JP2002014515A - Image forming device and belt device used for the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a tandem type laser printer capable of preventing a worm- eaten state image from being caused at the time of superposing a toner image on transfer paper, and preventing faulty transfer such as void without weakening the pressing of a pressing means for pressing the transfer paper to a photoreceptor drum too much. SOLUTION: The pressing force of the pressing means for pressing the transfer paper 100 to the photoreceptor drum 11 is set to be weaker as it comes nearer to a toner image forming part on a downstream side in a transfer paper moving direction. In the case of constituting the device by using backup rollers 68Y, 68M, 68C and 68K guiding the movement of a transfer carrying belt 60 so that the belt 60 may be partially wound round the peripheral surface of the drum 11 at a transfer position, the projecting amount of the backup roller in the direction of the drum 11 is set to obtain specified pressing force.

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, a printer, a facsimile, etc. and a belt device used in the apparatus. More specifically, the present invention relates to a plurality of toner image forming sections and a plurality of toner image forming sections. The present invention relates to an image forming apparatus provided with a plurality of pressing means for pressing a transfer object passing through a plurality of transfer positions in contact with and facing each image carrier against each image carrier, and a belt device used in the apparatus.

[0002]

2. Description of the Related Art Conventionally, as this type of image forming apparatus,
A paper as a transfer / conveying member that carries a plurality of toner image forming units each having a photosensitive drum or the like as an image carrier on which a toner image composed of a charged toner is formed on the surface, and carries and carries a transfer paper as a transfer material 2. Description of the Related Art There is known an image forming apparatus of a so-called tandem type, which is arranged in parallel along the moving direction of a conveyor belt (for example, see Japanese Patent Application Laid-Open No. 6-202430). In this image forming apparatus, the electrostatic latent images formed on the photosensitive drums of the respective toner image forming units are developed using charged toners of different colors.
Then, a transfer bias is applied to the paper transport belt at a transfer position where each photoconductor drum and the paper transport belt come into contact with each other, and the charged toner image on each photoconductor drum is transferred to the paper transport belt by a transfer electric field generated by the transfer bias. The images are sequentially transferred to the upper transfer paper to form a color image.

In such a tandem type image forming apparatus having a plurality of toner image forming units, a pressing means for pressing the transfer paper against the surface of the photosensitive drum may be provided for each toner image forming unit. By increasing the contact pressure between the transfer paper and the surface of the photosensitive drum by the pressing means, the transfer efficiency at each transfer position can be increased, and the occurrence of transfer defects such as white spots can be prevented. In addition, it is also possible to suppress the waviness of the transfer conveyance belt at each transfer position and bring the transfer paper into uniform contact with the surface of the photosensitive drum, thereby suppressing transfer unevenness.

[0004]

However, in an image forming apparatus provided with the above-mentioned pressing means, an abnormal image called an "worm-eating image" in which a part of a toner image to be transferred to a transfer-receiving member is missing in a worm-like manner. May occur. The present inventors have conducted intensive studies on the cause of this worm-eating image. As a result, it was found that the larger the number of toner images to be superimposed on the transfer paper, the more easily the above-mentioned worm-eating image is generated. In the case of forming a single-color image without the superposition of the toner images, the worm-like image hardly occurred. In addition, it is considered that the wormy image when the toner image is superimposed on the transfer paper is generated as follows. FIG.
As shown in (a), the first-color transferred toner image 502 of the transfer paper 501 carried and conveyed on the transfer conveyance belt 500.
When the second color toner image 504 on the photosensitive drum 503 is transferred in a superimposed manner, the toner image 502 on the transfer paper 501 and the photosensitive drum 5 are transferred at the transfer nip as shown in FIG.
03, the toner image 504 is in close contact. Here, if the pressing force of the transfer paper 501 against the photosensitive drum 503 is large,
The adhesion between the second color toner image 504 and the surface of the photosensitive drum is the first color toner image 502 and the second color toner image 5.
03 tends to be larger than the adhesive strength between them. Due to the magnitude relationship of the adhesion, when the transfer paper 501 passes through the transfer nip and separates from the surface of the photosensitive drum 503, a part 5 of the second color toner image 504 on the photosensitive drum
04 ′ is separated from the surface of the first color toner image 502 and remains on the photosensitive drum side, and is considered to be an insect-eating image on the transfer paper 501 as shown in FIG. 9C.

