JPH06110339A - Image forming device - Google Patents

Abstract

PURPOSE:To prevent image deterioration caused by the adhesion of the releasing agent of a fixing means to an image carrier by releasing pressing action by means of a pressing means at nearly the same time that a recording material separates from a transferring position after transferring process to the recording material is finished. CONSTITUTION:On the transferring position, a transfer charger 10 is operated and performs transfer charging at nearly the same time that the recording material arrives to the transferring position to the moving timing of the recording material held on a transferring drum and to the movement of the connecting member 9c of the transferring drum. On the other hand, a little before the transfer charging, or at nearly the same time, a pressing member 18 is brought into press-contact with a dielectric sheet 93. Thus, the transferring drum rotates nearly once and an M(magenta) image is transferred to the recording member on a condition where the dielectric sheet 93 is brought into press-contact with the photosensitive drum 3. The instant a series of transferring process for four colors is finished and the trailing edge of the recording material separates from the transferring position, the pressing member 18 separates from the dielectric sheet 93 to be a stand-by condition.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to an image forming apparatus, and more particularly to a toner image formed on an image carrier formed by an electrophotographic system or an electrostatic recording system.
The present invention relates to an image forming apparatus that obtains an image by transferring to a recording material supported on a recording material support. As such an image forming apparatus, there are black-and-white, mono-color or full-color electrophotographic copying machines, printers, and other various recording devices.

BACKGROUND ART In a color image forming apparatus, a toner image on a photosensitive drum, which is an image carrier, is transferred to a recording material supported by a transfer drum, which is a recording material support, and then offset is prevented. It is known to fix a toner on a recording material by a fixing unit coated with a release agent.

When the fixing means coated with the release agent is used, the toner is once transferred onto the surface of the recording material,
When the double-sided image is formed by transferring the toner to the back surface of the same recording material again after fixing the image by the fixing means, it was found from the experiments by the present inventors that the following problems occur. .

[0004]

That is, when the first image forming operation is completed and the first heating and fixing of the surface of the recording material is performed, the oil as the release agent adheres to the toner and the recording material. When the recording material is re-fed for double-sided copying, the toner surface (that is, the oil-adhered surface) on which the fixing is performed for the first time faces the transfer drum surface side.
Therefore, when the recording material is wound around the transfer drum, the oil adhering to the toner or the recording material is transferred to the recording material carrying sheet, and then the recording material is separated from the transfer drum. The rotation of causes the oil on the transfer drum to transfer to the photosensitive drum.

When the oil is transferred to the photosensitive drum in this manner, the oil cannot be sufficiently cleaned even by the cleaning device on the photosensitive drum, so that the toner cannot be cleaned, or the oil is not collected on a portion other than the desired latent image forming portion. Since the toner adheres to the sheet, there is a problem in that, in the subsequent copying, a dirty image is formed with the excessive toner attached.

[Object of the Invention] An object of the present invention is to provide an image forming apparatus which prevents image deterioration due to the release agent of the fixing means adhering to the image carrier.

Another object of the present invention is to provide an image forming apparatus capable of forming a good image on both sides of a recording material.

In order to achieve the above object, according to the present invention, an image carrier on which a toner image can be formed and a recording material for transferring the toner image on the image carrier to a recording material are provided. A recording material support that supports, a biasing means that biases the recording material support toward the image carrier when at least the recording material is at the transfer position, and a toner for the recording material coated with a release agent. In an image forming apparatus having a fixing unit for fixing to a recording material, the urging operation by the urging unit is released almost at the same time as the recording material is separated from the transfer position after the transfer process to the recording material is completed. It is characterized by.

[0009]

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

FIG. 6 shows a multicolor electrophotographic copying machine equipped with a developing device called a rotary developing device as an image forming apparatus.

Briefly referring to FIG. 6, in a multicolor electrophotographic copying apparatus, an image carrier, that is, a photosensitive drum 3, which is rotatably supported and rotates in the direction of an arrow, is arranged, and an image is formed on the outer periphery thereof. Forming means are arranged. The image forming means may be any means, but in the present example, the primary charger 4 for uniformly charging the photosensitive drum 3 and the color-separated optical image or an optical image corresponding to the primary charger 4 are irradiated to the photosensitive drum 3. An exposing unit 8 for forming an electrostatic latent image on the surface of the photosensitive drum 3 and a rotary developing device 1 for converting the electrostatic latent image on the photosensitive drum 3 into a visible image are provided.

