but there is a difference in the absolute value of the difference itself. There are an example in which the peripheral speed of the photosensitive drum is made faster than that in full color, and an example in which the peripheral speed of the transfer belt is made faster than in the case of color, and there is also an example in which the speed is almost constant.
The above-mentioned Patent Document 1 has a configuration that does not have a transfer contact / separation mechanism in the single color mode, and a transfer portion that forms an image has a linear velocity difference, and a transfer portion that does not form an image has no linear velocity difference. This technology is characterized by
Further, although the speed difference is made different between the monochromatic mode and the color mode, the peripheral speed of the photosensitive drum and the peripheral speed of the transfer belt are always the same and are not reversed.

In Patent Document 3, there is a speed difference between the photosensitive drum speed and the transfer belt speed, and the photosensitive drum may be faster, slower, or constant speed.
In Patent Document 4, the peripheral speed of the transfer belt is faster than that of the photosensitive drum in the color mode, and both are constant speed in the monochrome mode. Patent Document 4 does not describe reversing the linear velocity ratio between the color mode and the monochrome mode.
Patent Document 4 is characterized in that the linear velocity difference is set to be smaller in the monochrome mode than in the color mode, or the monochrome mode is set to a constant speed. Therefore, there is no mention that the photosensitive drum peripheral speed and the transfer belt peripheral speed are reversed depending on the mode.
The object of the present invention is to reduce the shock jitter (horizontal band density unevenness) that occurs particularly when the transfer paper enters the fixing section and to prevent the color mode from being overwritten in consideration of the above situation. An object is to provide a high-quality image forming apparatus.

In order to solve the above-mentioned problem, the invention according to claim 1 is an image forming apparatus having a plurality of paper passing modes and having a speed difference between the photosensitive drum and the transfer belt by the speed setting means. In the speed setting means, (sheet surface speed of the photosensitive drum)> (surface speed of the transfer belt) is set so that (surface speed of the photosensitive drum) <(surface speed of the transfer belt) is set. The sheet passing mode can be set.
According to a second aspect of the present invention, there is provided the image forming apparatus according to the first aspect, wherein the speed is set by the speed setting means in such a configuration that the transfer is sequentially performed while directly transferring the transfer paper in a four-tandem configuration. .
The invention according to claim 3 has a monochrome mode or a monochrome mode and a color mode, and the setting of (surface speed of the photosensitive drum) <(surface speed of the transfer belt) is set in the monochrome mode or the monochrome mode. 3. The image forming apparatus according to claim 1, wherein (image surface speed of photosensitive drum)> (surface speed of transfer belt) is set in the color mode.
According to a fourth aspect of the present invention, there is provided the image forming apparatus according to any one of the first to third aspects, wherein the photosensitive member and the transfer belt are in contact with only one color in the black and white or single color mode. And
According to a fifth aspect of the present invention, the image forming apparatus according to any one of the first to fourth aspects is characterized in that the sheet passing speed is faster in the monochrome mode or the monochrome mode than in the color mode.
According to a sixth aspect of the present invention, the image forming apparatus according to any one of the first to fifth aspects is characterized in that the roller for driving the transfer belt is located downstream of the transfer nip.
The invention according to claim 7 is the image forming apparatus according to any one of claims 1 to 6, wherein the transfer portion closest to the roller for driving the transfer belt is the most downstream transfer portion. .

According to the present invention, in the setting of (photosensitive member surface speed)> (transfer belt surface speed), the pressure is reduced microscopically because the downstream side of the transfer nip is the loose side. Missing characters such as overlapped characters become worse when the pressure at the transfer nip is high. Therefore, by setting such a setting, there is an effect of preventing the omission of overlapped characters in the color mode.
Incidentally, since the toner directly adheres to the paper in the monochromatic character, the adhesion force between the paper and the toner is larger than the adhesion force between the photosensitive member and the toner, and the character void does not occur (the margin is high).
On the other hand, in the black monochromatic mode in which the transfer unit has a contact / separation mechanism, since there is one transfer unit that comes into contact with the photosensitive drum, compared to the color mode that has four transfer units in contact with the photosensitive drum. It is easy to be affected by shock jitter that occurs when the fixing portion of the transfer paper enters (it is easy to cause horizontal band-like density unevenness due to paper speed fluctuations).
However, in the BK mode, as described above, there is no character missing, so it is not necessary to secure a margin for the missing as (photosensitive drum surface speed)> (transfer belt surface speed). Rather, as (photosensitive drum surface speed) <(transfer belt surface speed), it is possible to reduce shock jitter (paper speed fluctuation) upon entering the fixing belt by eliminating looseness on the downstream side of the transfer belt. .

