JP2000075685A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent toner from scattering or penetrating in a toner image transfer. SOLUTION: When assuming the height of a transfer nip T on the upper most side as A, the height of the upper most point 8a of a prior transfer guide 8 as B, and the height of a resist nip R of rollers pair as C, respective value of these is set so that the following relations are valid among these A, B and C, A<B, and B>=C. In such a manner, the leading end of the transfer material P allowed to get out of the resist nip R, is guided at its rear side by the upper most point 8a of the prior transfer guide 8, brought into contact with the photosensitive drum 1 surface on the most upstream side of the transfer nip T, and thereafter, fed to the transfer nip T so as to copy the photosensitive drum 1 surface, therefore the toner is prevented from electrostatic shift right front of the transfer nip T, the toner image is prevented from scattering or penetrating, and the toner image is clearly transferred.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color image forming apparatus using an electrophotographic system, such as a copying machine, a laser beam printer, a facsimile or the like.

[0002]

2. Description of the Related Art FIG. 5 shows a part of a schematic configuration of a conventional electrophotographic color image forming apparatus (for example, a copying machine, a laser beam printer, a facsimile, etc.). The image forming apparatus shown in FIG. 1 is of a type in which toner images of a plurality of colors sequentially superimposed on the image carrier 101 are collectively transferred to a transfer material P. The material P is nipped and conveyed by the transfer roller 10. Note that the above-described image carrier is a photosensitive drum in an image forming apparatus of a multiple development system, and is an intermediate transfer body in an image forming apparatus of a multiple transfer system.

In such an image forming apparatus, a transfer roller 10 is used as a charging roller at a portion where batch transfer is performed, and is synchronized with a plurality of color toner images on an image carrier 101.
A predetermined transfer bias is applied by a transfer bias power supply (not shown).

The transfer roller 10 keeps the image carrier 10 until a plurality of color toner images are superimposed on the image carrier 101.
The transfer nip T is configured to be in contact with the image carrier 101 at a timing at which the toner images of the plurality of colors are collectively transferred onto the transfer material P. Further, the transfer roller 10 is arranged such that the transfer nip T is formed at a horizontal position (same height) of a registration nip R described later.

In the registration unit 7, a registration nip R is constituted by a registration upper roller 18 and a registration lower roller 19 arranged in parallel with the axis of the image carrier 101. The registration unit 7 is disposed on the upstream side of the transfer roller 10 with respect to the transfer direction of the transfer material P, and the toner images of a plurality of colors on the image carrier 101 are transferred to predetermined positions on the transfer material P. The transfer material P is conveyed at a predetermined timing in synchronization with the image carrier 101 as described above. The transfer material P transported by the registration unit 7 is supplied to the transfer nip T while being guided by the pre-transfer guide 8.

[0006]

However, in the above-mentioned conventional example, there is a problem that the transfer is not performed well. This problem will be described with reference to FIG.

The transfer material P whose conveyance is started at a predetermined timing by the registration unit 7 (see FIG. 5) is conveyed to the transfer nip T while being guided by a pre-transfer guide 8 provided immediately before the transfer nip T. . Transfer roller 10
Presses the transfer material P against the image carrier 101 when the toner images on the image carrier 101 are collectively transferred to the transfer material P.
Thereafter, by applying a transfer bias synchronized with the toner image to the transfer roller 10, the toner image on the image carrier 101 is transferred to the transfer material P. When the transfer bias is applied, an electric field is generated due to a potential difference between the image carrier 101 and the transfer material P in a minute space between the image carrier 101 and the transfer material P immediately before the transfer nip T. And by this electric field,
For example, the toner t that has been charged negatively (−) and electrostatically attracted to the image carrier 101 moves electrostatically to the transfer material P side in the above-described minute space, and the toner image is disturbed on the transfer material P. That is the problem. This phenomenon is a state in which the toner t is scattered around the originally drawn line. In an extreme example, in a fine kanji or the like, the toner t is scattered in a blank portion.
And characters are difficult to identify.

Further, when the transfer material P has a high resistance (OHP sheet or the like), an electric field is generated between the transfer material P and the image carrier 101, and the toner image is not transferred normally and partially comes off (penetration). ).

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and provides an image forming apparatus capable of preventing a transfer failure in which a toner image is disturbed and transferred, or a punch-through occurs in a toner image. The purpose is to do so.

