JPH09211925A - Color image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent color slurring derived from thermal expansion or a belt walk from occurring without making a device large, and to obtain an excellent image over a long period, in a color image forming device of the so-called tandem type, severally provided with electrifying, exposing and developing means on respective periphery of photoreceptors, by which toner images on respective photoreceptor is successively transferred in a state of being laid over the other. SOLUTION: This device is provided with the plural photoreceptor drums 1, arrayed close to each other along a periphery of a transfer belt 7, and planer shield members 20 is respectively provided in-between the adjacent photoreceptor drums. Moreover, plural developing devices 4 are mounted on the opposite side of a position where a line of the photoreceptor drums 1 is opposing the transfer belt 7. In this way, a space between the transfer position of the respective toner image can be narrowed, and the color slurring derived from thermal expansion of a frame can be reduced, while the belt walk can be made small. Furthermore, mutual interference among the adjacent photoreceptor drums are eliminated, therefore the image defect is prevented from occurring.

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 such as a copying machine and a printer using an electrophotographic method, and in particular, a plurality of photoconductors are provided with charging, exposing, developing means, etc. The present invention relates to a color image forming apparatus that obtains a multicolor image by sequentially superposing and transferring toner images formed thereon.

[0002]

2. Description of the Related Art Two typical color image forming apparatuses are known as electrophotographic color image forming apparatuses. One is a so-called single system or a single-drum system. As shown in FIG. 8, one photosensitive drum 101 and a transfer drum 107 that carries and rotates recording paper are provided in the apparatus. ing. Around the photosensitive drum 101, a charging device 10 for uniformly charging the photosensitive drum.
An exposure device 103 that performs image exposure for two and four colors, a developing device 104 that stores toner of each color installed on a rotatable support, and a transfer that transfers a toner image on a photosensitive drum onto a recording sheet. An apparatus 105 and a cleaning apparatus 106 for removing the residual toner on the photosensitive drum are provided. Then, a color-separated electrostatic latent image is formed on the photoconductor drum 101, is developed with toner of a corresponding color by the developing device 104, and the transfer drum 107 is moving at substantially the same peripheral speed as the photoconductor drum 101. The image is transferred to the upper recording sheet by the transfer device 105. This operation is repeated for each color that has been color-separated, and the toner images of the respective colors are sequentially superimposed on the recording paper, and the composite image on the recording paper is fixed by heating and pressing by the fixing device 108. In other words, the transfer drum 107 conveys the recording paper to the position facing the photoconductor drum 101 a plurality of times, and transfers the toner image each time a toner image is formed on the photoconductor drum to form a toner image of each color on one recording paper. It is to be overlaid.

The other one is called a tandem system having a plurality of photosensitive drums as shown in FIG. This type of image forming apparatus includes a transfer belt 117 that carries and conveys a recording sheet and four photoconductor drums 111 around the transfer belt 117. Each photoconductor drum 111 has a charging device 112, an exposure device 113, and a photoconductor drum 111 around the photoconductor drum 111. Developing device 114,
A transfer device 115 and a cleaning device 116 are provided. In this image forming apparatus, a color-separated electrostatic latent image is formed on each photoconductor drum 111, and visualized by a developing device 114 having toner corresponding to each color. A transfer device 115 is provided at a position where each photoconductor drum 111 faces the transfer belt 117, and the toner images of the respective colors are sequentially superposed on the recording paper on the transfer belt 117 that passes at the same peripheral speed as the photoconductor drums. To go. At this time, writing on each photosensitive drum is performed with a certain delay time so that the toner images can be accurately superimposed on the recording paper. Then, the recording paper on which the toner images are superposed enters the fixing device 118, and the composite image is fixed by pressing and heating. That is, in this type of image forming apparatus, the transfer belt 117 sequentially conveys the recording paper to a position facing the photosensitive drums, and a plurality of colors formed by the charging device, the exposing device, and the developing device facing the respective photosensitive drums. The toner images of 1 are sequentially transferred onto the recording paper.

Comparing these two systems, the single system image forming apparatus has only one photoconductor drum and a charging device, an exposure device, and a cleaning device arranged around the photoconductor drum. Compared with this, there is an advantage that the size of the entire device is relatively small. However, there is a problem that it takes a long time to form an image because the color-separated images are sequentially visualized. On the other hand, the tandem-type image forming apparatus visualizes the color-separated images in parallel and superimposes them on the recording paper, so that the time required for image formation is short, which is about a quarter of that of the single-type apparatus. It only takes time.

[0005]

However, the above-mentioned tandem type color image forming apparatus has the following problems. The first problem is that this device is equipped with four photoconductor drums and a charging device, an exposure device, a developing device, and a cleaning device around the photoconductor drums, respectively, and the entire device becomes extremely large. In order to eliminate such an increase in the size of the device, it is conceivable to reduce the size of each component.However, the life of the photoconductor is determined by the mechanical wear of the surface. It gets shorter. If the developing device is also made smaller, the amount of the developer accommodated inside becomes smaller, and the replacement time must be advanced. Therefore, the easy miniaturization causes a new problem that the life of the device is shortened.

The second problem is that it is difficult to accurately align the transfer positions when the toner images on the respective photosensitive drums are superposed on the recording paper. It is known that the single-type device has a small displacement even if the toner images of the respective colors are superposed by making the diameter of the transfer drum an integral multiple of the diameter of the photosensitive drum and engaging them with a gear or the like. .
Further, even with respect to the positional deviation in the axial direction (lateral direction), since the toner image is transferred between the drum-shaped members having high rigidity, the color deviation is unlikely to occur. On the other hand, in the tandem system, it is necessary to align the writing start position of the image light of the next image so that it accurately overlaps the position of the previous image transferred onto the recording paper. Moreover, since the distance from the previous image changes due to thermal expansion of the frame of the device supporting the photosensitive drum, accurate alignment becomes extremely difficult. The color shift due to this thermal expansion occurs as follows.