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic configuration diagram showing an embodiment in which the present invention is applied to an electrophotographic direct color transfer color laser printer (hereinafter referred to as “laser printer”) as an image forming apparatus.
In FIG. 1, the laser printer includes four sets of toner image forming units 1Y, 1M, 1C, for forming images of each color of yellow (Y), magenta (M), cyan (C), and black (BK). 1BK (hereinafter, the subscripts Y, M, C, and BK of the respective symbols indicate yellow, magenta, cyan, and black members), respectively, along the moving direction of the transfer paper P (along arrow A in the figure). In the direction in which the transfer conveyance belt 60 travels).
Each of the toner image forming units 1Y, 1M, 1C, and 1BK includes photosensitive drums 11Y, 11M, 11C, and 11BK as image carriers and a developing unit. The toner image forming units 1Y, 1M, 1C, and 1BK are arranged so that the rotation axes of the photosensitive drums 11Y, 11M, 11C, and 11BK are parallel to each other and at a predetermined pitch in the transfer sheet moving direction. So that it is set.
1 carries the toner image forming units 1Y, 1M, 1C, and 1BK, the optical writing unit 2, the paper feed cassettes 3, 4, the registration roller pair 5, and the transfer paper P to form each toner image. A transfer unit 6 as a belt driving device having a transfer conveyance belt 60 as a transfer conveyance member for conveying the sections 1Y, 1M, 1C, and 1BK so as to pass through the transfer positions; a belt fixing type fixing unit 7; Etc.
In addition, a manual feed tray MF and a toner supply container TC are provided, and a waste toner bottle, a duplex / reversing unit, a power supply unit, and the like (not shown) are also provided in a space S indicated by a two-dot chain line.
The optical writing unit 2 includes a light source, a polygon mirror, an f-θ lens, a reflection mirror, and the like, and irradiates the surface of each of the photosensitive drums 11Y, 11M, 11C, and 11BK while scanning with laser light based on image data. .

FIG. 2 is an enlarged view showing a schematic configuration of a transfer unit of the color laser printer of FIG. The transfer conveyance belt 60 used in the transfer unit 6 is a high-resistance endless single-layer belt having a volume resistivity of 10 ¹⁰ to 10 ¹² Ωcm and a surface resistivity of 10 ¹² to 10 ¹⁴ Ω / □.
The material is PVDF (polyvinylidene fluoride). The transfer / conveying belt 60 is wound around support rollers 61 to 68 so as to pass through the transfer positions that are in contact with and opposed to the photosensitive drums 11Y, 11M, 11C, and 11BK of the toner image forming units.
Among these support rollers 61 to 68, the entrance roller 61 on the upstream side in the transfer sheet moving direction faces the outer peripheral surface of the transfer conveyance belt 60 so that the electrostatic adsorption roller 80 to which a predetermined voltage is applied from the power source 80 a faces. Has been placed. The transfer paper P that has passed between the two rollers 61 and 65 is electrostatically attracted onto the transfer conveyance belt 60.
A roller 63 is a drive roller that frictionally drives the transfer conveyance belt 60, and is connected to a drive source (not shown) and rotates in the direction of the arrow.
As a transfer electric field forming means for forming a transfer electric field at each transfer position, transfer bias applying members (transfer rollers) 67Y, 67M, and 67C are provided so as to be in contact with the back surface of the transfer conveyance belt 60 at positions facing the photosensitive drum. , 67BK.
These are bias rollers provided with a sponge or the like on the outer periphery, and a transfer bias is applied to the roller core from each transfer bias power source 9Y, 9M, 9C, 9BK. Due to the action of the applied transfer bias, a transfer charge is applied to the transfer conveyance belt 60.
As a result, a transfer electric field having a predetermined strength is formed between the transfer conveyance belt 60 and the surface of the photosensitive drums 11Y, 11M, 11C, and 11BK at each transfer position. In addition, a backup roller 68 is provided to keep the contact between the transfer paper and the photoconductor in the area where the transfer is performed, and to obtain the best transfer nip.

