JP3453541B2 - Image forming device - Google Patents

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 for forming an image on a recording material by using an electrophotographic system, for example, an image forming apparatus such as a copying machine, a printer or a FAX.

[0002]

2. Description of the Related Art Conventionally, various image forming apparatuses using an electrophotographic system have been proposed or implemented. For example, there is a system in which a toner image formed on a photosensitive drum is sequentially superimposed and transferred onto a recording material carried and conveyed by a transfer drum or a transfer belt.

The image forming process of the image forming apparatus using the transfer belt will be briefly described. First, the photosensitive drum rotates and its surface is uniformly charged by the charger. Next, the photosensitive drum is irradiated with laser light modulated by the image signal of the first color of the original, for example, magenta, to form a magenta electrostatic latent image on the photosensitive drum. The electrostatic latent image is developed by a magenta developing device to form a magenta toner image of the first color on the photosensitive drum.

On the other hand, the recording material stored in the paper feed cassette is conveyed to the transfer belt by a registration roller or the like. Simultaneously with the conveyance of the recording material, the adsorption roller is pressed against the surface of the transfer belt, and the recording material is electrostatically adsorbed from the back surface side by the adsorption charger to electrostatically adsorb the recording material on the transfer belt.
The transfer belt rotates in synchronization with the photosensitive drum, and the magenta toner image formed on the photosensitive drum is transferred to the recording material carried on the transfer belt by the transfer charger. The transfer belt continues to rotate in preparation for the transfer of the next cyan toner image of the second color.

Next, a cyan toner image of the second color is formed on the photosensitive drum, and the cyan toner image is transferred onto the recording material carried on the transfer belt by superimposing it on the magenta toner image. The same image forming process is repeated for the third and fourth colors of yellow and black to obtain a full-color image in which four color toner images of magenta, cyan, yellow and black are superposed on the recording material.

The recording material on which the four color toner images have been transferred is separated from the transfer belt and conveyed to the fixing device. The fuser is
The toner image and the recording material are heated and pressed by the fixing roller and the pressure roller to mix the toner images of the respective colors and fix the toner image on the recording material, and a full-color print image is formed, and the recording material is discharged to the outside of the machine. To be done.

[0007]

In the above transfer process,
From the standpoint of stability of image transfer, a constant current power source is connected to the transfer charger to control the transfer current at a constant current.
However, the following transfer failure may occur.

In this image forming apparatus, it is possible to form an image on a recording material having a size smaller than the maximum usable size (length in the direction orthogonal to the conveying direction of the recording material). Here, the volume resistivity of the recording material changes to about 2 × 10 ⁷ to 10 ¹⁴ Ωcm depending on the type of the recording material and the moisture absorption condition.

As shown in FIG. 12, when the transfer belt 130 transfers the recording material P of the maximum size to be used for transfer, the transfer current from the transfer charger 24 is Itr [μA], the width of the recording material (transfer). Assuming that the length in the thrust direction orthogonal to the moving direction of the belt 130) is χ [cm] and the rotation speed of the photosensitive drum 3, that is, the process speed is v “cm / s”, as shown in FIG. The transfer current flowing in the recording medium is Itr × Δt, and the target area of the recording material at that time is v × Δt ×
Since it is χ, the surface charge density on the recording material is Itr × Δt / (χ × Δt × v) = Itr / (χ · v) [μ
C / cm ² ].

The recording material is a normal temperature and low humidity environment (for example, 23
At 5 ° C. and 5% RH), the volume resistivity is relatively very high, but even in such a case, the constant current method changes the surface charge density on the maximum size recording material with the maximum width. do not do.

On the other hand, when a small-sized recording material having a small width is passed, as shown in FIG. 14, the current flowing in the portion of the recording material P is Iχ, and the non-sheet passing portion on both sides of the recording material is The flowing currents are Iw and Iz, respectively, and the width of the recording material is χ,
If the widths of the non-sheet passing portion are w and z, respectively, the surface charge densities on the recording material and on the non-sheet passing portion should be the same as when the recording material of the maximum size is passed (that is, Iw / (w · v) = Iχ / (χ · v) = Iz / (z
・ V)), but because the resistance of the recording material itself exists in the paper passing portion, the impedance is higher than in the non-paper passing portion, so the current per unit area flowing in the paper passing portion is also higher than in the non-paper passing portion. And the surface charge density on the recording material is smaller than that of the non-sheet passing portion.

For this reason, in the constant current method, the surface charge density on the recording material does not reach the necessary and sufficient level (that is, Iw / (w.
v) = Iz / (z.v)> I.chi ./ (. chi.v)), causing a transfer failure.

This is because, for example, when transferring to a recording material or the like which has become dry and has a high resistance, or when the transfer belt absorbs moisture under a high temperature and high humidity environment (for example, 30 ° C., 80% RH) and the resistance becomes low. Occurs remarkably.

Further, the surface potential of the portion corresponding to the non-sheet passing portion of the photosensitive drum is usually a non-image portion which is a high potential portion of the photosensitive drum as shown in FIG. 15 in order to prevent toner from adhering during development. It has the same potential as the potential (Vd).
Then, in the case of the constant current method, when a small size sheet is passed, a large amount of current flows in the non-sheet passing portion having a large potential difference, and the current flowing in the sheet passing portion is insufficient, resulting in a transfer failure.

Therefore, conversely, if the transfer bias is set so that a sufficient current flows through the paper passing portion when a small size paper is passed, an excessive current will flow when the recording material of the maximum width is passed. However, the toner is reversely charged and the transfer becomes defective.

An object of the present invention is to provide an image forming apparatus capable of preventing the occurrence of transfer failure when an image is transferred from an image carrier to a recording material carried on a recording material carrier. Is.

Other objects of the present invention will become apparent upon reading the following detailed description.