The rotary developing device 1 includes a magenta color developer,
Four developing devices 1M, 1M for separately storing four color developers of cyan color developer, yellow color developer, and black color developer
C, 1Y, 1Bk and these four developing devices 1M, 1C,
It is composed of a substantially cylindrical housing which holds 1Y and 1Bk and is rotatably supported. The rotary developing device 1
Is configured such that a desired developing device is conveyed to a position facing the outer peripheral surface of the photosensitive drum 3 by the rotation of the housing, and the electrostatic latent image on the photosensitive drum is developed to perform full-color development for four colors. .

A visible image on the photosensitive drum 3, that is, a toner image is transferred onto a transfer material P carried by a transfer device 9 and conveyed. In this example, the transfer device 9 is a transfer drum that is rotatably supported, and the transfer drum 9 includes cylinders 9a and 9b arranged at both ends and both the transfer drum 9 and the transfer drum 9, as will be understood with reference to FIG. A recording material holding member 93 for holding a recording material is stretched in the opening areas of the outer peripheral surfaces of the cylinders 9a and 9b. As the recording material carrying member 93, a film-shaped dielectric sheet such as polyethylene terephthalate or polyvinylidene fluoride resin film is usually used. or,
The recording material P fed from the paper feeder is connected to the connecting portion 9c.
It has a recording material gripper 7 for gripping. Further, a transfer discharger 10, and an inner charge-eliminating discharger 13 and outer charge-eliminating dischargers 11 and 14 that constitute a charge-eliminating unit are arranged inside and outside the transfer drum 9.

Further, in order to improve the transfer efficiency and obtain a clear transfer image, as shown in FIG. 8, the transfer discharger 10 is provided on the upstream side of the transfer drum 9 in the rotation direction, and the transfer drum 9 is provided. Of the pressing member as an urging unit that is supported by the supporting member on the upstream side of the sheet, extends in the downstream direction, contacts the dielectric sheet 93, and urges the dielectric sheet 93 toward the photosensitive drum, that is, elasticity. A sheet 18 is provided to improve transferability.

The process of forming a full-color image by the multicolor electrophotographic copying apparatus having the above-mentioned structure will be briefly described as follows.

The photosensitive drum 3 is uniformly charged by the primary charger 4, and the exposure means 8 irradiates the optical image E according to the image information to form an electrostatic latent image on the photosensitive drum 3. The electrostatic latent image is visualized as a toner image on the photosensitive drum 3 by the rotary developing device 1 using a resin-based toner having an average particle diameter of 8 μm.

On the other hand, the recording material P is sent to the transfer drum 9 by the registration rollers 6 in synchronism with the image, and the tip of the recording material P is gripped by the gripper 7 and the like, and the transfer drum 9 is held.
Is conveyed in the direction of the arrow in the figure.

Next, in the area in contact with the photosensitive drum 3, the transfer discharging device 10 receives a corona discharge having a polarity opposite to that of the toner from the back surface of the recording material carrying member of the transfer drum 9, that is, the dielectric sheet 93. The toner image on 3 is transferred onto the recording material P.

The recording material P is transferred a required number of times, and after the transfer process is completed, the conveyor belt 16 is peeled from the transfer drum 9 by the action of the separating claw 15 while being discharged by the discharging dischargers 11, 13, and 14. Is conveyed to the fixing device 17. After being fixed by heat in the fixing device 17, it is discharged to the outside of the apparatus.

On the other hand, the photosensitive drum 3 is subjected to the image forming process again after the residual toner on the surface is cleaned by the cleaning device 12.

The surface of the dielectric sheet of the transfer drum 9 is also cleaned by the action of the cleaning device 5 and the cleaning assisting means 8 such as a fur brush after the transfer process and the trailing edge of the recording material has passed from the transfer position. After that, the image forming process is performed again.

At this time, the fixing operation speed of the fixing device 17 is 90 mm / sec, which is lower than the process speed of the main body of 160 mm / sec. This is because when an unfixed image in which two to four layers of toner are stacked is melt-mixed, as described below, a sufficient heating amount must be given to the toner, so fixing is performed at a speed slower than the main body speed. As a result, the amount of heat applied to the toner is increased.

As described above, since the multicolor toner forms two to four layers in the color image, the electrophotographic apparatus for forming a color image is used for black and white in the following two points. It has different characteristics from the device.

The first feature relates to the toner used.

The toner has a melting property when heat is applied,
Since a good color mixing property is required, a sharp melt type toner having a low softening point and a low melt viscosity is used. By using such a sharp melt toner, it is possible to widen the color reproduction range of the copy and obtain a color copy faithful to the multicolor or full-color image of the original.