[0005] Further, from experiments conducted by the present inventors, it has been found that the insect-eating image becomes more likely to occur as the number of superimposed toner images on the transfer paper increases. When the number of superimposed toner images on the transfer paper increases, the toner image overlaps between the transfer paper and the photosensitive drum, and a portion where the toner layer is thickened is sandwiched, and the toner image is strongly compressed. When the thickened toner layer is strongly compressed, the adhesion between the toner image carried on the photoconductor drum and the surface of the photoconductor drum is increased by each of the transferred toner images on the transfer paper. It is considered that the adhesion is likely to be larger than the adhesion between the toner images. For this reason, it is considered that an experimental result was obtained in which the larger the number of superimposed toner images on the transfer paper, the easier the worm-like image was generated.

SUMMARY OF THE INVENTION The present invention has been made in view of the above background, and has as its object to provide a plurality of toner image forming units and a plurality of image bearing members for pressing a transfer target against an image carrier of each toner image forming unit. An image forming apparatus including a pressing unit and a belt device used in the device, wherein it is possible to prevent occurrence of a worm-like image when a toner image is superimposed on a transfer target, and to press each of the pressing units. An object of the present invention is to provide an image forming apparatus capable of preventing transfer defects such as white spots without weakening more than necessary, and a belt device used in the apparatus.

Japanese Patent Laid-Open No. Hei 6-202430 discloses that the pressing force of the backup member in each transfer position in the direction of the image carrier of the backup member corresponding to the pressing means is set for each image forming station corresponding to the toner image forming section. However, it is noted that the insect-eating image is more likely to occur as the number of superimposed toner images on the transfer paper increases, and a configuration for setting the pressing force is suggested. There is no such description.

[0008]

In order to achieve the above object, according to the present invention, a plurality of toner image forming sections each having an image carrier on which a toner image is formed, and the plurality of toner image forming sections are provided. A plurality of pressing means for pressing a transfer target body passing through a plurality of transfer positions facing each image carrier of the image forming unit to each image carrier, wherein each image carrier and the transfer target body are in contact with and opposed to each other. At the transfer position, in an image forming apparatus for sequentially transferring the toner images formed on the respective image carriers on the image receiving body by superimposing the toner images, the pressing force of the pressing unit against each toner image forming unit is changed to the image on the image receiving body. The toner image forming section on the downstream side in the moving direction is set to be weaker.

In this image forming apparatus, the pressing force of the pressing means is increased by increasing the number of superimposed toner images, thereby increasing the possibility of the occurrence of the worm-like image. The weaker the formed part. By weakening the pressing force in this manner, even in a portion where the amount of superimposed toner images is large, the adhesion between the image carrier and the toner image is more reliable than the adhesion between the toner images on the transfer-receiving body. Therefore, a part of the toner image does not remain on the surface of the image carrier that has passed the transfer position. Since the number of superimposed toner images on the transfer object passing through the transfer position is small for the toner image forming unit on the upstream side in the transfer object moving direction, the above-described pressure can be maintained without reducing the pressing force of the pressing unit. No wormy images occur. Therefore, the pressing force of the pressing means can be set so as to more reliably prevent the transfer failure such as the white spot.

According to a second aspect of the present invention, in the image forming apparatus of the first aspect, a belt-shaped transfer / transport member for carrying and transporting the transfer material so as to pass through the plurality of transfer positions is provided.
The transfer object is the transfer material conveyed by the transfer conveyance member.

In this image forming apparatus, even in a portion where the amount of superimposed toner image is large, the adhesive force between the image carrier and the toner image is transferred to a transfer material carried and conveyed by a belt-shaped transfer conveyance member. Since the adhesive force is surely smaller than the adhesive force between the upper toner images, a part of the toner image does not remain on the surface of the image carrier that has passed the transfer position.

According to a third aspect of the present invention, in the image forming apparatus of the first aspect, the object to be transferred is a belt-shaped intermediate transfer member.

In this image forming apparatus, the adhesion between the image carrier and the toner image is greater than the adhesion between the toner images on the belt-like intermediate transfer member even in a portion where the amount of toner image overlap is large. Therefore, a part of the toner image does not remain on the surface of the image carrier that has passed the transfer position.

According to a fourth aspect of the present invention, in the image forming apparatus of the second or third aspect, the image carrier is a roller-shaped member, and the pressing means is provided at the transfer position with the transfer / transport member or the intermediate transfer member. The transfer conveyance member or the guide member for guiding the movement of the intermediate transfer member is configured to partially wind the body around the peripheral surface of the image carrier, and each of the guide members is configured to have the predetermined pressing force. Is set.