[0010]

According to a first aspect of the present invention, there is provided an image forming apparatus for electrostatically transferring a toner image carried on a surface of an image carrier onto a transfer material. A transfer member that is arranged in contact with the image carrier to form a transfer nip between the image carrier and a transfer bias to which a transfer bias is applied to transfer a toner image on the image carrier to a transfer material; A transfer material transport guide that is disposed upstream of the transfer nip in the material transport direction and guides the transfer material supplied to the transfer nip from the back side thereof; and a transfer material transport guide that is disposed upstream of the transfer material transport guide. A registration unit having a pair of registration rollers for transporting the transfer material to the transfer nip via the transfer material transport guide, wherein the height of the most upstream side of the transfer nip is A, and the transfer material transport is guide Assuming that the height of the uppermost point is B and the height of the registration nip of the pair of registration rollers is C, these A, B, and C are set so that the relationship of A <B, B ≧ C is established. Is set,
It is characterized by the following.

According to a second aspect of the present invention, in the image forming apparatus of the first aspect, a transfer unit is configured by integrally supporting the transfer member and the transfer material transport guide, and the transfer unit is moved by moving the transfer unit. The transfer member may be selectively disposed at an operation position where the transfer member contacts the image carrier to form the transfer nip and a standby position where the transfer member is separated from the image carrier.

According to a third aspect of the present invention, in the image forming apparatus according to the first or second aspect, a connecting member for electrically connecting the transfer material transport guide to the registration unit to make the same potential as the registration unit. Comprising.

[0013] The invention of claim 4 is the invention of claim 1, 2, or 3.
Wherein the peripheral speed of the transfer member is V and the peripheral speed of the pair of registration rollers is v, such that the relationship of V ≦ v is established between V and v. These values are set.

According to a fifth aspect of the present invention, in the image forming apparatus according to the first, second, third, or fourth aspect, the transfer member comprises:
It is a belt-shaped transfer belt.

According to a sixth aspect of the present invention, in the image forming apparatus according to the first, second, third, or fourth aspect, the transfer member comprises:
It is a drum-shaped transfer drum.

[0016]

Embodiments of the present invention will be described below with reference to the drawings.

Embodiment 1 FIG. 1 shows an example of an image forming apparatus according to the present invention. The image forming apparatus shown in FIG.
There is a four-color full-color laser beam printer.
It is a longitudinal section showing the schematic structure.

A laser beam printer (hereinafter, referred to as "image forming apparatus") shown in FIG. 1 is a drum type electrophotographic photosensitive member (hereinafter, referred to as "photosensitive drum") 1 as an image carrier.
It has. The photosensitive drum 1 is driven to rotate at a predetermined process speed (peripheral speed) in the direction of arrow R1 by a driving unit (not shown).

The photosensitive drum 1 is uniformly charged to a predetermined polarity and a predetermined potential by a charger (primary corona charger) 2 during the rotation process, and then subjected to image exposure means (for example, color separation of a color original image). Receiving an image exposure 3 by an image forming exposure optical system or a scanning exposure system using a laser scanner that outputs a laser beam modulated in accordance with a time-series electric digital pixel signal of image information). An electrostatic latent image corresponding to a first color component image (for example, a yellow component image) is formed.

Next, the electrostatic latent image is transferred to the first
Is developed by the yellow toner as the first color by the developing device (yellow developing device) 4Y.

The yellow, magenta, cyan, and black developing units 4Y, 4M, 4C, and 4BK are mounted on a rotary 4a rotatable in the direction of arrow R4. By rotation in the R4 direction, the photosensitive drum 1 is arranged at a developing position D facing the photosensitive drum 1.

The above-described yellow toner image is not transferred to the transfer material P in the next process, and the photosensitive drum 1 is again charged to a predetermined polarity and a predetermined potential by the charger 2 in order to develop the next toner image. And an electrostatic latent image corresponding to a second color component image (for example, a magenta component image) of the color image is formed, and is developed with magenta toner of the second color.

Similarly, the third color (for example, cyan) and the fourth color (for example, black) are successively developed so as to overlap on the photosensitive drum 1 to form a composite color toner image corresponding to a target color image. .

The toner used in the first embodiment is a non-magnetic toner having an average particle diameter of 8 μm and an average toner tribo in a normal environment (temperature of 25 ° C., relative humidity of 50%) of about −25 μC / g.
It is.