FIG. 10 is a partially enlarged view showing the four photosensitive drums 111 and the transfer position A. First, the sensor 120 detects the leading edge position of the recording sheet, and the image light B of the first color is detected.
Writing to the photosensitive drum 111 of No. 1 is performed. In order to match the position of the image of the second color with the image of the first color, the recording sheet is delayed from the transfer position A1 of the first color to the transfer position A2 of the second color, and then the image light B2 of the second color is delayed. Must start writing. Third color, 4
The same applies to the writing of the image lights B3 and B4 of the colors. Here, as shown in FIG. 12, the inter-transfer distance (distance between photoconductor centers) is L (mm), and the moving speed of the recording sheet is s (m
m / s), the writing timing of the image light is expressed by the following equation. Image writing timing for the first color: t1 Image writing timing for the second color: t2 = t1 + (L /
s) Third color image writing timing: t3 = t1 + 2 * (L
/ S) Timing of writing the image of the fourth color: t4 = t1 + 3 × (L
/ S)

In such a case, if the center-to-center distance L between the photoconductors changes due to the thermal expansion of the frame, the expansion leads to the displacement of the position unless the delay time for the second and subsequent colors is changed. Such thermal expansion becomes remarkable when the distance from the fixing device is short, and the image is misaligned in the vertical direction. On the other hand, there is a technique in which the arrangement of the gears of the drive system is devised in order to perform accurate alignment.
Although it is disclosed in Japanese Patent Laid-Open No. 156160 and Japanese Patent Laid-Open No. 61-156159, none of them is effective for positional displacement due to thermal expansion.

A third problem is the occurrence of color shift due to the flexibility of the transfer belt. In the tandem type apparatus, as shown in FIG. 11, a flexible transfer belt is stretched around a support roll and driven by the rotation of the support roll, so that the belt meanders due to slight eccentricity of the roll. To do. Unless the movement of the belt called the walk in the horizontal direction is suppressed, the image is displaced in the horizontal direction. Such a phenomenon occurs because the tension of the belt is insufficient and the lateral movement cannot be suppressed. The longer the distance between the two support rolls, the greater the tension required to press the walk. Become. However, tension cannot be applied unnecessarily from the standpoint of the proof strength of the belt, and this problem becomes remarkable when the length of the transfer belt stretched becomes large.

A fourth problem of the tandem system is that it is caused by the arrangement of each component and device.
That is, as shown in FIG. 9, since the transfer position is below the developing device, there is a problem that the developer leaked from the developing device adheres to the transfer belt and recording paper below. In particular, if a carrier or a toner agglomerate larger than the toner particle size is attached, the adhesion between the recording sheet and the photoconductor drum at the transfer position becomes non-uniform, causing uneven transfer. In the tandem system, the toner image is transferred onto the recording paper multiple times so that this effect becomes remarkable.

Further, since the exposure device is located above the fixing device, the exposure device is easily affected by heat generated from the fixing device, and the optical components in the exposure device are distorted by heat. This effect becomes larger as the exposure device is closer to the fixing device, and the difference is larger at the device farthest away from the fixing device. Therefore, the magnification of the electrostatic latent image shifts for each color, and the color of the composite image is not reproduced correctly, causing color unevenness. There is also a problem that the life of the element for generating image light and the motor for scanning the image light is shortened due to the temperature rise.

Further, since the charging device is located above the fixing device, the component of the releasing agent such as silicone oil used for enhancing the releasing property in the fixing device is evaporated by heat, and the vapor which rises by the heat is evaporated. It adheres to the charging device along the flow. The charging device utilizes discharge of a wire to which a high voltage is applied, and if the component of the release agent adheres, the discharge state becomes non-uniform. Therefore, the electrostatic latent image formed on the photoconductor is uneven, and a high-quality image cannot be obtained.

The present invention has been made in view of the above problems, and an object of the present invention is not to impair the advantages of the tandem system, such as a short image forming time, but to increase the size of the apparatus and to reduce the heat. It is an object of the present invention to provide a color image forming apparatus which can prevent color misregistration due to expansion or belt walk, adhesion of a developer to recording paper, occurrence of image defects due to charging failure, and the like, and which can obtain good image quality for a long time.

[0014]

In order to solve the above problems, the invention according to claim 1 is characterized in that a plurality of photosensitive drums having electrostatic latent images formed on their peripheral surfaces, and the photosensitive drums are provided. A charger that uniformly charges the image, an image writing device that irradiates the photosensitive drums with image light, a plurality of developing devices that form toner images of different colors on each of the plurality of photosensitive drums, and a plurality of the plurality of developing devices. In a color image forming apparatus having a transfer belt which is in close proximity to or in contact with a photosensitive drum and is stretched so as to circulate, and a fixing device which fixes toner images of a plurality of colors superposed on the transfer belt onto a recording sheet. ,
The plurality of photoconductor drums are arranged along the outer peripheral surface of the transfer belt and are positioned so as to closely face and oppose adjacent photoconductor drums, and the plurality of developing devices are arranged in rows of the photoconductor drums. Is installed on the side opposite to the side facing the transfer belt.

In the above structure, the adjacent photosensitive drums are in a range in which the toner images on the photosensitive drums are not disturbed by the airflow generated by the mutual rotation or the electric field or magnetic field generated between the adjacent photosensitive drums. Therefore, it is arranged as close as possible. Further, when a shield plate that shields an electric field or a magnetic field is arranged between adjacent photoconductor drums, a range in which an air flow that disturbs a toner image does not occur between the photoconductor drum and the shield plate, and between both regions. The adjacent photosensitive drums are arranged as close to each other as possible within a range in which the toner image is not disturbed by the generated electric field and magnetic field.