In this image forming apparatus, when the tension of the belt-like transfer / transport member or the belt-like intermediate transfer member is the same, the amount of winding of the transfer / transport member around the image carrier made of the roller-like member is controlled. The more
The pressing force with which the transfer / conveying member presses the peripheral surface of the image carrier increases. Therefore, each guide member is arranged at a different position for each toner image forming unit, and the amount of winding of the transfer conveyance member or the like around the image carrier is set. As a result, the pressing force for pressing the transfer material or the intermediate transfer member on the transfer / conveying member against the peripheral surface of the image carrier is reduced in the toner image forming portion on the downstream side in the transfer object moving direction.

According to a fifth aspect of the present invention, in the image forming apparatus of the second or third aspect, the image carrier is a roller-shaped member, and the pressing means is configured to move the transfer member or the intermediate transfer member at the transfer position. A guide member for guiding the movement of the transfer / transfer member or the intermediate transfer member so as to partially wind the body around the peripheral surface of the image carrier, and an urging means for urging the guide member toward the image carrier, The biasing force of each biasing means is set so as to achieve the predetermined pressing force.

In this image forming apparatus, the urging force of the urging means for urging the guide member toward the image carrier is set for each toner image forming section, so that the toner image on the downstream side in the moving direction of the transfer object is set. The pressing force for pressing the transfer material or the intermediate transfer member on the transfer / conveying member against the peripheral surface of the image carrier is reduced in the forming unit.

According to a sixth aspect of the present invention, in the image forming apparatus of the second or third aspect, a transfer bias applying member which comes into contact with the transfer conveying member or the intermediate transfer member from the opposite side of the transfer position from the image carrier. And the pressing means is constituted by using the transfer bias applying member, and an urging means for urging the transfer bias applying member toward the image carrier, so that each of the pressing means has the predetermined pressing force. It is characterized in that the urging force of the urging means is set.

In this image forming apparatus, the urging force of the urging means for urging the transfer bias applying member, which also serves as one component of the pressing means, toward the image carrier is set for each toner image forming unit. As a result, the pressing force for pressing the transfer material or the intermediate transfer member on the transfer / conveying member against the peripheral surface of the image carrier is reduced in the toner image forming portion on the downstream side in the transfer object moving direction.

The invention according to claim 7 is based on claims 1, 2, 3,
In the image forming apparatus of 4, 5, or 6, each of the plurality of toner image forming units forms a toner image on the image carrier using toner of a different color. The color image is formed by superimposing and transferring the formed toner image on the transfer object.

In this image forming apparatus, the generation of the worm-like image for the color of the toner used in the toner image forming section located on the downstream side in the moving direction of the transfer object is suppressed. This prevents the occurrence of an abnormal image in which a specific color image is partially missing in a color image in which toner images of each color are superimposed on a transfer target.

According to a further aspect of the present invention, a belt-shaped transfer / transporter which carries and conveys the transfer material so that the transfer material passes through a plurality of transfer positions facing each image carrier of the plurality of toner image forming units. In a belt device comprising a member and a plurality of pressing means for pressing the transfer material against each image carrier at each transfer position, the pressing force of the pressing means against each toner image forming portion is set to a downstream side in a transfer material moving direction. The toner image forming portion is set to be weaker.

In this belt device, even in a portion where the amount of toner image overlap is large, the adhesion force between the image carrier and the toner image is increased by the transfer material carried and conveyed by the belt-shaped transfer / conveyance member. Therefore, a part of the toner image does not remain on the surface of the image carrier that has passed the transfer position. Since the number of superimposed toner images on the transfer paper passing through the transfer position is small for the toner image forming portion on the upstream side in the transfer body moving direction, the insect-eating image can be obtained without reducing the pressing force of the pressing means. Does not occur. Therefore, the pressing force of the pressing means can be set so as to more reliably prevent the transfer failure such as the white spot.

According to a ninth aspect of the present invention, there is provided a belt-like intermediate transfer member which is driven so as to pass through a plurality of transfer positions facing each image carrier of a plurality of toner image forming units, and the intermediate transfer member at each transfer position. In a belt device having a plurality of pressing means for pressing the transfer body against each image carrier, the pressing force of the pressing means on each toner image forming unit is weaker toward the toner image forming unit on the downstream side in the intermediate transfer body moving direction. It is characterized by having been set so as to be.

In this belt device, the adhesion between the image carrier and the toner image is greater than the adhesion between the toner images on the belt-shaped intermediate transfer member even in a portion where the amount of toner image overlap is large. Since the size is reliably reduced, a part of the toner image does not remain on the surface of the image carrier that has passed the transfer position. Since the number of superimposed toner images on the intermediate transfer member passing through the transfer position is small for the toner image forming portion on the upstream side in the transfer object moving direction, the toner image forming portion does not need to weaken the pressing force of the pressing means. No wormy images occur. Therefore, the pressing force of the pressing means can be set so as to more reliably prevent the transfer failure such as the white spot.