The resist unit 7 is arranged upstream of the transfer nip T, and performs alignment between the image and the transfer material P in the transfer nip T during the batch transfer process. The registration unit 7 includes two rollers (a registration upper roller 18 and a registration lower roller 1) arranged in parallel with the axis of the photosensitive drum 1.
9) and a pair of registration rollers 1
A resist pressure spring (not shown) for pressing the roller 8 against the resist lower roller 19 is provided. A belt-shaped resist nip R is formed between the rollers 18 and 19. The registration upper roller 18 and the registration lower roller 19 can be repeatedly rotated and stopped by an electromagnetic clutch (not shown). A resist upper roller 18 that moves up and down in accordance with the thickness of the transfer material P, and a resist upper guide 20 and a resist lower roller 19 that move following the resist 18 are each moved by a resist lower guide 21 via a sintered bearing (not shown). The resist upper roller 18 is supported, and is configured so that the upper resist roller 18 is pressed by bridging the resist pressure spring over the bearing. The resist lower guide 21 is grounded to the ground via a varistor or resistor 24, and is configured to prevent the transfer current from flowing out of the transfer nip T via the transfer material P. The transfer material P is stored in a paper feed cassette 13 and is conveyed via a paper feed roller 26, a feed roller 27, a conveyance roller 28, and the like. It is sent to the transfer unit 5 at the same timing.

The transfer unit 5 is disposed below the photosensitive drum 1 and forms a transfer unit. In the transfer unit 5, a transfer belt 12 as a transfer member is arranged so as to rotate (move) in the direction of arrow R12. As shown in FIG. 1, the transfer belt 12 is hung on two rollers arranged in parallel with the axis of the photosensitive drum 1, namely, a transfer roller (driven roller) 10 and a transfer drive roller (transfer material drive roller) 11. Has been passed. Transfer roller 10 and drive roller 11
Is constituted by a core metal such as stainless steel and a surface layer (resistance: about 10 ⁴ to 10 ⁷ Ω) of conductive urethane rubber, and is set so that the overall resistance is 10 ⁴ Ω or less. A predetermined transfer bias is applied to both rollers by a power supply (not shown). The transfer belt 12 has a two-layer structure in which a base layer is made of a thermosetting urethane elastomer and a surface layer is made of vinylidene fluoride rubber, and has a thickness t = 0.
At 3 mm, the total resistance is 10 ¹⁰ Ω. The degree of expansion when the transfer belt 12 is stretched over the transfer roller 10 and the drive roller 11 is about 5%.

In the transfer unit 5, a transfer roller 10 and a drive roller 11 are supported by a transfer frame 9 via bearings (not shown), and the transfer roller 10 side is configured to be able to swing substantially vertically. I have. One end of the transfer frame 9 is supported by a pressure piece 15 via a pressure spring (compression spring) 14.
The transfer unit 5 is moved up and down by the rotation of the cam member 16, so that the entire transfer unit 5 performs an operation of moving toward and away from the photosensitive drum 1. That is, when the cam member 16 is rotated clockwise in the figure around the shaft 16a, the pressure piece 15 is lowered, and the transfer roller 10 side of the transfer frame 9 is lowered via the pressure spring 14, and Thereby, the transfer belt 12 is separated from the surface of the photosensitive drum 1. On the other hand, when the cam member 16 is disposed at the position shown in the figure, the pressing piece 15 is pushed up, and the transfer roller 10 side of the transfer frame 9 is pushed up via the pressing spring 14,
Thereby, the transfer belt 12 is brought into contact with the surface of the photosensitive drum 1 to form a transfer nip T. The contact pressure (transfer pressure) at this time is set to be 4 kg in total pressure. Note that the contact and separation operation of the transfer unit 5 is performed at a predetermined timing by an electromagnetic clutch (not shown) and a pressing cam 16 provided in the image forming apparatus.

The pre-transfer guide (transfer material transport guide) 8
The transfer frame 9 is supported on the upstream side of the transfer roller 10 at a predetermined distance from the transfer roller 10, and stabilizes the transfer path of the transfer material P sent from the registration unit 7. The pre-transfer guide 8 swings up and down following the transfer roller 10 and approaches the photosensitive drum 1 at a predetermined interval in accordance with the timing at which the transfer material P enters the transfer nip T. Is ensured. Further, the pre-transfer guide 8 is connected to the resist unit 7 via a contact plate (connecting member) 25 and has the same potential as the resist unit 7.