According to a second aspect of the present invention, in the color image forming apparatus according to the first aspect, when the diameter of the photosensitive drum is D and the center distance between adjacent photosensitive drums is L, L / D. Is 1.3 or less.

According to a third aspect of the present invention, in the color image forming apparatus according to the first or second aspect, the transfer belt is stretched so that a lower portion of its circulation path is substantially horizontal. The plurality of photoconductor drums are arranged so as to be close to or in contact with a horizontal portion, the plurality of developing devices and the image writing device are arranged below the photoconductor drum array, and the fixing device is arranged on the photoconductor drums. It is assumed to be located above the central axis.

According to a fourth aspect of the present invention, in the color image forming apparatus according to the first aspect, the second aspect, or the third aspect, a plate-shaped shielding member is provided between the photoconductor drums arranged adjacent to each other. Shall have.

According to a fifth aspect of the present invention, in the color image forming apparatus according to the fourth aspect, the volume resistance value is 10 ⁶ Ω · cm to 10 at least near the surface of the shielding member.
^It shall be made of a semi-conductive material of about ¹¹ Ω · cm.

According to a sixth aspect of the present invention, in the color image forming apparatus according to the fourth or fifth aspect, the shielding member is electrically grounded.

According to a seventh aspect of the present invention, in the color image forming apparatus according to the fourth or fifth aspect, the shielding member is provided in a direction in which the toner on the photoconductor drum is attracted to the photoconductor drum side. A DC voltage forming an electric field is applied.

The invention according to claim 8 is the color image forming apparatus according to any one of claims 1 to 7.
The photoconductor layer on the photoconductor drum exhibits light attenuation characteristics in which potential attenuation is unlikely to occur when the exposure energy of the image light irradiated after uniform charging is small, and large potential attenuation occurs when the exposure energy exceeds a certain level. It is assumed that the developing device is set to collect the toner remaining after the toner image formed on the photosensitive drum is transferred.

[Operation] The invention according to the present application has the above-mentioned configuration, and operates as follows. In the image forming apparatus according to the first aspect of the present invention, the plurality of photosensitive drums are arranged along the transfer belt, and the developing device facing each of the photosensitive drums is installed on the side opposite to the position of the transfer belt. Therefore, it is possible to arrange the photoconductor drums close to each other with a slight gap without reducing the diameter of the photoconductor drums. Therefore, as shown in FIG. 13, the center distance L between the photosensitive drums can be made shorter than that of the conventional apparatus shown in FIG. As a result, the frame that supports the photoconductor drum becomes shorter, and the degree of misalignment due to thermal expansion of the frame can be reduced. Further, since the distance from the transfer position of the first color to the transfer position of the fourth color is shortened, the belt walk can be reduced without increasing the tension of the belt. Therefore, it is possible to obtain a good image without misalignment without shortening the life of the photosensitive drum.

In the image forming apparatus according to the second aspect of the present invention, L / D is set to 1.3 or less, where D is the diameter of the photosensitive drum and L is the center distance between adjacent photosensitive drums. Therefore, it is possible to prevent the positional deviation from occurring by sufficiently reducing the distance between the photosensitive drums. Then L
If / D is closer to 1, the displacement of the photosensitive drums can be prevented while maximizing the life of the photosensitive drums. However, if the photosensitive drums are too close to each other, the electrostatic latent images formed on the both surfaces are prevented. If a shielding member is provided between the photosensitive drums, or between the electrostatic latent image and the shielding member, an electric field is formed to disturb the toner image on the adjacent photosensitive drum, or the toner is not exposed to light. The phenomenon of transfer between body drums occurs. Considering the image quality deterioration due to the interference between the adjacent photoconductor drums, the L / D is preferably 1.1 to 1.2.

In the image forming apparatus according to the third aspect of the present invention, the plurality of photoconductor drums are arranged under the transfer belt, and the plurality of developing devices and the image writing device are arranged under the photoconductor drum array. Therefore, it is possible to prevent the developer from leaking from the developing device and adhering to the transfer belt or the like. Therefore, in addition to reducing the occurrence of misalignment due to the small L as shown in FIG. 14 or 15,
In principle, white spots due to poor adhesion between the photoconductor drum and the recording sheet at the transfer portion, which has been a serious problem in color images, do not occur, and good color images can be obtained. Further, the leakage of the developer does not interfere with the image light from the image writing device, and the deterioration of the image quality at the time of image writing is also eliminated.

Further, in the above image forming apparatus, since the fixing device is arranged above the central axis of the photosensitive drum,
The heat generated from the fixing device is difficult to be transmitted downward, and the temperature of the frame of the photosensitive drum is prevented from rising abnormally. Therefore, it is possible to reduce the occurrence of color shift due to thermal expansion of the frame. In addition, the optical components of the image writing device are prevented from being distorted by heat, and positional deviation and magnification change due to exposure are unlikely to occur. Therefore, the influence of heat on the motor and the light beam generating element is reduced, the life can be extended, and a stable color image can be obtained for a long period of time.

Further, in the conventional apparatus, a releasing aid such as silicone oil is vaporized and mixed in the hot airflow generated from the fixing device, and when this airflow enters the charging device utilizing electric discharge, the releasing agent is released. The auxiliary agent deposits on the discharge member and hinders discharge, causing uneven density and uneven color.However, since the fixing device is arranged above the charging device with the above configuration, It becomes difficult for the hot air flow to enter the charging device, and a stable discharge capacity can be maintained for a long period of time.