[0026]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment in which the present invention is applied to an electrophotographic color laser printer (hereinafter, referred to as "laser printer") as an image forming apparatus will be described below. FIG. 1 is a schematic configuration diagram of a laser printer according to the present embodiment. This laser printer includes four sets of toner image forming units 1Y for forming images of each color of yellow (Y), magenta (M), cyan (C), and black (K).
1M, 1C, 1K (hereinafter, suffixes Y, M, C, K
Indicate the members for yellow, magenta, cyan, and black, respectively) are arranged in order from the upstream side in the moving direction (direction of arrow A in the figure) of the transfer paper as the transfer target (transfer material). ing. The toner image forming units 1Y, 1
Each of M, 1C, and 1K includes a photosensitive unit having photosensitive drums 11Y, 11M, 11C, and 11K as image carriers, and a developing unit. The toner image forming units 1Y, 1M, 1C, and 1K are arranged so that the rotation axes of the photosensitive drums in the respective photosensitive units are parallel and are arranged at a predetermined pitch in the transfer paper moving direction. ,
Is set.

The laser printer of the present invention carries an optical writing unit 2, paper feed cassettes 3, 4, a pair of registration rollers 5, and transfer paper in addition to the toner image forming units 1Y, 1M, 1C, and 1K. The image forming apparatus includes a transfer unit 6 as a belt device having a transfer conveyance belt 60 as a transfer conveyance member that conveys the toner image so as to pass through the transfer position of the toner image forming unit, a fixing unit 7 of a belt fixing type, a paper discharge tray 8, and the like. .
The laser printer also includes a manual feed tray (not shown), a toner supply container, a waste toner bottle, a duplex / reversing unit, a power supply unit, and the like. 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 11K with a laser beam while scanning based on image data. I do. A dashed line in FIG. 2 indicates a transfer path of the transfer paper. The transfer paper fed from the paper feed cassettes 3 and 4 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 sent out by the registration roller pair 5 at a predetermined timing is carried by the transfer conveyance belt 60, and the toner image forming units 1Y, 1Y
The sheet is conveyed so as to pass through the transfer positions of 1M, 1C, and 1K. The transfer paper on which the toner images of the respective toner image forming units 1Y, 1M, 1C, and 1K are transferred and the color image is formed is discharged onto a discharge tray 8 after the fixing unit 7 fixes the toner images.

FIG. 3 shows the toner image forming sections 1Y, 1M,
FIG. 3 is an enlarged view showing a schematic configuration of a yellow toner image forming unit 1Y of 1C and 1K. Other toner image forming units 1M,
1C and 1K have the same configuration, respectively, and thus description thereof is omitted. In FIG. 3, the toner image forming unit 1Y includes the photoconductor unit 10Y and the developing unit 20Y as described above. Photoconductor unit 10
Y denotes a photosensitive drum 11Y, a brush roller 12Y for applying a lubricant to the surface of the photosensitive drum, a swingable counter blade 13Y for cleaning the surface of the photosensitive drum, and a neutralization for removing the surface of the photosensitive drum. Lamp 14
And a non-contact type charging roller 15Y for uniformly charging the surface of the photosensitive drum. As the photoconductor drum of each photoconductor unit, a photoconductor drum having an organic photoconductor (OPC) layer on the surface and having a diameter of 30 mm was used. In the photoconductor unit having the above configuration, the photoconductor drum 11Y is uniformly charged by the charging roller 15Y to which the AC voltage is applied.
Is irradiated with the laser beam modulated and deflected by the optical writing unit 2 while being scanned, an electrostatic latent image is formed on the surface of the photosensitive drum 11Y. The electrostatic latent image on the photoreceptor drum 11Y is developed by a later-described developing unit 20Y to become a yellow toner image. Transfer conveyor belt 6
At the transfer position Pt where the transfer paper 100 on the transfer roller 100 passes through the transfer paper 100, the toner image on the photosensitive drum 11Y is transferred to the transfer paper 100. After a predetermined amount of lubricant is applied by a brush roller 12Y, the surface of the photosensitive drum 11Y after the transfer of the toner image is cleaned by a counter blade 13Y, and is discharged by light emitted from a discharge lamp 14Y. It is prepared for the formation of the next electrostatic latent image.