The multicolor developed four-color toner image on the photosensitive drum 1 is transferred to a transfer material P at a transfer nip T by applying a transfer bias to a transfer roller 10 and a drive roller 11.

The registration unit 7, the pre-transfer guide 8, the transfer unit 5 and the like will be described later in detail.

In FIG. 1, a cleaning device 6 is disposed on the left side of the photosensitive drum 1. The cleaning device 6
The cleaning blade 6a is disposed so as to be able to freely contact and separate from the photosensitive drum 1 in the direction of arrow K6. ) Is removed.

The transfer unit 5 and the cleaning device 6 are separated from the surface of the photosensitive drum 1 while the toner image on the photosensitive drum 1 is being developed.

The transfer material P to which the toner image has been transferred is conveyed to a fixing device 17, where it is heated and pressed by a fixing roller 17a and a pressure roller 17b to fix the toner image on the surface. Is switched by the discharge roller 32 so that the toner image is directed upward.
The toner image is discharged onto the discharge tray 37 on the discharge tray 37 by the transfer rollers 34 and 35 and the discharge roller 36.

Next, the registration unit 7, the pre-transfer guide 8, the transfer unit 5, and the like will be described in detail.

The surface of the photosensitive drum 1 facing the transfer belt 12, which is the transfer means, is charged by the charger 2 and then subjected to image exposure to remove the charge and to have a potential V _L (hereinafter referred to as a “bright portion”). called. a) potential without receiving image exposure is V _D region (hereinafter referred to as "dark portion".) is formed. There is usually a potential difference of 300 to 600 V between the bright part and the dark part. At the time of transfer, the toner on the transfer material P scatters after the transfer unless the transfer is performed with a voltage that absorbs these potential differences, in other words, the difference in load impedance.

This is because when the transfer voltage is low, the transfer charge is supplied to the dark part and the light part is supplied to the dark part due to the difference of the load impedance described above. This occurs because toner on the transfer material P after transfer scatters from a bright portion to a dark portion. Therefore, it is necessary to use a transfer belt 12 having a relatively high resistance, which generates a transfer voltage high enough to absorb the difference in load impedance.

When a composite color toner image of four colors (yellow, magenta, cyan, and black) is formed on the photosensitive drum 1, the transfer material P is adjusted by the registration unit 7 to the toner image on the photosensitive drum 1. The sheet is conveyed to the transfer section via the pre-transfer guide 8, and at this time, the transfer unit 5 is also brought into contact with the photosensitive drum 1 in time with the toner image on the photosensitive drum 1. That is, the pressing cam 16 rotates, and the pressing unit 15 in the transfer unit 5 is pushed up to rotate the transfer unit 5 upward, so that the transfer belt 12 and the transfer roller 10 abut on the photosensitive drum 1. However, at the moment when the transfer belt 12 and the transfer roller 10 contact the photosensitive drum 1, the pressure spring 14 is hardly compressed yet, so that the contact shock between the transfer belt 12 and the transfer roller 10 is not applied to the photosensitive drum 1. rare. Therefore, there is no adverse effect such as vibration or displacement due to a shock caused by contact with the photosensitive drum 1 which continues the exposure / development process.
The toner images of the colors are accurately synthesized without displacement, and a synthesized color toner image having high resolution and high color reproducibility is formed.

When the pressure cam 16 is further rotated and its top is fixed by contact with the pressure piece 15, the pressure spring 14 is also gradually compressed, and finally the transfer belt 12 and the transfer roller are set at a set pressure of 4 kg. 10 is brought into contact with the photosensitive drum 1. When the contact operation is completed, the transfer material P conveyed by the registration unit 7 is transferred onto the photosensitive drum 1 by the above-described transfer bias at a transfer nip T, which is a contact portion between the photosensitive drum 1 and the transfer belt 12 and the transfer roller 10. Are transferred collectively.

The transfer material P after the transfer is separated from the photosensitive drum 1 by the transfer belt 12 having a large capacitance.
Adsorbed and transported. In the first embodiment,
Under a normal environment (temperature 25 ° C., relative humidity 50%), the potential V _D of the dark portion of the photosensitive drum 1 is −600 to −660 V, and the potential V _{L of the} bright portion where the toner is developed is −170 to −180 V.
The transfer bias in the batch transfer step is +200 to +300 V under constant voltage control. The transfer material P conveyed while being attracted to the transfer belt 12 is separated by curvature at the most downstream side of the transfer belt 12, and the fixing device 17 heats and fixes the toner image on the surface.