The transfer bias applying members 67Y, 67M, and 67C and the backup roller 68 disposed in the vicinity thereof are integrally held by the swing bracket 93 so as to be rotatable, and are rotatable about a rotation shaft 94. This rotation is clockwise as the cam 96 fixed to the cam shaft 97 rotates in the direction of the arrow.
The entrance roller 61 and the suction roller 80 are integrally supported by the entrance roller bracket 90, and can be rotated clockwise from the state shown in FIG.
A hole 95 provided in the swing bracket 93 and a pin 92 fixed to the entrance roller bracket 90 are engaged with each other, and rotate in conjunction with the rotation of the swing bracket 93.
By the clockwise rotation of these brackets 90, 93, the bias applying members 67Y, 67M, 67C and the backup roller 68 disposed in the vicinity thereof are separated from the photoconductors 11Y, 11M, 11C, and the entrance roller 61 and the suction roller 80 also moves downward.
It is possible to avoid contact between the photosensitive drums 11Y, 11M, and 11C and the transfer / conveying belt 60 when forming a black-only image.
On the other hand, the transfer bias applying member 67BK and the backup roller 68 adjacent to the transfer bias applying member 67BK are rotatably supported by the outlet bracket 98, and are rotatable about a shaft 99 coaxial with the separation roller 62.
When the transfer unit 6 is attached to or detached from the main body, the transfer unit 6 is rotated clockwise by the operation of a handle (not shown) to transfer the transfer bias applying member 67BK and the backup roller 68 adjacent thereto from the photosensitive drum 11BK for black image formation. Are separated from each other.
A cleaning device 85 composed of a brush roller and a cleaning blade is disposed on the outer peripheral surface of the transfer conveyance belt 60 wound around the driving roller 63. The cleaning device 85 removes foreign matters such as toner adhering to the transfer conveyance belt 60.
A roller 64 is provided downstream of the drive roller 63 in the traveling direction of the transfer conveyance belt 60 in a direction to push the outer peripheral surface of the transfer conveyance belt, and a winding angle around the drive roller 63 is secured.
In the loop of the transfer conveyance belt 60 further downstream from the roller 64, a tension roller 65 that applies tension to the transfer conveyance belt 60 by a pressing member (spring) 69 is provided.
The one-dot chain line in FIG. 1 shown above indicates the conveyance path of the transfer paper P. The transfer paper P fed from the paper feed cassettes 3 and 4 or the manual feed tray MF is transported by transport rollers while being guided by a transport guide (not shown), and is transported to a temporary stop position where the registration roller pair 5 is provided.
The transfer paper P delivered by the registration roller pair 5 at a predetermined timing is carried on the transfer conveyance belt 60, conveyed toward the toner image forming portions 1Y, 1M, 1C, and 1BK, and passes through the transfer nips. .
The toner images developed on the photosensitive drums 11Y, 11M, 11C, and 11BK of the toner image forming portions 1Y, 1M, 1C, and 1BK are superimposed on the transfer paper P at the transfer nips, respectively. It is transferred onto the transfer paper P under the action of pressure. A full color toner image is formed on the transfer paper P by this superposition transfer.
The surfaces of the photoconductive drums 11Y, 11M, 11C, and 11BK after the toner image transfer are cleaned by the cleaning device 85, and are further discharged to prepare for the formation of the next electrostatic latent image.
On the other hand, the transfer paper P on which the full-color toner image is formed is fixed in the first paper discharge direction B or the second in accordance with the rotation posture of the switching guide G after the full-color toner image is fixed by the fixing unit 7. In the paper discharge direction C.
When the paper is discharged from the first paper discharge direction B onto the paper discharge tray 8, it is stacked in a so-called face-down state with the image surface down. On the other hand, when the paper is discharged in the second paper discharge direction C, it is conveyed toward another post-processing device (such as a sorter or a binding device) (not shown), or is registered again for double-sided printing via a switchback unit. It is conveyed to the roller pair 5.

Next, an embodiment of the present invention will be described in the case of color image formation. FIG. 3 is a schematic view showing a color image forming time. The four photosensitive drums 11BK, 11Y, 11C, and 11M are in contact with the transfer belt 60. Charge is applied to the transfer paper by the paper suction roller 80, and the transfer paper is sucked to the transfer belt 60.
The transfer paper is conveyed while adsorbed to the transfer belt 60, and the colors are superposed in the order of M, C, Y from the upstream drum, and finally passes through the BK drum 11BK. At this time, the transfer paper is more strongly attracted to the transfer belt 60 as it goes downstream by the transfer current of each color.
At this time, for example, when the speed of the surface of the transfer belt 60 is set to 125 mm / second, the speed of the surface of the photosensitive drums 11BK, 11Y, 11C, and 11M is set to 127 mm / second. Reference numeral 100 in the figure denotes a CPU that controls the entire apparatus.
In this way, the CPU 100, in other words, the speed setting means sets (surface speed of the photosensitive drums 11BK, 11Y, 11C, and 11M)> (surface speed of the transfer belt 60), so that the downstream side of the transfer nip becomes the loose side and is fine. The transfer nip pressure is reduced visually. The reduction of the transfer nip pressure is particularly effective in preventing a missing image in a character portion where two or more colors overlap.
At this time, there is a concern that the transfer speed of the transfer paper becomes unstable when the leading edge of the transfer paper enters the fixing portion. However, since the transfer paper is supported by the four-color transfer nip, the transfer of one color is performed. The transfer speed of the transfer paper does not become unstable compared to the configuration in which the transfer paper is supported by the nip.