The developing unit 20Y uses a two-component developer containing a magnetic carrier and a negatively charged toner. Then, a developing roller 22Y as a developer carrying member, which is disposed so as to be partially exposed from an opening on the photosensitive drum side of the developing case 21Y, and conveying screws 23Y and 2Y.
4Y, a developing doctor 25Y, a toner density sensor (T sensor) 26Y, a powder pump 27Y, and the like. The developer contained in the developing case 21 </ b> Y is supplied to the transfer screw 23.
The toner is agitated and conveyed in Y and 24Y to be charged by friction. Then, after a part of the developer is carried on the surface of the developing roller 22Y and the layer thickness is regulated by the developing doctor 25Y,
The photosensitive drum 11Y is transported to a developing position facing the photosensitive drum 11Y.
At the developing position, the electrostatic latent image on the photosensitive drum 11Y is developed by the toner in the developer on the developing roller. The toner concentration of the developer in the developing case 21Y is detected by the toner concentration sensor 26Y, and if necessary, the powder pump 27Y
Supplies toner.

FIG. 1 is an enlarged view showing a schematic configuration of the transfer unit 6. The transfer conveyance belt 60 used in the transfer unit 6 has a volume resistivity of 10 ⁹ to 10 ¹¹ Ωc.
m, a high-resistance endless single-layer belt made of PVDF (polyvinylidene fluoride). The transfer / conveyance belt 60 is connected to the photosensitive drum 11 of each toner image forming unit.
Y, 11M, 11C, and 11K, four grounded support rollers 61 to
It is multiplied by 64. Of these support rollers,
A power supply 65 is provided at the entrance roller 61 on the upstream side in the transfer paper moving direction.
The electrostatic attraction roller 65 to which a predetermined voltage has been applied from a. These two rollers 61, 6
The transfer paper that has passed between 5 is electrostatically attracted onto the transfer / conveyance belt 60. The exit roller 62 on the downstream side in the transfer paper moving direction is a drive roller that frictionally drives the transfer conveyance belt 60, and is connected to a drive source (not shown). A bias roller 66 to which a predetermined cleaning voltage is applied from a power source 66a is arranged so as to come into contact with the outer peripheral surface of the transfer conveyance belt 60 stretched between the support rollers 61c and 61d. The bias roller 66 removes foreign matters such as toner adhered to the transfer conveyance belt 60.

Further, as a transfer electric field forming means for forming a transfer electric field at each transfer position, a transfer bias is applied so as to come into contact with the back surface of the transfer conveying belt 60 forming the transfer nip in contact with the photosensitive drum. Member 67Y, 6
7M, 67C and 67K are provided. The transfer bias applying members 67Y, 67M, 67C, and 67K are fixed brushes made of Mylar, and each of the transfer bias power supplies 9Y, 9M, and 9K.
A transfer bias is applied from C and 9K. By the transfer bias applied by the transfer bias applying member, a transfer charge is applied to the transfer / conveyance belt 60, and a transfer electric field having a predetermined strength is formed between the transfer / conveyance belt 60 and the surface of the photosensitive drum at each transfer position. You.

In the present embodiment, a plurality of pressing means for pressing the transfer paper 100 passing through each transfer position against each of the photosensitive drums 11Y, 11M, 11C, and 11K together with the transfer conveyance belt 60 is used. Backup rollers 68Y, 68M as guide members for guiding movement,
It is configured using 68C and 68K. FIGS. 4 and 5 are a front view and a side view of the photosensitive drum 11Y and the backup roller 68Y in the yellow toner image forming unit 1Y, respectively. 4 and 5, the backup roller 68 for the yellow toner image forming unit 1Y is used.
Y, the other backup rollers 68
M, 68C, and 68K have the same configuration except for the protrusion amount described later.

Rotation shafts at both ends of the backup roller 68Y are supported by a unit side plate (not shown) via bearings 681Y and 682Y, and the backup roller 68Y can rotate with the movement of the transfer conveyance belt 60. The backup roller 68Y projects to the toner image forming unit 1Y side by a predetermined projection amount indicated by D in the figure, and moves the transfer conveyance belt 60 to the peripheral surface of the photosensitive drum 11Y at a position upstream of the transfer position in the transfer paper moving direction. The transfer and conveyance belt 60 is guided so as to be partially wound. Here, the “projection amount” D is the distance between a virtual plane passing through each transfer position and the vertex of the backup roller 68Y on the photosensitive drum side.

The transfer / transport belt 60 is moved to the photosensitive drum 1
Since the transfer conveyance belt 60 is pressed against the peripheral surface of the photosensitive drum 11Y by being wound around 1Y, the contact pressure between the transfer paper 100 and the photosensitive drum 11Y at the transfer nip increases, and the transfer efficiency at the transfer position increases. . Transfer defects such as white spots can be prevented. In particular, in the case of the present embodiment, since the transfer conveyance belt 60 is wound around the peripheral surface of the photosensitive drum 11Y on the upstream side of the transfer position in the transfer paper moving direction, the photosensitive drum 11Y formed on the upstream side of the transfer nip is formed. The transfer electric field does not reach the gap between the transfer belt 60 and the transfer belt 60, so that the transfer dust due to the pre-transfer in the gap hardly occurs.