In the image forming apparatus according to the fourth aspect of the present invention, since the plate-shaped shielding member is provided between the photoconductor drums arranged adjacent to each other, the electrostatic latent image on the photoconductor drum is electrically charged. The toner image on the adjacent photoconductor drum is not disturbed by the physical force. Further, when the photoconductors are brought too close to each other, there is a problem that the rotation of the photoconductor drums causes airflows in mutually opposite directions between adjacent photoconductor drums, which causes turbulent flow in the proximity portion. By the shielding member, the air flows in the opposite direction do not interfere with each other, and the disturbance of the toner image can be reduced. Therefore, image quality deterioration due to mutual interference between the photoconductor drums is prevented.

In the image forming apparatus according to the present invention, the volume resistance value near the surface of the shielding member is 10 ⁶ Ω.
Since it is made of a semi-conductive material of about 10 cm to 10 ¹¹ Ω · cm, even if the toner peeled off due to the rotation of the photosensitive drum rubs against the surface of the shielding member, the electric charge generated by frictional charging is localized. And the surface potential is prevented from locally increasing. Therefore, unlike the case where the insulating member is used, the surface potential becomes unstable and a Coulomb force is not electrically exerted on the toner particles, and a stable image can be obtained. Furthermore, by selecting the volume resistance value as described above, it acts as an overcurrent preventing resistance, and it is possible to prevent the leakage of charges from the surface of the photoconductor.

In the image forming apparatus according to the sixth aspect of the present invention, since the shielding member is electrically grounded, even if the surface of the shielding member is rubbed with toner, the charge generated thereby is eliminated. can do. Therefore, the surface potential of the shielding member is prevented from becoming unstable, and the toner image is not disturbed by an electric force.

In the image forming apparatus according to the seventh aspect of the present invention, since a DC voltage that forms an electric field in the direction of attracting the toner to the photosensitive drum side is applied to the shielding member, the toner image on the photosensitive drum is formed. When passing through the position facing the shielding member, the toner is attracted onto the photosensitive drum, and the disturbance of the toner due to the air flow or electric force does not occur. The effect of this shielding member is remarkable when the gap between the photosensitive drums is 30 mm or less, and the length of the shielding member can be set appropriately.

In the image forming apparatus according to the eighth aspect of the present invention, when the exposure energy of the image light applied to the photoconductor layer on the photoconductor drum is small, the potential is less likely to be attenuated, and the exposure energy is not less than a certain level. Therefore, even if the peak of the energy received from the image light on the photoconductor due to uneven rotation of the photoconductor drum becomes slightly higher or lower, The surface potential of is attenuated by exposure energy above a certain level, and the electrostatic latent image does not faithfully reproduce a slight difference in exposure energy. For this reason, the difference in exposure energy does not appear as the potential difference of the electrostatic latent image, and an image without density unevenness can be obtained.

Further, the developing device is set so as to collect the toner remaining after the toner image on the photosensitive drum is transferred, and it is not necessary to provide a cleaning device. In this way, when the developing device collects the residual toner without removing the residual toner by the cleaning device, the photosensitive member in the state where the residual toner is present is irradiated with the image light. However, due to the potential attenuation characteristic of the photoconductor layer, it is possible to reduce the fluctuation of the potential of the exposed portion due to the light shielding effect of the residual toner. That is, since the surface potential of the photoconductor is attenuated by the exposure energy above a certain level, even if the amount of the image light irradiated onto the photoconductor by the residual toner slightly fluctuates, it is stable without being affected by the fluctuation. The electrostatic latent image can be obtained. Therefore, the occurrence of so-called ghost in which traces of residual toner appear in the next image is eliminated. Further, since the cleaning device can be omitted, the ratio of the diameter of the photoconductor drum to the center distance of the photoconductor L /
R can be brought close to 1, and color misregistration can be suppressed to a minimum without shortening the life of the photoconductor.

[0034]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic configuration diagram showing an image forming apparatus which is an embodiment of the invention described in claim 1, claim 2, claim 4 or claim 5. This image forming apparatus has a transfer belt 7 that conveys recording paper while electrostatically adhering it, and four photosensitive drums 1 at positions facing the transfer belt 7, and surrounds each photosensitive drum 1. A charging device 2 for uniformly charging the surface of the photoconductor drum, an image writing device 3 for irradiating each photoconductor drum with a laser beam to form a latent image, and a charging device 2 formed on the photoconductor. A developing device 4 that visualizes a latent image by attaching toner to it, and a transfer device 5 that transfers the toner image formed on the photoconductor onto a recording sheet.
And a cleaning device 6 for cleaning the toner remaining on the photosensitive drum after the toner image is transferred.

Further, in the apparatus, a paper tray 9 for accommodating the recording paper 8, a paper conveying path 10 for conveying the recording paper 8 to the transfer belt 7, and a suction charger for adsorbing the recording paper on the transfer belt 7. 11, a peeling charger 12 for peeling the recording paper from the transfer belt 7, and a fixing device 13 for fixing the toner image transferred on the recording paper. Also,
A plate-shaped shield member 20 is provided between the photoconductor drums 1 arranged adjacent to each other.

The photoconductor drum 1 is provided with a photoconductor layer on a conductive substrate which is electrically grounded. Four photoconductor drums are arranged at positions facing the transfer belt 7 in a horizontal direction at substantially equal intervals. It is located in. The diameter D of the photosensitive drum 1 is 84 mm, and the center interval L of the photosensitive drums is 100.
mm, and the ratio L / D of these is approximately 1.2. Further, the shielding member 20 arranged between the photosensitive drums 1 is
A plate-shaped member made of polycarbonate and having a thickness of 1 mm.
It is adjusted to ¹⁰ Ω · cm.

The charging device 2 is a so-called scorotron type charger including a discharge wire surrounded by a shield member and a grid electrode for controlling the potential. The charging device 2 uniformly charges the surface of the photosensitive drum 1 to approximately -650V.