As shown in FIG. 3, the transfer belt 12, the transfer roller 10, and the drive roller 11 are provided on the transfer path of the transfer material P in the transfer nip T from the registration unit 7.
A transfer unit 5 having the following. A registration unit 7 for aligning the toner image on the photosensitive drum 1 with the transfer material P during the batch transfer process; A pre-transfer guide 8 for regulating the transfer direction is provided. The relation between these guides in the height direction is as follows: A is the height of the most upstream side of the transfer nip T formed by the photosensitive drum 1 and the transfer belt 12; The height of the highest point 8a of 8 is B,
When the height of the resist nip R is C, these A,
These values are set so that the relationship of A <B, B ≧ C holds between B and C.
As a result, the leading end of the transfer material P that has exited the resist nip R is guided on its back surface by the uppermost point 8a of the pre-transfer guide 8, and is located upstream of the transfer nip T in the rotation direction of the photosensitive drum 1 (the direction of arrow R1). After that, the photosensitive drum 1 is supplied to the transfer nip T so as to follow the surface of the photosensitive drum 1 moving in the direction of arrow R1. That is, between the uppermost point 8a of the pre-transfer guide 8 and the transfer nip T, the transfer material P
It curves to almost the same shape as the surface. Accordingly, it is possible to suppress the electrostatic movement of the toner t immediately before the transfer nip T in a region where the electrostatic attraction occurs. Therefore, in the present embodiment shown in FIG. 2, unlike the conventional example shown in FIG. 6, the toner is electrostatically moved immediately before the transfer nip T to cause toner scattering or penetration, A sharp color image can be output by preventing scattering and penetration of the toner image due to the discharge immediately before the nip T.

In the present embodiment, as shown in FIG. 3, when the height C of the registration nip R is set to 0 mm, the most upstream side of the transfer nip T in the transfer belt 12 stretched over the transfer roller 10. By setting the height A to 9.1 mm and the height B of the uppermost point 8a of the pre-transfer guide 8 to 10.8 mm, it was possible to perform good transfer without scattering of the toner image.

Further, in order to keep the transfer material P along the surface of the photosensitive drum 1 at all times, the relationship between the peripheral speeds of the transfer belt 12 and the registration upper roller 18 and the registration lower roller 19 provided in the registration unit 7 is transferred. When the peripheral speed of the belt 12 is V and the peripheral speeds of the registration upper roller 18 and the registration lower roller 19 are both v, these values are set so that the relationship of V ≦ v is satisfied. By maintaining the above-mentioned peripheral speed relationship, the transfer material P is prevented from being stretched (pulled) between the resist nip R and the transfer nip T, and the transfer material P is stably lightened toward the photosensitive drum 1. It can be transported while pressing. In the present embodiment, when the process speed (moving speed of the surface of the photosensitive drum 1) is 117.1213 mm / sec, the peripheral speed V of the transfer belt 12 is 117.0525 mm / sec, the upper resist roller 18 and the lower resist. Peripheral speed v of roller 19
Is set to 117.2445 mm / sec.

Further, the pre-transfer guide 8 is connected to the transfer unit 5.
The pre-transfer guide 8 is moved in and out of contact with the photosensitive drum 1 in conjunction with the transfer unit 5. Thereby, the relative position of the pre-transfer guide 8 with respect to the transfer belt 12 is always kept constant, and the path of the transfer material P entering the transfer nip T can be stabilized.

Further, by setting the pre-transfer guide 8 to the same potential as the resist unit 7 and grounding it to ground via a varistor or resistor 24, the outflow of the transfer current via the transfer material P to the ground is prevented. Thereby, the transfer material P
Regardless of the difference in properties such as resistance value, material, size, etc.
Scattering, penetration, and the like are effectively prevented, and a stable output image can be obtained at all times.
It is possible to prevent the pre-transfer guide 8 and the resist unit 7 from being overcharged due to discharge from the printer or the outflow current via the transfer material P.

<Embodiment 2> FIG. 4 shows an image forming apparatus according to Embodiment 2 of the present invention. The image forming apparatus shown in FIG. 1 is a four-color full-color laser beam printer using an electrophotographic method, and FIG. 1 is a longitudinal sectional view showing a schematic configuration thereof.