By providing the pressing means as described above, it is possible to prevent transfer defects such as white spots on the transfer / conveyance belt 60 and occurrence of transfer dust due to pre-transfer on the upstream side of the transfer position. However, if the pressing force of the pressing unit is set in the same manner for each toner image forming unit, when the toner image forming unit on the downstream side in the transfer paper moving direction transfers the toner image, the above-mentioned insect-eating image is not obtained. There was a risk of occurrence.

Therefore, in the present embodiment, the pressing force of the pressing means on each of the toner image forming units 1Y, 1M, 1C, and 1K is set to be lower toward the downstream side in the transfer paper moving direction. Specifically, the backup roller 68Y for the yellow toner image forming unit 1Y on the most upstream side
Is set to 1 mm, and magenta,
The protrusion amounts D of the backup rollers 68M, 68C, 68K with respect to the cyan and black toner image forming units 1M, 1C, 1K are 0.8 mm, 0.6 mm, and 0.4 mm, respectively.
Set to. By setting the protrusion amount D, the pressing force with which the transfer paper 100 presses the surface of the photosensitive drum at each transfer nip becomes smaller toward the toner image forming portion on the downstream side in the transfer paper movement direction. Therefore, transfer paper 1
The pressing force of the transfer paper 100 pressing the surface of the photosensitive drum does not become excessively large in the portion where the amount of the toner image on the image No. 00 is large. Therefore, when the transfer paper passes through the transfer nip and separates from the surface of the photoconductor drum, a part of the superimposed toner image on the transfer paper does not adhere to the surface of the photoconductor drum in a state where the transfer paper is peeled off. Occurrence can be prevented.

As described above, according to the present embodiment, the transfer paper 100
It is possible to prevent the occurrence of an insect-eating image when a toner image is superimposed on the toner image. In addition, in the toner image forming portion on the upstream side in the transfer paper moving direction, transfer failure such as white spots can be more reliably prevented without unnecessarily reducing the pressing force for pressing the transfer paper 100 against the photosensitive drum collecting surface. be able to.

In the pressing means in the above embodiment, the backup rollers 68Y, 68M, 68
Although the predetermined pressing force is set by changing the positions of C and 68K, as shown in FIG. 6, the pressing means includes backup rollers 68Y, 68M, 68C and 68K, and each backup roller is connected to a photosensitive drum. It may be configured using pressure springs 69Y, 69M, 69C, 69K as urging means for urging to the side. Each pressure spring is provided at each of both ends of the backup roller, and urges a bearing attached to a rotation shaft at each of the both ends to transfer the transfer conveyance belt 6.
0 is pressed against the surface of the photosensitive drum. Further, the backup rollers are urged by the respective pressure springs so as to press the transfer paper transport belt 60 toward a transfer paper guide member (not shown) provided above the backup roller with the transfer paper 100 interposed therebetween. Is also good.

In the structure shown in FIG.
The pressing force for pushing up the backup rollers 68Y, 68M, 68C, 68K by 9Y, 69M, 69C, 69K is set so as to become smaller toward the toner image forming portion on the downstream side in the transfer paper moving direction. Specifically, the backup roller 68Y in the yellow toner image forming unit 1Y on the most upstream side
To 2.45 N for the rotation axis at each end.
(= 250 gf), and a total of 4.9 N (=
500 gf). And magenta on the downstream side,
The pressure applied to the backup rollers 68M, 68C, 68K in the cyan and black toner image forming units 1M, 1C, 1K is set to 4N, 3N, and 2N, respectively, at both ends.

As shown in FIG. 7, a transfer roller 670 serving as a transfer bias applying member is used as the pressing means.
Y, 670M, 670C, and 670K may also be used. Each transfer roller has an elastic layer around a cored bar to which a transfer bias is applied. To the extent that the elastic layer is slightly deformed, each transfer roller is pressed into contact with the corresponding photosensitive drum 11Y, 11M, 11C, 11K. The predetermined pressing force for each toner image forming unit can be set by changing the elastic characteristic of the elastic layer of each transfer roller or changing the amount of bite into the photosensitive drum.

Further, in the above-described embodiment, the case where the transfer object is the transfer paper supported and conveyed on the transfer conveyance belt has been described. May also occur on the belt-like intermediate transfer member. Therefore, the present invention can be applied to an image forming apparatus using such a belt-shaped intermediate transfer member, and a similar effect can be obtained.