The image writing device 3 includes a laser generating element for generating a laser beam based on an image signal, a polygon mirror for reflecting the laser beam to scan the peripheral surface of the photosensitive drum 1 in the axial direction, and the polygon. It is provided with a motor for rotating the mirror. This gives 6
A laser beam having a spot diameter of 4 μm is applied to the photosensitive drum 1
It is designed to irradiate the surface of, and form an electrostatic latent image on the surface. At this time, the exposure energy of the laser light is set so as to attenuate the potential of the photoconductor to about -200V.

The developing device 4 has a particle size of about 50 inside.
A developer containing a magnetic carrier of μm and a resin toner having an average particle size of 7 μm is contained, and the contained amount is about 650.
g. Further, a rotatable magnet roll 4a having a diameter of 30 mm is disposed at a position facing the photoconductor drum 1, and the developer is adsorbed on the surface to be conveyed to a region facing the photoconductor drum. . A developing bias voltage of about -500V is applied to the magnet roll 4a, and the toner having a negative charge is transferred to the exposed portion of the photoconductor by the electric field formed between the magnet roll 4a and the photoconductor drum. .

The transfer device 5 is a corotron charger composed of a discharge wire surrounded by a shield member, and transfers the toner image on the photosensitive drum onto a recording sheet conveyed on a transfer belt by the action of an electric field. Is.

The cleaning device 6 is provided so that a blade made of urethane rubber is brought into contact with the photosensitive drum 1, and scrapes off the untransferred toner on the surface of the photosensitive drum.

The transfer belt 7 is a sheet-shaped belt member stretched by support rollers 15 and 16 so as to be movable endlessly. The recording paper is placed on the transfer belt 7 and conveyed, and the speed of the recording paper on the transfer belt 7 is equal to the peripheral speed of the photosensitive drum 160.
It is set to mm / s. The center distance between the support rollers 15 and 16 is set to be larger than the length between all the transfers to provide a suction portion and a peeling portion for the recording sheet, and is about 600 mm. The center interval L of the photoconductor is 100 mm, which is the same as each transfer interval, but the laser light writing to the photoconductor drums is started sequentially while maintaining the delay time of 0.625 seconds to superimpose the images on the recording paper. It is set to continue.

The fixing device 13 comprises two rolls having a heat source inside and heated to about 180.degree. The recording paper on which the toner image has been transferred enters the pressing portions of the two rolls and is heated and pressed to melt the toner and fix the toner on the paper. At this time, silicone oil is applied to the surface of the roll in order to improve the releasing property between the melted toner and the roll.

In this image forming apparatus, the charging device 2 is placed at a position of about 11:00 on the photosensitive drum 1, a portion of the photosensitive drum 1 is irradiated with laser light at a position of about 12:00, and the developing device 4 is placed at a position of about 1 o'clock. Cleaning device 6 at the 7 o'clock position
Is installed. The length between the transfer positions of the first and fourth colors is 300 mm, and the distance between the supporting rolls of the transfer belt 7 that conveys the recording paper is about 600 mm including the paper suction portion and the peeling portion. The fixing device also has a width of about 150 mm, and the width of the entire device including this is about 900.
mm. That is, by disposing the transfer device 2, the developing device 4, and the cleaning device 6 on the side opposite to the side where the photoconductor drum 1 faces the transfer belt 7 as described above, the center interval of the photoconductor drums can be shortened. it can. As a result, the size of the device can be reduced as compared with the conventional device.

Next, the operation of the image forming apparatus will be described. When the image forming process is started, the photoconductor drums 1 are uniformly charged by the charging devices 2, and thereafter, the laser light corresponding to each color is irradiated from the image writing device 3 so that the surface of each photoconductor drum is latently irradiated. An image is formed. At this time, the photosensitive drum on the downstream side in the transport direction of the recording paper is
Writing is sequentially started by laser light while maintaining a delay time of 0.625 seconds each. The latent image thus formed is developed into a toner image by each developing device 4, and then sequentially transferred onto each recording sheet conveyed on the transfer belt 7 by each transfer device 5. As a result, a composite image in which toner images of four colors are superimposed is formed on the recording sheet, and the toner images are fixed by the fixing device 13 to form a full-color image.

In such an image forming process, since the shielding member 20 is provided between the photoconductor drums 1, it is formed on the adjacent photoconductor drums by the air flow generated by the rotation of the photoconductor drums. The toner image is not disturbed. Further, the shielding member 20 has a volume resistance value of about 1
It is set to 0 ¹⁰ Ω · cm, the electric charge is not localized on the surface, and the toner image is less likely to be electrically affected. Therefore, the center interval of the photoconductor drums can be narrowed, and the occurrence of displacement when transferring the toner image can be reduced. Further, since the transfer interval from the first color to the fourth color is also shortened, the belt walk can be reduced without increasing the belt tension.

FIG. 2 shows claims 1, 2, 3 and
FIG. 6 is a schematic configuration diagram showing an image forming apparatus which is an embodiment of the invention described in claim 4 or claim 5. In this image forming apparatus, the lower portion of the transfer belt 27 is stretched so as to be substantially horizontal, and four photoconductor drums 21 are arranged so as to contact the horizontal portion, and the charging device 22 and the image writing device 23 are arranged. , And the developing device 24 are disposed below the respective photosensitive drums 21. That is, the charging device 22 at the position of about 7 o'clock of the photosensitive drum 21, the portion where the laser beam is irradiated at the position of about 6 o'clock, the developing device 24 at the position of about 5 o'clock, and the cleaning device 26 at the position of about 11 o'clock. Is installed. Further, a fixing roll 36 that supports the transfer belt 27 and has a heat source inside is provided on the downstream side of the position where the toner image is transferred, and the pressing roll 33a is brought into pressure contact with this fixing roll to form the fixing device 33. . Further, below the fixing device, a paper conveyance path 30 for conveying the recording paper 28 in the paper tray 29 to the pressure contact position between the fixing roll 36 and the pressing roll 33a is provided. The other configuration of the image forming apparatus is the same as that of the image forming apparatus shown in FIG.