In the second embodiment, the first embodiment is used.
Unlike the above, the step of temporarily transferring the toner image formed on the photosensitive drum 1 to the intermediate transfer drum (intermediate transfer body) 20 once is repeated for the toner images of a plurality of colors (four colors). Superimpose a plurality of toner images on the intermediate transfer drum 2
The secondary transfer is collectively performed on the transfer material P from the top. In the present embodiment, the intermediate transfer drum 20 as an intermediate transfer member corresponds to an image carrier.

In the second embodiment, the same members and the like as those in the above-described first embodiment are denoted by the same reference numerals, and the description thereof will not be repeated.

A laser beam printer (hereinafter, referred to as an “image forming apparatus”) shown in FIG. 1 includes a drum type electrophotographic photosensitive member (photosensitive drum) 1.
Is driven to rotate at a predetermined process speed (peripheral speed) in the direction of arrow R1 by a driving unit (not shown).

In the course of rotation, the photosensitive drum 1 is uniformly charged to a predetermined polarity and a predetermined potential by a charger (primary charging roller) 2A, and then exposed to exposure means (for example, color separation / composition of a color original image). The image exposure 3 by an image exposure optical system or a scanning exposure system using a laser scanner that outputs a laser beam modulated in accordance with a time-series electric digital pixel signal of image information. (For example, a yellow component image) is formed.

Next, the electrostatic latent image is transferred to the first
Is developed by the yellow toner as the first color by the developing device (yellow developing device) 4Y. Yellow, magenta, cyan and black developing units 4Y, 4M, 4C, 4
BK is a rotary 4a that turns clockwise R4 in FIG.
And the rotation of the rotary 4a
A developing device used for development is arranged at a developing position D facing the surface of the photosensitive drum 1.

[0051] In the present embodiment, normal environment (temperature 25 ° C., 50% relative humidity) under the potential V _D of the charged areas (dark portion) of the photosensitive drum 1 is -500-55
0V, the potential V _L region (bright area) in which toner is developed -
170 to -180V.

An intermediate transfer drum 20 as an intermediate transfer body is arranged in contact with the photosensitive drum 1, and is arranged in the direction of arrow R20.
The photosensitive drum 1 is driven to rotate at the same peripheral speed as the rotation in the direction of arrow R1.

The first color yellow toner image formed and carried on the photosensitive drum 1 is composed of a photosensitive drum 1 and an intermediate transfer drum 2.
In the process of passing through the primary transfer nip T ₁ between the intermediate transfer drum 20 and the intermediate transfer drum 20, the electric field and pressure generated by the primary transfer bias applied from the primary transfer bias power supply (not shown) apply to the intermediate transfer drum 20. Primary transfer (intermediate transfer) is performed on the surface.

Hereinafter, similarly, a magenta toner image of the second color,
The cyan toner image of the third color and the black toner image of the fourth color are primary-transferred sequentially onto the intermediate transfer drum 20 so as to be superimposed on each other, and a composite color toner image corresponding to a target color image is formed on the surface of the intermediate transfer drum 20. It is formed.

In the present embodiment, the primary transfer bias is +300 V in a normal environment (temperature: 25 ° C., relative humidity: 50%).

The transfer belt 12 is disposed below the intermediate transfer drum 20. The transfer belt 12 is supported in parallel with the intermediate transfer drum 20 and is
0 contacts from below. The transfer belt 12 is supported by a transfer roller (bias roller) 10 and a drive roller (tension roller) 11.
A desired secondary transfer bias is applied by a secondary transfer bias source (not shown).

The secondary transfer bias in the present embodiment is +20 μA under constant voltage control, and is usually in an environment (temperature 2).
(5 ° C., 50% relative humidity)
A voltage of 500 V is generated.

The primary transfer bias for the sequential superposition transfer of the toner images of the first to fourth colors from the photosensitive drum 1 to the intermediate transfer drum 20 has a polarity (+) opposite to that of the toner and has a primary transfer bias power supply (not shown). ). The transfer belt 12 and the intermediate transfer body cleaning roller 23 are configured to be able to contact and separate from the intermediate transfer drum 20, and sequentially transfer toner images of the first to fourth colors from the photosensitive drum 1 to the intermediate transfer drum 20. In the execution process, it is separated from the intermediate transfer drum 20.