FIG. 8 shows an example of the configuration of an intermediate transfer belt unit 200 in a laser printer using an endless intermediate transfer belt 201 as an intermediate transfer member. In this laser printer, the intermediate transfer belt 201
It is wrapped around a plurality of support rollers 202 to 205 and is driven to rotate in the direction of the arrow in the figure. The toner images formed on the photosensitive drums 11Y, 11M, 11C, and 11K of the respective toner image forming units are transferred to a primary transfer roller 206 provided to face each photosensitive drum with the intermediate transfer belt 201 interposed therebetween.
The image is sequentially transferred onto the intermediate transfer belt 201 by the primary transfer electric field formed by Y, 206M, 206C, and 206K. The color image on the intermediate transfer belt 201 is provided near the black toner image forming portion of the fourth color.
In the next transfer section, the grounded support roller 203 and the secondary transfer roller 20 to which a predetermined secondary transfer bias is applied
7 are collectively transferred onto transfer paper 100 conveyed so as to pass between them.

In the intermediate transfer unit shown in FIG. 8, the backup rollers 208Y, 208M, 208C, and 20K are formed by the pressing springs 209Y, 209M, 209C, and 209K, similarly to the transfer unit shown in FIG.
The pressing force for depressing 8K is set to be smaller at the toner image forming portion on the downstream side in the intermediate transfer belt moving direction.
With this setting, the pressing force of the intermediate transfer belt 201 pressing the surface of the photosensitive drum becomes weaker toward the toner image forming portion on the downstream side. Can be suppressed.

In the above embodiment, a case where a two-component developer is used has been described as an example. However, the present invention is also applicable to a case where a one-component developer is used.

Further, in the above-described embodiment, the case of a laser printer having four sets of toner image forming units for forming images of four colors of yellow, magenta, cyan, and black has been described. The present invention can be applied without being limited to the number of toner image forming units, and a similar effect can be obtained. Then, a plurality of toner image forming units are provided, and the toner images formed on the image carriers of the respective toner image forming units are sequentially transferred to a transfer target such as an intermediate transfer member or a transfer sheet to form a superimposed image. As long as the image forming apparatus is not limited to a laser printer, the present invention can be applied to other image forming apparatuses such as a copying machine.

[0046]

According to the first to ninth aspects of the present invention, it is possible to prevent the occurrence of a worm-like image when a toner image is superimposed on a transfer object, and to weaken the pressing force of each pressing means more than necessary. There is an effect that transfer defects such as white spots can be prevented without any problem.

In particular, according to the second and eighth aspects of the present invention, it is possible to prevent the occurrence of a worm-like image when a toner image is transferred onto a transfer material carried and conveyed by a transfer / conveyance member. .

In particular, according to the third and ninth aspects of the present invention, it is possible to prevent the occurrence of a worm-like image when a toner image is superimposed and transferred onto an intermediate transfer member.

In particular, according to the fourth aspect of the present invention, the arrangement of the guide member is set for each of the toner image forming sections, so that the more the toner image forming section is located on the downstream side in the moving direction of the transfer object, the more the transfer conveyance is performed. There is an effect that the pressing force for pressing the transfer material on the member or the intermediate transfer body against the peripheral surface of the image carrier can be reduced.

In particular, according to the fifth aspect of the present invention, the urging force of the urging means for urging the guide member toward the image carrier is set for each of the toner image forming units, so that the transfer conveyance member can be moved. There is an effect that the pressing force of pressing the transfer material or the intermediate transfer body against the peripheral surface of the image carrier can be reduced.

In particular, according to the invention of claim 6, since the transfer bias applying member is also used as one component of the pressing means, the size and cost of the apparatus can be reduced. Then, by setting the urging force of urging means for urging the transfer bias applying member toward the image carrier for each toner image forming unit, the transfer material on the transfer / transport member or the intermediate transfer member is image-supported. There is an effect that the pressing force for pressing the peripheral surface of the body can be reduced.

According to the seventh aspect of the present invention, in the color image in which the toner images of each color are superimposed on the transfer object,
There is an effect that it is possible to prevent occurrence of an abnormal image in which an image of a specific color is partially omitted.

[Brief description of the drawings]

FIG. 1 is an enlarged view of a main part around a transfer belt unit of a laser printer according to an embodiment.

FIG. 2 is a schematic configuration diagram of the laser printer.

FIG. 3 is a schematic configuration diagram of a toner image forming unit of the laser printer.

FIG. 4 is an enlarged front view around a transfer position in a yellow toner image forming unit.