In such an image forming apparatus, the toner image on the photosensitive drum 21 is directly transferred onto the transfer belt 27, and the transferred toner image is conveyed to a position facing the fixing device 33. This toner image is brought into contact with the recording paper conveyed to the fixing device 33, and the toner image is melted and pressed onto the recording paper.

In the above image forming apparatus, the center distance L between the photosensitive drums is 100 mm, and the ratio L / D to the drum diameter D is L / D.
Is about 1.2. The length between the transfer positions of the first color and the fourth color is 300 mm, and the transfer belt 27 needs to be at least 300 mm which is the length between the transfer positions, and is the center of the support roll 35 and the fixing roll 36. The interval is 350 mm. In addition, since a part of the fixing device is also used as one roll that supports the transfer belt, the width of the entire device is reduced, and in this example, it is about 600 mm. In addition, the image writing device 23 also has one polygon mirror 4
The method of sharing by color is adopted.

Even in such an image forming apparatus, the distance between the centers of the photosensitive drums can be made narrower than that in the conventional apparatus.
It is possible to reduce the occurrence of displacement when transferring the toner image. In addition, the shielding member 4 is provided between the photoconductor drums 21.
0 is provided so that the toner image formed on the adjacent photoconductor drum is not disturbed by the air flow or the electric force caused by the rotation of the photoconductor drum.

On the other hand, the image writing device 23 is replaced with the fixing device 33.
It is provided below, and each component in the image writing device 23 is less likely to be affected by the heat of the fixing device 33, and the occurrence of color shift due to distortion or the like is reduced. Further, the drawback that the release agent contaminates the inside of the charging device 22 due to the hot air flow from the fixing device 33 is eliminated, and the occurrence of image defects such as color unevenness parallel to the moving direction of the transfer belt is reduced. It further,
The transfer position is located above the developing device 24, so that the developer spill on the transfer belt 27 can be prevented. Therefore, a uniform contact state with the photoconductor can be obtained at the transfer portion, and the occurrence of image loss due to transfer failure can be eliminated. Therefore, a stable image can be obtained for a long period of time.

FIG. 3 shows claims 1, 2, 3 and
It is a schematic block diagram which shows the image forming apparatus which is one Embodiment of invention of Claim 6, Claim 7, or Claim 8. In this image forming apparatus, four photoconductor drums 41 are arranged so as to come into contact with the lower portion of the transfer belt 47. Below the photoconductor drums 41, each charging device 42 and the image writing device 4 are provided.
3 and the developing device 44 are arranged, but unlike the image forming apparatus shown in FIG. 2, the cleaning device is not provided. That is, the charging device 42 is installed at the position of about 7 o'clock of the photosensitive drum 21, the portion where the laser beam is irradiated at the position of about 6 o'clock, and the developing device 44 is installed at the position of about 5 o'clock. The shielding member 60 is made of a plate-shaped conductive member and is connected to a power source 61 that applies a DC voltage of -1000V as shown in FIG. As a result, an electric field is formed between the photoconductor drum 41 and the shield member 60 in a direction to attract the toner onto the photoconductor drum. Further, the shielding member 60 may be electrically grounded as shown in FIG.

Further, the photoconductor layer formed on the photoconductor drum 41 is less likely to cause potential attenuation when the exposure energy of the laser light is small, and exhibits large potential attenuation when the exposure energy is above a certain level. Is used. The other configuration of the image forming apparatus is the same as that of the image forming apparatus shown in FIG.

In such an image forming apparatus, the toner image on the photosensitive drum 41 is directly transferred onto the transfer belt 47, and the transferred toner image enters the fixing device 53 and at the same time is brought into contact with the recording paper, and the toner image is transferred. The image is fixed by heating and pressing. On the other hand, the photosensitive drum 41 is charged and irradiated with laser light while holding the residual toner after transfer, the residual toner is conveyed to a position facing the developing device 44, and the residual toner is transferred to a magnetic brush formed on the magnet roll. It is recovered by the scraping force and the electric field force. At the same time, the toner on the magnet roll is transferred to the electrostatic latent image, and development is performed. As a result, the cleaning device can be omitted, and the photoconductor drums can be brought closer to each other.

In the above image forming apparatus, the distance between the centers of the photosensitive drums is 92 mm, and the ratio L / D between the diameter D of the drums and the distance L between the photosensitive bodies is approximately 1.1. The length between the transfer positions of the first color and the fourth color is 276 mm, and the transfer belt 47 is at least the length between the transfer positions.
The distance between the rolls supporting the transfer belt is 326 mm. Further, by using a part of the fixing device also as a roll for supporting the transfer belt, the width of the entire device is reduced to about 580 mm.

As described above, the distance between the centers of the photosensitive drums can be made narrower than that of the device shown in FIG. 2, and the occurrence of positional deviation at the time of transferring the toner image can be further reduced. Further, as shown in FIG. 4A, since a DC voltage in the direction of attracting the toner to the photoconductor drum 41 is applied to the shielding member 60, the toner image on the photoconductor drum is held on the surface as it is, and the toner image is retained. Is not disturbed. Therefore, it is possible to prevent the image quality from being deteriorated due to the mutual interference between the photoconductor drums.