In the secondary transfer of the composite color toner image superimposedly transferred onto the intermediate transfer drum 20 to the transfer material P, the transfer belt 12 is brought into contact with the intermediate transfer drum 20 and the transfer material P resist unit 7 from 13 pre-transfer is through the guide 8 and the like to the secondary transfer nip T ₂ of the between the intermediate transfer drum 20 and the transfer belt 12 feeds a predetermined timing, at the same time the secondary transfer bias is secondary This is performed by applying a voltage to the transfer roller 10 from a transfer bias power supply (not shown). The composite transfer toner image is collectively and secondarily transferred from the intermediate transfer drum 20 to the transfer material P by the secondary transfer bias.

The transfer material P having received the secondary transfer of the toner image is introduced into the fixing device 17 and is fixed by heating. Toner remaining on the surface of the intermediate transfer drum 20 without being transferred onto the transfer material P after the secondary transfer (secondary transfer residual toner) is cleaned by the intermediate transfer body cleaning roller 23 being in contact with the surface of the intermediate transfer drum 20. .

Hereinafter, the transfer in the image forming apparatus having the above configuration will be described in detail.

The intermediate transfer drum 20 used in this embodiment
For example, at least rubber on a cylindrical conductive support,
It has a roller shape having an elastic layer made of an elastomer or a resin, and further has a roller shape having one or more coating layers on the elastic layer.

As the cylindrical conductive support, a metal or alloy such as aluminum, iron, copper and stainless steel, a conductive resin in which carbon or metal particles are dispersed, or the like can be used. , A cylindrical shape as described above,
Examples include a cylinder having a shaft penetrating the center thereof, and a cylinder having a reinforced interior. The cylindrical conductive support used in the present embodiment is one in which a 3 mm-thick aluminum cylinder is reinforced inside.

The thickness of the elastic layer used for the intermediate transfer drum 20 is 0.5 to 7 in terms of the formation of the primary transfer nip T ₁ and the secondary transfer nip T ₂ , color shift due to rotation, material cost, and the like.
mm, and the thickness of the surface layer is preferably a thin layer in order to further convey the flexibility of the lower elastic layer to the upper layer or the surface of the photosensitive drum 1.
00 μm is desirable. In the present embodiment, the thickness of the elastic layer is 5 mm, the thickness of the surface layer is 30 μm,
Was 186 mm in total outer diameter. The elastic layer was made of a material in which the volume resistivity was controlled to 10 ⁵ 10 ⁸ Ω · cm by dispersing ketjen black as a conductive material in acrylonitrile butadiene rubber (NBR) with emphasis on only the resistance value. In the present embodiment, it is set to 10 ⁷ Ω · cm.

For the surface layer of the intermediate transfer drum 20, a material in which urethane resin was used as a binder and 200 parts of PTFE powder was dispersed was used. As a molding method, spray coating and polishing are performed on the elastic layer. The resistance value of the intermediate transfer drum 20 thus manufactured is 2 × 10 ⁷ Ω.
Met.

On the intermediate transfer drum 20, four colors (yellow,
When the composite color toner image (magenta, cyan, black) is formed, the transfer material P is secondarily transferred via the pre-transfer guide 8 by the registration unit 7 in synchronization with the toner image on the intermediate transfer drum 20. It is conveyed to the nip T _2, the transfer unit 5 at this time, in the first embodiment
The intermediate transfer drum 20 is pressurized at a predetermined transfer pressure (total pressure of 4 kg in the present embodiment) by the same process as that of contacting and pressurizing the photosensitive drum with the four-color toner image on the transfer material P. Are collectively secondary-transferred.

In this embodiment, the transfer unit 5,
The configurations of the registration unit 7, the pre-transfer guide 8, and the like are the same as those in the first embodiment. That is, the relationship of these height direction, of the intermediate transfer drum 20 and the transfer belt 12 and the most upstream side of the constructed secondary transfer nip T ₂ the height A, of the uppermost point 8a of the pre-transfer guide 8 High A, B, and C, when the height is B and the height of the resist nip R is C.
These values are set so that the relationship of A <B, B ≧ C is established.

As described above, according to the present embodiment, a four-color full-color image forming apparatus using the intermediate transfer drum 20 is also similar to the first embodiment.
In the transport path of the transfer material P, thereby enters the secondary transfer nip T ₂ in the form of and along the transfer material P to the intermediate transfer drum 20 surface becomes Kaze', the secondary transfer nip T ₂ immediately before the intermediate transfer drum The electrostatic movement of the toner in the electric field generated between the transfer material 20 and the transfer material P can be suppressed to prevent the toner image transferred to the transfer material P from scattering or piercing, and the resistance value, material, size, etc. of the transfer material P Irrespective of the properties of the toner, scattering and penetration of the toner image can be prevented, and a stable output image can always be obtained.