Such a sharp melt toner includes, for example, a binder resin such as polyester resin or styrene-acrylic ester resin, a coloring agent (dye, sublimable dye),
It is produced by melt-kneading, pulverizing and classifying a toner forming material such as a charge control agent. If necessary, an external addition step of adding various external additives (for example, hydrophobic colloidal silica) to the toner may be added. As such a color toner, it is particularly preferable to use a polyester resin as the binder resin in consideration of the fixability and the sharp melt property. As the sharp melt polyester resin, a polymer compound having an ester bond in the main chain of a molecule synthesized from a diol compound and a dicarboxylic acid is exemplified.

In particular, the following equation

[0028]

[Outer 1] (In the formula, R is an ethylene or propylene group, and x, y
Each is a positive integer of 1 or more, and the average value of x + y is 2 to 10. ), A bisphenol derivative represented by the formula (1) or a substituted product thereof is used as a diol component, and a carboxylic acid component composed of a divalent or higher carboxylic acid or an acid anhydride thereof or a lower alkyl ester thereof (for example, fumaric acid, maleic acid, maleic anhydride, Phthalic acid, terephthalic acid,
A polyester resin obtained by copolycondensing at least trimellitic acid or pyromellitic acid) is more preferable because it has sharp melting characteristics.

The softening point of the polyester resin is 75 to 15
0 degreeC, Preferably 80-120 degreeC is good.

FIG. 9 shows an example of the softening characteristics of a sharp melt toner containing this polyester resin as a binder resin. The measurement conditions are as follows.

A flow tester CFT-500A type (manufactured by Shimadzu Corporation) was used, and the diameter of the die (nozzle) was 0.2 m.
m, thickness 1.0 mm, 20 kg extrusion load, initial temperature 70 ° C., preheating time 300 sec, and constant temperature increase at a rate of 6 ° C./min. A curve (hereinafter referred to as a softened S-shaped curve) was obtained. As a sample toner, fine powder precisely weighed from 1 to 3 g is used, and the cross-sectional area of the plunger is 1.0 cm ² . Softening S
The curved curve is as shown in FIG. As the temperature rises at a constant rate, the toner is gradually heated and begins to flow (plunger descending A → B). When the temperature is further raised, the melted toner largely flows out (B → C → D), and the plunger lowering is stopped and completed (D → E).

The height H of the S-shaped curve indicates the total outflow amount, and H /
The temperature T0 corresponding to the point C of 2 indicates the softening point of the toner.

Whether or not the toner and the binder resin have a sharp melting property can be determined by measuring the apparent melt viscosity of the toner or the binder resin.

The toner or the binder resin having such a sharp melting property means that the temperature at which the apparent melt viscosity is 10 ³ poise is T1, and the temperature at which the apparent melt viscosity is 5 × 10 ² poise is T2. , T1 = 90 to 150 ° C. | ΔT | = | T1-T2 | = 5 to 20 ° C.

The sharp-melting resin having these temperature-melt viscosity characteristics is characterized by extremely sharply decreasing the viscosity when heated. Such a decrease in viscosity causes a proper mixing of the uppermost toner layer and the lowermost toner layer, and further sharply increases the transparency of the toner layer itself, thereby causing good color-reducing mixing.

Such a sharp melt color toner has a large affinity and has a property of being easily offset to the fixing roller.

Therefore, in the fixing device of the image forming apparatus using the color toner as described above, it is necessary to exhibit high releasability for a long period of time. The fixing device 17, which is the second feature of the electrophotographic apparatus for forming a color image, will be described below with reference to FIG.

In FIG. 10, a fixing roller 29 as a fixing means is an HTV (high temperature vulcanization type) silicone rubber layer 32 on an aluminum cored bar 31 and an RTV (room temperature vulcanization type) on the outside thereof.
Has a silicone rubber layer 33, thickness 3 mm, diameter 40 m
It is formed in m.

On the other hand, the pressure roller 30 as a pressure means has a 1 mm thick HTV on an aluminum cored bar 34 and an RTV silicone rubber layer 35 on the surface thereof, and has a diameter of 40 mm.
Has become.

The fixing roller 29 is provided with a halogen heater 36 which is a heat generating means, and the pressure roller 30 is also provided with a heater 37 in the core bar to perform heating from both sides. The thermistor 38 in contact with the pressure roller 30
Detects the temperature of the pressure roller, and the controller 39 controls the halogen heaters 36 and 37 based on the detected temperature so that both the temperature of the fixing roller 29 and the temperature of the pressure roller 30 are kept constant at about 170 ° C. Controlled by. The fixing roller 29 and the pressure roller 30 are pressed by a pressure mechanism (not shown) at a total pressure of about 40 kg.