FIG. 5 is an enlarged side view around a transfer position in the yellow toner image forming unit.

FIG. 6 is an enlarged view of a main part around a transfer belt unit of a laser printer according to another embodiment.

FIG. 7 is an enlarged view of a main part around a transfer belt unit of a laser printer according to still another embodiment.

FIG. 8 is an enlarged view of a main part around an intermediate transfer belt unit of a laser printer according to still another embodiment.

FIGS. 9A to 9C are explanatory diagrams showing the appearance of an insect-eating image on transfer paper.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Toner image forming part 6 Transfer unit 9 Transfer bias power supply 10 Photoconductor unit 11 Photoconductor drum 20 Developing unit 60 Transfer conveyance belt 61-64 Support roller 67 Transfer bias application member (fixed brush) 68 Backup roller 69 Pressure spring 100 Transfer paper

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Hitoshi Ishibashi 1-3-6 Nakamagome, Ota-ku, Tokyo F-term in Ricoh Co., Ltd. (Reference) 2H030 AA04 AB02 AD05 BB02 BB42 BB46 BB54 BB71 2H032 AA05 AA14 AA15 BA01 BA09 BA11 BA18 BA23 CA02

Claims (9)

[Claims] A plurality of toner image forming portions each having an image carrier on which a toner image is formed, and a transfer object passing through a plurality of transfer positions of the plurality of toner image forming portions facing the respective image carriers. A plurality of pressing means for pressing each image carrier against each image carrier, and transferring the toner image formed on each image carrier at each transfer position where each image carrier and the object to be transferred come into contact with each other. In the image forming apparatus for sequentially transferring images onto a body, the pressing force of the pressing means on each toner image forming unit is set to be weaker toward the toner image forming unit on the downstream side in the moving direction of the transferred object. Image forming apparatus. 2. The image forming apparatus according to claim 1, further comprising: a belt-shaped transfer / transport member for carrying and transporting the transfer material so as to pass through the plurality of transfer positions; An image forming apparatus comprising the transfer material conveyed by a member. 3. The image forming apparatus according to claim 1, wherein the object to be transferred is a belt-shaped intermediate transfer member. 4. The image forming apparatus according to claim 2, wherein said image carrier is a roller-shaped member, and said pressing means is configured to move said transfer conveyance member or said intermediate transfer body at said transfer position to said image carrier. The transfer conveying member or the guide member for guiding the movement of the intermediate transfer member is configured to be partially wound around the peripheral surface of the body, and the arrangement of each guide member is set so that the predetermined pressing force is obtained. An image forming apparatus comprising: 5. The image forming apparatus according to claim 2, wherein the image carrier is a roller-shaped member, and the pressing means is configured to move the transfer conveying member or the intermediate transfer member at the transfer position. A guide member for guiding the movement of the transfer / transport member or the intermediate transfer member so as to be partially wound around the peripheral surface of the body, and an urging means for urging the guide member toward the image carrier; An image forming apparatus, wherein the urging force of each urging means is set so that the predetermined pressing force is obtained. 6. The image forming apparatus according to claim 2, further comprising a transfer bias applying member that comes into contact with the transfer conveying member or the intermediate transfer member from a side of the transfer position opposite to the image carrier, and The transfer bias applying member and biasing means for biasing the transfer bias applying member toward the image bearing member; and applying the biasing means to each of the biasing means so that the predetermined pressing force is obtained. An image forming apparatus, wherein power is set. 7. The image forming apparatus according to claim 1, wherein each of the plurality of toner image forming units forms a toner image on the image carrier using a different color toner. An image forming apparatus for forming a color image by superposing and transferring a toner image formed by each toner image forming unit onto the transfer object. 8. A belt-shaped transfer / transport member for carrying and transporting the transfer material so that the transfer material passes through a plurality of transfer positions facing each image carrier of the plurality of toner image forming units, and each transfer device. A plurality of pressing means for pressing the transfer material against each image carrier at a position, wherein the pressing force of the pressing means against each toner image forming unit is changed to a toner image forming unit on the downstream side in the transfer material moving direction. A belt device characterized in that the belt device is set so as to be smaller. 9. A belt-shaped intermediate transfer member driven to pass through a plurality of transfer positions facing each image carrier of a plurality of toner image forming units, and the intermediate transfer member is transferred to each image at each transfer position. In a belt device provided with a plurality of pressing means for pressing against a carrier, the pressing force of the pressing means on each toner image forming unit is set so as to be smaller toward the toner image forming unit on the downstream side in the intermediate transfer body moving direction. A belt device characterized by the above-mentioned.