In the first embodiment, the drum-shaped photosensitive drum 1 is used as the image carrier, and in the second embodiment, the drum-shaped intermediate transfer drum 20 is used as the image carrier. Although the case has been described, a belt-shaped photosensitive belt or a belt-shaped intermediate transfer belt can be used instead.

Further, the present invention is not limited to the case where the transfer member is the belt-like transfer belt 12 shown in FIGS. 1 and 4, but also the case where the transfer member is a roller-like transfer roller as shown in FIG. Can also be applied.

[0071]

As described above, according to the present invention,
When the height of the uppermost stream side of the transfer nip is A, the height of the uppermost point of the transfer material transport guide is B, and the height of the resist nip of the pair of resist rollers is C, the height is between A, B, and C. , A <B, B ≧ C, the leading edge of the transfer material exiting the resist nip is guided by the uppermost point of the transfer material transport guide, and the transfer is performed. The toner comes into contact with the surface of the image carrier on the upstream side of the nip, and thereafter, is supplied to the transfer nip so as to follow the surface of the image carrier. The transfer of the toner image can be performed sharply while preventing the transfer and the penetration.

[Brief description of the drawings]

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

FIG. 2 is a view for explaining that a transfer material has a shape following the surface of a photosensitive drum between an uppermost point of a pre-transfer guide and a transfer nip.

FIG. 3 is a diagram illustrating a configuration of a transfer unit, a pre-transfer guide, a registration unit, and the like according to the first embodiment.

FIG. 4 is a longitudinal sectional view illustrating a schematic configuration of an image forming apparatus according to a second embodiment.

FIG. 5 is a diagram illustrating a configuration of a conventional transfer roller, a pre-transfer guide, a registration unit, and the like.

FIG. 6 is a view for explaining how a toner image scatters between a top point of a pre-transfer guide and a transfer nip according to the related art.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 image carrier (photosensitive drum) 5 transfer unit 7 registration unit 8 transfer material transport guide (pre-transfer guide) 8a highest point of transfer material transport guide 12 transfer member (transfer belt) 18, 19 registration roller pair (registration upper roller 1)
8, resist lower roller 19) 20 image carrier (intermediate transfer body, intermediate transfer drum) 25 connecting member (contact plate) P transfer material R resist nip T transfer nip T ₁ primary transfer nip T ₂ transfer nip (secondary transfer nip) )

Claims (6)

[Claims] 1. A toner image carried on the surface of an image carrier,
In an image forming apparatus for electrostatically transferring an image on a transfer material, a transfer nip is formed between the image carrier and the image carrier, and a transfer bias is applied to the image carrier to apply a transfer bias to the image carrier. A transfer member for transferring a toner image to a transfer material; a transfer material disposed upstream of the transfer nip in the transfer direction of the transfer material and guiding the transfer material supplied to the transfer nip from the back side thereof A transfer guide, and a registration unit disposed upstream of the transfer material transfer guide and having a pair of registration rollers for transferring the transfer material to the transfer nip via the transfer material transfer guide. Let A be the height on the most upstream side, B be the height of the uppermost point of the transfer material transport guide, and C be the height of the registration nip of the registration roller pair.
An image forming apparatus characterized in that these values are set so that the following relationship is established between C: A <B, B ≧ C. A transfer unit configured to integrally support the transfer member and the transfer material transport guide; and by moving the transfer unit, the transfer member is brought into contact with the image carrier to transfer the image. The image forming apparatus according to claim 1, wherein the image forming apparatus is selectively disposed at an operation position that forms a nip and a standby position that is separated from the image carrier. 3. The image forming apparatus according to claim 1, further comprising a connection member that electrically connects the transfer material transport guide to the registration unit to make the same potential as the registration unit. . 4. When the peripheral speed of the transfer member is V and the peripheral speed of the pair of registration rollers is v, these values are set so that the relationship of V ≦ v is established between V and v. 4. The image forming apparatus according to claim 1, wherein: 5. The image forming apparatus according to claim 1, wherein the transfer member is a belt-shaped transfer belt. 6. The image forming apparatus according to claim 1, wherein the transfer member is a roller-shaped transfer roller.