Further, in FIG. 10, O is an oil applying device as a releasing agent applying means, C is a cleaning device, and C1 is a cleaning blade for removing oil and dirt of the pressure roller. The oil application device O is a dimethyl silicone oil 41 (KF96 30 manufactured by Shin-Etsu Chemical Co., Ltd.) in an oil pan 40.
0 cs) is passed through the oil scooping roller 42 and the oil application roller 43, and the oil application amount is regulated by the oil application amount adjusting blade 44 to apply it on the fixing roller 29.
In the apparatus shown in FIG. 10, the coating amount is 0.08 g / A4 by the measuring method described later.

The amount of silicone oil applied by the oil applying device O is determined as follows.

First, the weight of 50 sheets of A4 size blank paper is A
1 (g), without transferring the image onto the blank sheet and without applying silicone oil to the rubber layer of the fixing roller,
Blank sheet 50 after passing between the fixing roller and the pressure roller
Let the weight of the sheet be B (g). Next, similarly, another 50 sheets of A4 size blank paper are set to A2 (g), and the image is not transferred onto this blank paper, but the silicone oil is applied to the rubber layer of the fixing roller to fix it. Let C (g) be the weight of 50 blank sheets after passing between the roller and the pressure roller. When the above A1, B, A2, and C are used, the amount of silicone oil applied per white sheet of A4 size X (g)
Is calculated by the following equation.

X = (C + A1-B-A2) / 50 On the other hand, the cleaning device C cleans the non-woven fabric web 46 made of Nomex (product name) by pressing it against the fixing roller 29 with the pressing roller 45. The web 46 is appropriately wound by a winding device (not shown) so that toner or the like does not accumulate on the contact portion.

This apparatus once transfers the toner onto the recording material,
After fixing the image by the fixing device, the same recording material is fed again to the electrophotographic apparatus for color image formation, the toner image surface of the recording material on which the image is formed is held on the transfer drum surface side, and A double-sided image can be formed by transferring and fixing the toner image again on the opposite side.

When forming a double-sided image, the recording material is placed on the tray T as shown in FIG. 6, and after the recording material is transferred and fixed, the recording material is ejected. The recording material is again placed on the tray T so that an image is formed on the surface, transferred, fixed and discharged.

Next, with reference to FIG. 1, the pressing member 18 in the transfer device and the driving means for moving the pressing member 18 to and from the transfer drum will be specifically described.

As described above, at the transfer position where the photosensitive drum 3 and the transfer drum 9 are close to each other, within the discharge width of the transfer charger 10, a dielectric made of polyvinylidene fluoride resin (PVdF) which is a recording material support. A pressing member 18 having elasticity extending from the introduction side of the sheet 93 toward the downstream side in the moving direction (forward direction with respect to the moving direction of the transfer drum) is provided. The pressing member 18 is a pressing sheet made of a synthetic resin film such as polyethylene, polypropylene, polyester, or polyethylene terephthalate, and is arranged over the entire transfer portion. The volume resistivity of these resin films is 10 ¹⁴ Ω · cm or more. In this example, polyethylene terephthalate resin was used.

The pressing sheet 18 presses the dielectric sheet 93 by its own elastic force, and the tip of the pressing sheet 18 on the side of the dielectric sheet is at a position where the transfer material P has come into contact with the photosensitive drum. , Or a position at which contact is started, or a position as close to such a position as possible.

If the dielectric sheet 93 is simply pressed, the backup member 18 is moved from the side opposite to the introduction side of the dielectric sheet 93 toward the upstream side in the moving direction (counter direction with respect to the moving direction of the transfer drum). May be provided,
With such a structure, the effect of suppressing the image scattering caused by the start of the transfer before the photosensitive drum and the recording material come into contact cannot be obtained, so that the pressing member 18 is made of a dielectric material as shown in this embodiment. It is preferable to provide the sheet 93 from the introduction side toward the downstream side in the moving direction.

In the present embodiment, a driving means for moving the pressing member 18 in and out is further provided.

The driving means includes three link members 181, 182, 183 for supporting the pressing member 18 and the link member 1.
A cam 185 that moves 83 around the shaft 183a, a motor 186 that rotationally drives the cam 185, and the bracket 2
4 and the link member 181 and a tension spring 184 which is an elastic holding means for exerting an elastic force in a direction separating the pressing member 18 from the recording material support 93. The motor 186 is a main body controller unit (not shown).
Is driven asynchronously with the rotation of the transfer drum 9.