Further, since the photoconductor layer on the photoconductor drum 41 has a light attenuation characteristic in which a large potential attenuation occurs when the exposure energy exceeds a certain level, even if toner remains on the photoconductor drum. The toner does not affect the electrostatic latent image formed during exposure. For this reason,
The fluctuation of the surface potential is prevented, and an image having no density unevenness can be obtained.

On the other hand, as shown in FIG. 4B, when the shielding member 60 is electrically grounded, even if the toner is rubbed against the shielding member 60, the charged charges due to friction are localized. In addition, the toner image on the photoconductor is not disturbed by an electric force. Therefore, similarly to FIG. 4A, it is possible to prevent the deterioration of image quality.

Next, in order to confirm the effect of the present invention, Table 1 shows the dimensions and conditions of each of the three image forming apparatuses (Embodiments 1, 2 and 3) and the conventional image forming apparatus (Comparative Example). 5 and 6 show the results of examining the state of color misregistration of color images by conducting an image forming test for 1 hour continuously in a room temperature environment of 23 ° C. using these image forming apparatuses. Here, Example 1 is the image forming apparatus shown in FIG. 1, Example 2 is the image forming apparatus shown in FIG. 2, Example 3 is the image forming apparatus shown in FIG. 3, and Comparative Example is the conventional image forming apparatus shown in FIG. The device was used.

[Table 1]

As shown in this table, in the conventional image forming apparatus, the width of the entire apparatus is 1230 mm, whereas in Example 1, the width is 900 mm even if the developing device having the same photosensitive member and the same function is used. In Examples 2 and 3, the distance is 600 mm, and it can be seen that the device size can be improved by devising the arrangement of each member and device.

The color misregistration in the vertical direction which occurs in such an image forming apparatus is due to the thermal expansion of the frame supporting the photosensitive drum. A temperature sensor was attached between the photoconductors for the second and third colors in the conventional image forming apparatus and in the vicinity of the image writing device to measure the temperature transition. As shown in FIG. Confirmed to rise. The frame supporting the photoconductor drum is made of iron, and its expansion coefficient is 15 ×
It is 10 ^-6 . When the thermal expansion of the frame due to the temperature increase is calculated, the distance between the transfer positions from the first color to the fourth color is 588 mm in the conventional example.
At 3 ° C., there is a thermal expansion of 114 μm. On the other hand, in Examples 1 and 2, the distance between the transfer positions from the first color to the fourth color was 300 mm, so the thermal expansion was 5
It is about half, which is 8 μm. Therefore, as shown in FIG. 6A, in the conventional example, the color misregistration in the vertical direction after one hour has passed is 1
In contrast to 20 μm to 295 μm, in Example 1, the similar color shift is 250 μm as shown in FIG. 5A, which shows that the occurrence of color shift is improved.

Further, it is considered that each component of the image writing device is also distorted by heat and causes color misregistration. When the image writing apparatus is cooled in order to reduce the distortion due to heat in the conventional example, as shown in FIG. 6A, the color misregistration after one hour is 230 μm, and it is confirmed that the color misregistration in the vertical direction is reduced. To be done. In the second and third embodiments, the fixing device is located above the position of the frame that supports the photosensitive drum and the image writing device, and thus is less affected by the heat discharged from the fixing device than in the first embodiment. Therefore,
As shown in FIG. 5A, it can be seen that the occurrence of color misregistration in the vertical direction is further improved in the second and third embodiments.

Further, in the second and third embodiments, image loss and color unevenness parallel to the moving direction of the transfer belt are improved as compared with the first embodiment. This is because the developer spill on the transfer belt is eliminated and a uniform contact state with the photosensitive drum is obtained at the transfer portion. Further, it is considered that the charging device is less likely to be affected by the hot air flow from the fixing device and the charging failure is eliminated.

Further, in Example 3, the degree of color misregistration is almost the same as in Example 2, but the occurrence of uneven density due to uneven rotation of the photosensitive drum is remarkably improved. This is because by making the light attenuation characteristics of the photoconductor layer appropriate, a slight fluctuation in energy due to uneven rotation of the photoconductor drum does not appear as a potential difference in the latent image potential, and stable density can be obtained. Is. Although the cleaning device is omitted in the image forming apparatus of the third embodiment, the so-called ghost in which the history of the previous image appears is not generated. At this time, the developing weight is 0.5 mg / cm ² and the developing efficiency is 9
0%, the residual toner weight is 0.05 mg / c
the m ^2, but by the light-shielding of the residual toner of this order is because not reach the point vary the attenuation of the potential.

On the other hand, the lateral color shift is remarkably improved by reducing the distance between the supporting rolls of the transfer belt. That is, as shown in FIG. 6B, in the conventional example, the distance between the support rolls of the transfer belt is 900 mm, and the color shift in the lateral direction is about 60 to 70 μm.
As shown in (b), Example 1 (distance between supporting rolls: 6
It is understood that the color misregistration in the lateral direction is improved as much as the distance between the support rollers is shortened to about 30 to 40 μm. Further, in Example 2, since the distance between the support rolls is as small as 350 mm, it can be seen that the lateral color shift is further improved to 20 μm as shown in FIG. 5B.

Further, in Examples 1, 2, and 3, when the shielding member was removed in order to confirm the effect, and a color image was formed, turbulence occurred at the trailing edge of the image. When the apparatus was stopped during image formation and the state of the photosensitive drum was observed, a part of the toner image was attached to the adjacent photosensitive drum. At this time, in Example 3, the toners of different colors adhere to the photoconductor, and if the cleaning device is omitted, color mixing occurs in the developing device, and a good image cannot be obtained. Therefore, in Examples 1, 2, and 3, it was confirmed that the shielding member can prevent the apparatus from becoming large and can largely eliminate the color misregistration, and a good image can be obtained for a long period of time.