FIG. 4 is a schematic view showing a monochrome (single color) image formation. As an embodiment here, a case where a BK image is formed as a single color will be described. The C, M, and Y photoconductive drums are separated from the transfer belt 60, and only the BK photoconductive drum 11BK contacts the transfer belt.
As in the color image formation described above, the transfer paper suction roller 80 applies charge to the transfer paper and causes the transfer belt 60 to suck the transfer paper. When the speed of the surface of the transfer belt 60 is 160 mm / second, the photosensitive drum driving motor is switched so that the surface speed of the photosensitive drum 11BK is 157 mm / second.
Thus, by setting (surface speed of the photosensitive drum 11BK) <(surface speed of the transfer belt 60), the transfer nip pressure increases microscopically because the downstream side of the transfer nip becomes the tension side.
For this reason, when (surface speed of the photosensitive drum 11BK)> (surface speed of the transfer belt 60) is set, the loose portion generated on the downstream side of the transfer nip extends when the leading edge of the transfer paper enters the fixing portion. As a result, the speed of the transfer paper fluctuates, resulting in horizontal band-shaped density unevenness (shock jitter image).
As in the above-described configuration, if (surface speed of the photosensitive drum 11BK) <(surface speed of the transfer belt 60), the upstream side of the transfer nip is loosened and the downstream side is a tight side. Therefore, even if the leading edge of the transfer paper enters the fixing portion, the transfer belt 60 and the transfer paper are not loosened and stretched between the fixing and the transfer portion, so that the shock jitter does not occur.
Although productivity can be improved by increasing the monochrome monochrome sheet passing speed, the image forming apparatus is susceptible to fluctuations in the speed of the transfer sheet when entering the fixing nip due to the increased conveyance speed. With this configuration, fluctuations in the speed of the transfer paper can be suppressed, and the occurrence of abnormal images can be suppressed.

FIG. 5 is a schematic view showing the position of the driving roller in the configuration at the time of color image formation shown in FIG. FIG. 6 is a schematic view showing the position of the drive roller having a structure changed during black-and-white (single color) image formation shown in FIG.
5 and 6, the position of the driving roller 60 is set to the downstream side of the transfer unit. When the position of the driving roller 60 is immediately after the final color, the relationship between the speed of the photosensitive drum 11BK and the transfer belt 60 is set as (surface speed of the photosensitive drum 11BK)> (surface speed of the transfer belt 60). To do.
As a result, the transfer belt 60 is gripped by the drive roller 63 having a high surface friction coefficient in the vicinity of the drive roller 63, so that the transfer belt 60 does not sag, and the nip is loosened microscopically downstream of the transfer nip. Occur.
Since the looseness on the downstream side of the transfer nip reduces the transfer nip pressure, it is possible to prevent abnormal images such as missing two-color superimposed characters. On the other hand, when (surface speed of the photosensitive drum 11BK) <(surface speed of the transfer belt 60) is set, the transfer belt 60 is gripped by the drive roller 63 having a high surface friction coefficient in the vicinity of the drive roller 63.
For this reason, the transfer belt 60 does not sag, and the nip loosens microscopically upstream of the transfer nip. Therefore, the downstream side of the transfer nip is a tension side and is a disadvantageous side for missing two-color superimposed characters, but is less susceptible to fluctuations in the speed of the transfer portion when the leading edge of the transfer paper is shocked.
Therefore, in the present invention, in the color mode: (surface speed of the photosensitive drum) 160 mm / second, (surface speed of the transfer belt) 157 mm / second In the monochrome mode: (surface speed of the photosensitive drum) 160 mm / second, (transfer Belt surface speed) is set to 163 mm / sec.
Further, as shown in FIG. 1, the position of the drive roller 63 is not immediately after the final color, “the transfer portion closest to the roller driving the transfer belt 60 is the most downstream transfer portion. Therefore, it is possible to make the conditions easy to obtain the effect of this patent.