With such a configuration, the primary charging, the exposure, the development, the transfer charging, and the contacting / separating operation of the pressing member by the driving of the motor 186, which are based on the rotation of the transfer drum 9 during the image formation, will be described. This will be described with reference to 2.

FIG. 2 is an example of a timing chart when the size of the recording material P on the transfer drum 9 is, for example, A4 size paper.

In order to form a magenta (M) image of the first color, corona discharge is performed by the primary charger 4 to uniformly charge the surface of the photosensitive drum 3. And according to the image information,
Laser light is irradiated, and a latent image of M image is formed on the photosensitive drum 3.

At the developing position, the magenta developing unit 1M, which has been moved in advance, is waiting, and the developing drive is started.
Visualize the M image. The developing drive is performed by a developer carrier that carries and conveys the developer in the developing device toward the photosensitive drum, that is,
Allowing for the time t for the developing sleeve to rise to a certain number of revolutions,
Driving is started earlier than the image front end at the developing position by time t.

At the transfer position, the transfer charger 10 is moved almost at the same time when the recording material reaches the transfer position in accordance with the movement timing of the recording material held on the transfer drum 9 and the movement of the connecting member 9c of the transfer drum 9. Operates and performs transfer charging.
On the other hand, slightly before or almost at the same time as this transfer charging, the pressing member 18 is pressed against the dielectric sheet 93. As a result, the dielectric sheet 93 is pressed against the photosensitive drum 3, and in this state, the transfer drum makes one rotation and the M image is transferred to the recording material. The transfer charger operates correspondingly when the recording material is at the transfer position.

When the pressing member 18 is pressed against the dielectric sheet 93, as better seen in FIG. 2,
Since the latent image is being formed on the photosensitive drum 3 by exposure, if the vibration generated during the pressure contact is large, the latent image is greatly affected and an image defect such as pitch unevenness occurs. Therefore, as shown in this embodiment, it is preferable that the pressing sheet 18 is smoothly brought into contact with the dielectric sheet 93 by combining the motor and the cam so that vibration is not generated as much as possible. When using a pressure contact mechanism such as a tension spring and solenoid to simplify the mechanism,
The recording material is carried on the transfer drum from above the connecting member by a gripper provided on the connecting member 9c, or
It is preferable that an electrostatic force of a suction corona discharger or the like is used from the vicinity of the above to carry the toner on the transfer drum, and the contact timing of the pressing member with the drum is performed when the connecting member 9c is contacted. Further, it is preferable that a buffer member such as a sponge is provided on the pressing member 18 side of the connecting member 9c so that vibration when the pressing member 18 contacts the transfer drum is minimized.

Then, at the second rotation of the same transfer drum, the rotary developing device 1 rotates to move the cyan developing device 1C to the developing position in preparation for developing a cyan (C) image. After that, when the transfer drum enters the third rotation, the above process is repeated to transfer the C image onto the recording material.

By performing the above steps for the yellow image and the black image in the same manner, a four-color superimposed transfer image can be obtained on the recording material carried on the transfer drum. Almost at the same time that the rear end of the recording material is separated from the transfer position after the series of transfer processes for four colors is completed, the pressing member 18 is moved by the above-mentioned mechanism.
Is placed in a standby state apart from the dielectric sheet 93. In this standby state, the pressing member 18 does not contact the dielectric sheet 93, and the pressing force does not act on the dielectric sheet 93. Therefore, in this state, the dielectric sheet 93 and the photosensitive drum 3 maintain a gap of about 100 μm. Is configured.

Further, almost at the same time when the transfer of the black image of the fourth color is started, the charger 11 for removing the charge of the dielectric sheet 93,
13 is operated, and the dielectric sheet 93 is discharged at the sixth and seventh rotations of the transfer drum.

When double-sided copying is performed in the conventional color image forming apparatus, the oil adhering to the surface of the recording material is transferred to the dielectric sheet during fixing, and the transfer of the toner image is completed. During rotation, or immediately before the start of image formation on the back surface of the recording material, during so-called pre-multi-rotation, the dielectric sheet with oil adhered to the photosensitive drum is pressed and adhered directly to the photosensitive drum by the pressing member. The oil on the body sheet re-transferred to the photosensitive drum, resulting in a defective image. However, when the dielectric sheet and the drum are separated from each other as described above, when the dielectric sheet directly contacts the photosensitive drum during the rear multiple rotation or the front multiple rotation, the pressing member is pressed. Since the dielectric sheet is kept in contact with the photosensitive drum as much as possible in the standby state, it is possible to prevent the image defect caused by the oil adhering to the photosensitive drum, which has been conventionally generated.