[0067]

As described above, in the image forming apparatus of the present invention, the distance between the photoconductor drums can be reduced without reducing the diameter of the photoconductor drums. Can solve the problem. Further, by providing the shielding member, it is possible to reduce the influence of the electric force and the air flow by the adjacent photoconductor drums, which is a problem when the photoconductor drums are brought close to each other, and it is possible to prevent the occurrence of image quality defects.

By providing the image writing device and the charging device below the fixing device, the influence of heat and dirt from the fixing device can be avoided and a stable image can be obtained for a long period of time. Further, by arranging the opening of the developing device to face upward so that the transfer position is located above the developing device, it is possible to eliminate the occurrence of transfer failure due to developer spill.

Further, by making the photosensitive layer of the photosensitive drum have the characteristic that the potential is less likely to be attenuated when the exposure energy of the laser light is weak and the potential is attenuated when the exposure energy exceeds a certain level, It is possible to suppress density unevenness due to speed fluctuations of the photoconductor drum and obtain good image quality for a long time. Further, the cleaning device can be omitted by utilizing the characteristics of the photoconductor layer,
The distance between the photoconductor drums can be made smaller.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram illustrating an image forming apparatus according to a first embodiment of the present invention;

FIG. 2 is a schematic configuration diagram showing an image forming apparatus according to a second embodiment of the invention related to the present application.

FIG. 3 is a schematic configuration diagram showing an image forming apparatus according to a third embodiment of the invention related to the present application.

FIG. 4 is a diagram showing a configuration of a shielding member used in the image forming apparatus shown in FIG.

FIG. 5 is a diagram showing changes over time in color misregistration in the three image forming apparatuses.

FIG. 6 is a diagram showing changes over time in color misregistration in an image forming apparatus which is a comparative example.

FIG. 7 is a diagram showing a temperature change in an image forming apparatus as a comparative example.

FIG. 8 is a schematic configuration diagram showing an example of a conventional image forming apparatus.

FIG. 9 is a schematic configuration diagram showing another example of a conventional image forming apparatus.

FIG. 10 is a diagram illustrating vertical color misregistration in the image forming apparatus shown in FIG.

FIG. 11 is a diagram illustrating a lateral color shift due to a walk of the transfer belt in the image forming apparatus illustrated in FIG.

FIG. 12 is a diagram showing a problem of the conventional image forming apparatus.

FIG. 13 is a diagram showing a configuration of an image forming apparatus according to the present invention.

FIG. 14 is a diagram showing a configuration of an image forming apparatus according to the present invention.

FIG. 15 is a diagram showing a configuration of an image forming apparatus according to the present invention.

[Explanation of symbols]

 1, 21, 41 Photosensitive drum 2, 22, 42 Charging device 3, 23, 43 Image writing device 4, 24, 44 Developing device 5, 25, 45 Transfer device 6, 26 Cleaning device 7, 27, 47 Transfer belt 8 , 28, 48 Recording paper 9, 29, 49 Paper tray 10, 30, 50 Paper transport path 11 Adsorption charger 12 Separation charger 13, 33, 53 Fixing device 15, 35, 55 Support roll 16 Support roll 36, 56 fixing roll 20, 40, 60 shielding member 61 power supply

Claims (8)

[Claims] 1. A plurality of photosensitive drums on which an electrostatic latent image is formed on a peripheral surface thereof, a charger for uniformly charging the photosensitive drums, and an image writing device for irradiating the photosensitive drums with image light. A plurality of developing devices that form toner images of different colors on the plurality of photoconductor drums, a transfer belt that is in close proximity to or in contact with the plurality of photoconductor drums, and is stretched in a loopable manner; In a color image forming apparatus having a fixing device for fixing toner images of a plurality of colors superposed on a belt onto a recording sheet, the plurality of photoconductor drums are arranged along an outer peripheral surface of the transfer belt and are adjacent to each other. And a plurality of developing devices are installed on the side opposite to the side where the row of the photosensitive drums faces the transfer belt. Image formation Location. 2. The color image forming apparatus according to claim 1, wherein L / D is 1.3 or less, where D is a diameter of the photosensitive drum and L is a center distance between adjacent photosensitive drums. An image forming apparatus characterized by the above. 3. The color image forming apparatus according to claim 1, wherein the transfer belt is stretched so that a lower portion of a circulation path of the transfer belt is substantially horizontal, and the transfer belt is close to or in contact with the horizontal portion. As described above, the plurality of photosensitive drums are arranged, the plurality of developing devices and the image writing devices are arranged below the photosensitive drum row, and the fixing device is arranged above the central axis of the photosensitive drums. And a color image forming apparatus. 4. The color image forming apparatus according to claim 1, 2, or 3, wherein a plate-shaped shielding member is provided between the photoconductor drums arranged adjacent to each other. Color image forming apparatus. 5. The color image forming apparatus according to claim 4, wherein the volume resistance value is 1 at least near the surface of the shielding member.
A color image forming apparatus characterized by being formed of a semi-conductive material of about 0 ⁶ Ω · cm to 10 ¹¹ Ω · cm. 6. The color image forming apparatus according to claim 4 or 5, wherein the shielding member is electrically grounded. 7. The color image forming apparatus according to claim 4, wherein the shielding member is provided with a DC voltage that forms an electric field in a direction to attract the toner on the photosensitive drum toward the photosensitive drum. A color image forming apparatus characterized by being applied. 8. The color image forming apparatus according to claim 1, wherein the photosensitive layer on the photosensitive drum has a potential when the exposure energy of the image light irradiated after uniform charging is small. Attenuation is less likely to occur, and shows a light attenuation characteristic in which a large potential attenuation occurs when the exposure energy exceeds a certain level, and the developing device removes the toner remaining after the toner image formed on the photosensitive drum is transferred. A color image forming apparatus, which is set to collect.