FIG. 7 is a diagram showing the relationship between the missing characters, “(surface speed of the photosensitive drum), and (surface speed of the transfer belt)”. As shown in FIG. 7, when the void level of the two-color overlay is viewed, the surface speed of the photosensitive drum> (surface speed of the transfer belt) <(transfer surface speed of the photoconductor) <(transfer) It was found that the hollow-out level was better than the case of the belt surface speed.
This can improve the character dropout level that is greatly influenced by the nip pressure by microscopically relaxing the transfer nip pressure on the downstream side of the transfer nip. Similarly, with respect to the single-color void level, as shown in FIG. 7, in the case of a single color, the toner holding power of the transfer paper is high and it is difficult for character void to occur.
FIG. 8 is a diagram showing the relationship between the shock jitter level and “(surface speed of the photosensitive drum) and (surface speed of the transfer belt)”. As shown in FIG. 8, in printing on a single photoconductive drum, the transfer belt 60 and the transfer paper on the downstream side of the transfer unit are transferred by setting (surface speed of photoconductor drum) <(surface speed of transfer belt). It can be seen that it is possible to improve the shock jitter level when the transfer paper leading edge enters the fixing side.
In addition, it was found that when printing was performed with four color photoconductors, the shock jitter level was good regardless of the relationship between the (surface speed of the photoconductor drum) and the (surface speed of the transfer belt). This is because the transfer paper is held by the four-color transfer nip, so that the shock when the front end of the transfer paper enters the fixing portion can be alleviated.
By setting the driving roller 63 for driving the transfer belt 60 to the downstream side of the transfer nip, if (surface speed of the photosensitive drum)> (surface speed of the transfer belt) is surely established, the downstream side of the transfer nip becomes the loose side, and (photosensitive If the surface speed of the body drum) <(surface speed of the transfer belt), the downstream side of the transfer nip is the tension side.
Furthermore, this effect can be obtained even when a configuration in which a roller that is most susceptible to the influence of thermal fluctuations closest to the fixing unit is not used as a driving roller is avoided.

1 is a schematic configuration diagram illustrating an embodiment in which the present invention is applied to a color laser printer using an electrophotographic direct transfer system that is an image forming apparatus. FIG. 2 is an enlarged view showing a schematic configuration of a transfer unit of the color laser printer of FIG. 1. It is the schematic which shows the time of color image formation. It is the schematic which shows the time of monochrome (monochrome) image formation. FIG. 4 is a schematic view showing a position of a driving roller having a configuration changed when forming a color image in FIG. 3. FIG. 5 is a schematic diagram showing the position of a driving roller having a configuration changed during black-and-white (single color) image formation in FIG. 4. FIG. 5 is a diagram illustrating a relationship between a letter dropout, “(surface speed of photosensitive drum), and (surface speed of transfer belt)”. FIG. 6 is a diagram illustrating a relationship between a shock jitter level, “(surface speed of photosensitive drum), and (surface speed of transfer belt)”.

Explanation of symbols

  1BK toner image forming unit (photosensitive drum), 1Y toner image forming unit (photosensitive drum), 1C toner image forming unit (photosensitive drum), 1M toner image forming unit (photosensitive drum), 6 transfer unit (transfer unit) , Transfer nip), 7 fixing unit (fixing unit, fixing nip), 10 photosensitive unit, 60 transfer conveyance belt, 63 drive roller, 80 electrostatic adsorption roller, 80a power supply, 100 CPU, P transfer paper

Claims (7)

In an image forming apparatus having a plurality of paper passing modes and having a speed difference between the photosensitive drum and the transfer belt by the speed setting means,
In the speed setting means, (sheet surface speed of the photosensitive drum)> (surface speed of the transfer belt) is set so that (surface speed of the photosensitive drum) <(surface speed of the transfer belt) is set. An image forming apparatus capable of setting a paper passing mode.   2. The image forming apparatus according to claim 1, wherein speed is set by a speed setting unit in a four-tandem configuration in which transfer is sequentially performed while directly transferring transfer paper.   It has black and white or single color mode and color mode. In the black and white or single color mode, (surface speed of photosensitive drum) <(surface speed of transfer belt) is set, and in color mode (surface speed of photosensitive drum) 3. The image forming apparatus according to claim 1, wherein: (surface speed of transfer belt) is set.   4. The image forming apparatus according to claim 1, wherein in the monochrome or single color mode, the photosensitive member and the transfer belt are in contact with only one color. 5.   5. The image forming apparatus according to claim 1, wherein the sheet passing speed is faster in the monochrome mode or the monochrome mode than in the color mode.   6. The image forming apparatus according to claim 1, wherein the roller for driving the transfer belt is located downstream of the transfer nip.   The image forming apparatus according to claim 1, wherein the transfer portion closest to the roller for driving the transfer belt is a transfer portion on the most downstream side.

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