In the present embodiment, the pressing member 18 is contacted and separated at the timing shown in FIG.
As shown in FIG. 3, contact and separation are performed in synchronization with transfer charging so that the surface portion of the dielectric sheet on which the recording material does not exist does not come into contact with the photosensitive drum at all. It goes without saying that you can get it. However, in this case, the number of times of contact and separation increases, and as described above, there is a risk that image defects such as pitch unevenness may occur due to the effect on the latent image due to vibration during pressure contact. At the time of contact, it is preferable that the motor is driven so as to rotate at a low speed immediately before contact, for example, so that vibration at contact is prevented as much as possible.

Further, in the present embodiment, the transfer means is composed of a transfer charger capable of forming a transfer electric field by corona discharge and a pressing member. However, the electrode 20 as shown in FIGS. A brush 19 composed of a fiber spring having a volume resistivity of 10 ⁰ to 10 ⁸ Ω · cm capable of applying a voltage or a blade 19 having an equivalent resistivity.
May be used, or a voltage can be applied via the electrode 22,
Also in the system using the rubber roller 21 having the same resistivity, the same effect as that of the above-described embodiment can be obtained by providing the contacting / separating mechanism and contacting and separating the same.

Next, another embodiment in which the recording material carrying member and the photosensitive drum are separated from each other will be described.

In this embodiment, as will be better understood by referring to FIG. 11, in the image forming apparatus shown in the above embodiment, the entire transfer drum 9 can be brought into contact with and separated from the photosensitive drum 3.

As a driving means for making the transfer drum 9 contactable and separable, as shown in FIG. 11, a fixed shaft 190 and a driving support member 187 fixed to the central axis of the transfer drum 9.
A cam 188 for moving the support member 187 around the shaft 190, and a motor 189 for rotationally driving the cam 188.

With such a configuration, the primary charging, the exposure, the development, the transfer charging, and the contacting / separating operation of the transfer drum 9 by driving the motor 189 based on the rotation of the transfer drum 9 during image formation will be described. This will be described with reference to 12.

FIG. 12 is an example of a timing chart when the size of the recording material P on the transfer drum 9 is A4 size paper, for example.

In order to form a magenta (M) image of the first color, corona discharge is performed by the primary charger 4 to uniformly charge the surface of the photosensitive drum 3. And according to the image information,
Laser light is irradiated, and a latent image of M image is formed on the photosensitive drum 3.

At the developing position, the magenta developing unit 1M, which has been moved in advance, is waiting, and the developing drive is started.
Visualize the M image. The developing drive is performed by a developer carrier that carries and conveys the developer in the developing device toward the photosensitive drum, that is,
Allowing for the time t for the developing sleeve to rise to a certain number of revolutions,
Driving is started earlier than the image front end at the developing position by time t.

At the transfer position, the transfer charger 10 is moved almost at the same time when the recording material reaches the transfer position in accordance with the movement timing of the recording material held on the transfer drum 9 and the movement of the connecting member 9c of the transfer drum 9. Operates and performs transfer charging.
On the other hand, prior to this transfer charging, or almost simultaneously, the transfer drum 9 and the photosensitive drum 3 are pressed against each other. In this state, the transfer drum makes one rotation, and the M image is transferred to the recording material. The transfer charger operates correspondingly when the recording material is at the transfer position. Then, at the second rotation of the same transfer drum, the rotary developing device 1 rotates to move the cyan developing device 1C to the developing position in preparation for developing a cyan (C) image. After that, when the transfer drum enters the third rotation, the above process is repeated to transfer the C image onto the recording material.

By carrying out the above steps for the yellow image and the black image in the same manner, a transferred image of four colors can be obtained. The transfer drum 9 separates from the photosensitive drum 3 and is placed in a standby state almost at the same time that the rear end of the recording material is separated from the transfer position after the transfer charging for the four colors is completed. In this standby state, the transfer drum 9 does not come into contact with the photosensitive drum 3, and a gap of approximately 100 μm is maintained between them.

With this structure, it is possible to minimize the direct contact of the recording material carrying sheet to which the oil adheres with the photosensitive drum, as in the above-described embodiment. It has become possible to prevent the image defect caused by the adherence of.

In the above embodiment, the oil of the recording material adheres to the transfer drum when the image is formed on the back surface of the recording material. Therefore, at least after this, the photosensitive drum and the transfer drum are separated from each other. It is desirable to prevent oil from transferring to the photosensitive drum. That is, it is desirable that the recording material carrying member of the transfer drum and the photosensitive drum be separated by post-rotation after finishing the image formation on at least the back surface of the recording material. According to the sequences of FIGS. 2 and 3, it is desirable to perform the separation after the transfer of the fourth color is completed and at least the trailing edge of the recording material has passed the transfer position. If the cleaning member is used to clean the oil adhering to the transfer drum during the subsequent rotation, the transfer drum and the photosensitive drum may come into contact with each other because the amount of oil on the transfer drum becomes considerably small in the previous rotation during the next image formation. .

Further, according to the present invention, as shown in FIG.
It can also be embodied in a multicolor electrophotographic copying machine having one image forming unit I to IV.

In this embodiment, each image forming unit I to IV is used.
Has photosensitive drums 3a to 3d, around which primary charging devices 4a to 4d, exposing means 8a to 8d, developing devices 1a to 1d,
Transfer chargers 10a to 10d, static eliminators 11a to 11d
And 13a to 13d and cleaners 12a to 12d, and further, an endless belt-shaped conveying unit 23 is arranged below the photosensitive drum so as to penetrate each image forming unit.

On the other hand, pressing members 18a to 18d are provided as urging means for urging the endless belt-shaped conveying means 23 toward the respective photosensitive drums within the respective discharge widths of the transfer dischargers 10a to 10d. Further, the contacting / separating mechanisms 180a to 180d for pressing and separating the pressing member against the conveying means 23.
Is provided.

In this embodiment, at the same time that the leading edge of the recording material reaches the transfer position at the same timing as the recording material passes through the transfer position for each image forming station, the transfer charging is started and the pressing member is pressed. The contact charging is performed, and the transfer charging is stopped and the pressing member is separated almost at the same time when the recording material finishes passing through each transfer position. When the pressing member is separated, the photosensitive drum and the transfer belt are separated.

With this structure, in this embodiment as well, as in the previous embodiment, after double-sided copying, the oil adhering to the surface of the belt-shaped conveying means re-adheres to the photosensitive drum, thus causing an image defect. It came to be able to prevent it.

In this embodiment, as will be better understood by referring to FIG. 14, in the image forming apparatus shown in the previous embodiment, instead of separating the photosensitive drum and the conveyor belt by separating the pressing member, the recording material is conveyed. The entire belt unit is configured to be able to come into contact with and separate from the photosensitive drums 3a to 3d.

As a driving means for making the entire conveyance belt unit contactable and separable, as shown in FIG. 14, a drive fixed to another conveyance belt support roller 26 with a roller 25 supporting the conveyance belt as a fixed shaft. Support member 26
1 and a cam 262 for moving the support member 261 up and down, and a motor 263 for rotationally driving the cam 262.

In such a structure, the recording material is fed from the paper feeding portion onto the conveyor belt, and at the same time as or immediately before the recording material reaches the transfer position of the first station, the conveyor belt unit performs the above-mentioned conveyance. By the operation of the belt driving means, the belt is brought into contact with the photosensitive drums 3a to 3d. And
Transfer charging is performed and the magenta image is transferred. Further, the transfer belt unit is separated from the photosensitive drum by the action of the transfer belt driving means almost at the same time when the transfer material is sequentially transferred to the recording material at each image forming station and the recording material passes through the fourth station. did.

With this structure, in this embodiment as well, as in the previous embodiment, the oil adhering to the surface of the belt-shaped conveying means re-adheres to the photosensitive drum after double-sided copying, causing an image defect. It came to be able to prevent it.

In the embodiments shown in FIGS. 13 and 14, the oil of the recording material adheres to the conveyor belt when the image is formed on the back surface of the recording material. It is desirable to separate the rollers from each other to prevent the transfer of oil from the conveyor belt to the photosensitive drum. That is, it is desirable to separate the conveying belt and the photosensitive drum by post-rotation after finishing image formation on at least the back surface of the recording material. If the oil adhering to the transport belt is cleaned during the subsequent rotation, the transport belt will be almost free of oil in the previous rotation during the next image formation, so the transport belt may contact the photosensitive drum.

FIG. 15 shows another embodiment of the present invention. The present inventors continued the research and experiment, and as a result, FIG.
7, a member 28 such as a sensor for detecting the position of the transfer material supported on the transfer drum 9 is provided in the vicinity of the transfer drum 9, and the pressing member 18 is based on a signal from the position detection member 28. It has been found that the further effect can be obtained by adopting a configuration in which is contacted with and separated from the dielectric sheet 93. The details will be described below.

As can be seen from FIG. 15, according to this embodiment, the transfer drum 9 is constructed by concentrically arranging the cylindrical members 9a and 9b and integrally connecting them by the connecting member 9c. A transfer material position detection member 28 is provided between the transfer plate and a guide plate 27, which is a part of the recording material transport system, in the vicinity of.

The recording material which has passed through the guide plate 27 and is supplied to the transfer drum 9 is attracted to the dielectric sheet 93 by electrostatic means (not shown). The attracted recording material moves as the transfer drum 9 moves, and when the leading edge of the recording material passes through the transfer position detection member 28, the leading edge position of the recording material is taken into the storage device on the main body side. The recording material continues to move, and when the trailing edge of the recording material passes through the recording material position detecting member 28, the trailing edge position is captured again in the storage device on the main body side.

When the leading and trailing edge positions of the recording material taken into these storage devices and the distance from the recording material position detection member 28 to the transfer position are calculated, when the leading edge of the recording material reaches the transfer position, The pressing member 18 is brought into contact with the dielectric sheet 93, and when the rear end of the transfer material passes the transfer position, the pressing member 18 is immediately separated from the transfer sheet, and is set in a standby state.

With this structure, even if the recording material of any size or the position of the recording material is slightly deviated on the recording material support, the occurrence of the image defect is prevented as in the above embodiment. It is possible to do.

[0091]

As described above, according to the present invention, the biasing of the recording material support toward the image carrier is performed at least after the transfer step to the recording material is completed and the recording material is separated from the transfer position. Since the operation is not performed, it is possible to prevent the occurrence of an image defect due to the oil adhering to the surface of the recording material support transferred to the image carrier after the image is formed on the second surface of the recording material.

[Brief description of drawings]

FIG. 1 is a sectional view showing a biasing mechanism of a pressing member of an image forming apparatus of the present invention.

FIG. 2 is an operation sequence of the image forming apparatus of the present invention.

FIG. 3 is an operation sequence of the image forming apparatus of the present invention.

FIG. 4 is a cross-sectional view of a modified example of the pressing member of the image forming apparatus of the present invention.

FIG. 5 is a cross-sectional view of a modified example of the pressing member of the image forming apparatus of the present invention.

FIG. 6 is a sectional view showing an embodiment of the image forming apparatus of the present invention.

FIG. 7 is a perspective view of a transfer device.

FIG. 8 is an enlarged cross-sectional view of the vicinity of a transfer portion.

FIG. 9 is a graph showing the softening characteristics of the sharp melt toner used in this apparatus.

FIG. 10 is a sectional view of a fixing device used in the present apparatus.

FIG. 11 is a cross-sectional view of essential parts showing a recording material carrier releasing mechanism of the image forming apparatus of the present invention.

12 is an operation sequence in the apparatus of FIG.

FIG. 13 is a sectional view showing another embodiment of the image forming apparatus of the invention.

FIG. 14 is a sectional view showing another embodiment of the image forming apparatus of the invention.

FIG. 15 is a cross-sectional view of essential parts showing another embodiment of the image forming apparatus of the present invention.

[Explanation of symbols]

3, 3a, 3b, 3c, 3d Photoconductor drum 9 Transfer device (transfer drum) 93 Dielectric sheet 10 Transfer corona discharger 18 Pressing member 19 Transfer charging brush 21 Transfer charging roller 23 Recording material conveying belt 28 Recording material Position detecting member 29, 30 Fixing tora 181-186, 180a-180b Pressing member driving means 187-190 Transfer drum contact / separation driving means 261-263 Recording material conveying belt driving means

Claims (2)

[Claims] 1. An image carrier on which a toner image can be formed, a recording material support for supporting a recording material for transferring the toner image on the image carrier to a recording material, and a recording material at least at a transfer position. An image forming apparatus comprising: a biasing unit that biases the recording material support toward the image carrier, and a fixing unit that fixes the toner of the recording material coated with a release agent onto the recording material. An image forming apparatus characterized in that the urging operation by the urging means is released almost at the same time when the recording material is removed from the transfer position after the transfer step to the recording material is completed. 2. The recording material support is flexible, and the urging means is provided on the opposite side of the recording material support from the image carrier. Image forming apparatus.