[0018]

The above object can be achieved by an image forming apparatus according to the present invention. In summary, the present invention is
An image carrier that carries an image, a recording material carrier that carries a recording material, and in order to electrostatically transfer the image on the image carrier to the recording material carried on the recording material carrier, First charge applying means for applying a charge of a predetermined polarity on the side of the pre-recording material carrier opposite to the side carrying the recording material, and the recording material before the recording material is carried on the recording material carrier. An image forming apparatus comprising: a second charge applying unit that applies an electric charge to a side of a material carrying body opposite to a side carrying a recording material, the image forming apparatus comprising: Of the area of the recording material carrier to which the electric charge is applied by the first electric charge applying means, the second area outside the first area of the recording material carrier, which carries the recording material, has a charge amount of The second electric potential is set so as to be larger toward the predetermined polarity than the electric charge amount of the first region. Applying means is an image forming apparatus characterized by imparting a charge to said recording material bearing member.

According to the present invention, depending on the type of recording material,
It has a control means which controls the voltage applied to the said 2nd electric charge provision means. A recording material detection unit that detects the type of recording material is provided, and the control unit controls the voltage applied to the second charge applying unit according to the detection result of the recording material detection unit. It has a control means for controlling the amount of charge applied to the recording material carrier by the second charge applying means according to the type of recording material. A recording material detection unit that detects the type of recording material is provided, and the control unit supplies the amount of electric charge applied to the recording material carrier by the second electric charge application unit according to the detection result of the recording material detection unit. To control.

There is provided control means for controlling the voltage applied to the second charge applying means in accordance with the humidity. The control means controls the voltage applied to the second charge applying means in accordance with the detection result of the humidity detecting means. Depending on temperature and humidity,
It has a control means which controls the voltage applied to the said 2nd electric charge provision means. The control means has a temperature / humidity detection means for detecting temperature and humidity, and the control means controls the voltage applied to the second charge applying means in accordance with the detection result of the temperature / humidity detection means.

There is provided control means for controlling the amount of electric charge applied to the recording material carrier by the second electric charge applying means in accordance with the humidity. It has humidity detection means to detect humidity,
The control unit controls the amount of electric charge applied to the recording material carrier by the second electric charge applying unit according to the detection result of the humidity detecting unit. It has a control means for controlling the amount of electric charge applied to the recording material carrier by the second electric charge applying means according to temperature and humidity. The control means has a temperature / humidity detecting means for detecting temperature and humidity, and the control means controls the amount of electric charge applied to the recording material carrier by the second electric charge applying means in accordance with the detection result of the temperature / humidity detecting means. Control.

The amount of charge applied to the first region of the recording material carrier by the second charge applying means is zero. The polarity of the charge applied to the first area of the recording material carrier by the second charge applying means is the polarity of the charge applied to the recording material carrier by the first charge applying means. It has opposite polarity. The polarity of the charge applied to the second area of the recording material carrier by the second charge applying means is the polarity of the charge applied to the recording material carrier by the first charge applying means. It has opposite polarity. The absolute value of the amount of charge applied to the second area of the recording material carrier by the second charge applying means is the same as the absolute value of the amount of charge applied to the first area of the recording material carrier by the second charge applying means. It is smaller than the absolute value of the applied charge amount.

The polarity of the charge applied to the first region of the recording material carrier by the second charge applying means is the polarity of the charge applied to the recording material carrier by the first charge applying means. It is the same polarity as the polarity. The polarity of the charge applied to the second area of the recording material carrier by the second charge applying means is the same as the polarity of the charge applied to the recording material carrier by the first charge applying means. It is polar. The absolute value of the amount of charge applied to the second area of the recording material carrier by the second charge applying means is the same as the absolute value of the amount of charge applied to the first area of the recording material carrier by the second charge applying means. It is larger than the absolute value of the amount of charge given. The polarity of the charge applied to the second area of the recording material carrier by the second charge applying means is the polarity of the charge applied to the recording material carrier by the first charge applying means. It has opposite polarity.

The second charge imparting means includes a first charge imparting member for imparting an electric charge to the recording material carrying side of the recording material carrying body, and the first electric charge through the recording material carrying body. A second member that is provided so as to face the applying member and applies an electric charge to the opposite side of the recording material carrying body from the side carrying the recording material.
Charge imparting member. An area in which the second charge applying unit applies an electric charge to the recording material carrier in a direction substantially orthogonal to the recording material conveyance direction is variable according to the length of the recording material. In the direction substantially orthogonal to the recording material conveyance direction, the second charge applying means applies charges only to the second region of the recording material carrier. The charging polarity of the image carrier is opposite to the predetermined polarity.

The volume resistivity of the recording material carrier is from 10 ¹³ to
It is 10 ¹⁷ Ωcm. Images of a plurality of colors are sequentially superimposed and transferred onto the recording material carried by the recording material carrier. A plurality of the image carriers are provided to respectively carry images of a plurality of colors, and the images of a plurality of colors are sequentially superposed and transferred from the plurality of image carriers to the recording material carried by the recording material carrier.

[0026]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will now be described in more detail with reference to the drawings.

Embodiment 1 FIG. 1 is a block diagram showing an image forming apparatus to which the present invention can be applied.

As shown in FIG. 1, first, second, third, and fourth image forming sections Pa, Pb, Pc, and Pd are arranged side by side in the apparatus, and each of the image forming sections Pa to Pd. The toner images of different colors are formed through the processes of latent image formation, development, and transfer.

The image forming sections Pa, Pb, Pc and Pd are respectively dedicated image carriers, in this example, the electrophotographic photosensitive drum 3.
a, 3b, 3c, 3d, and each photosensitive drum 3a, 3
Toner images of respective colors are formed on b, 3c, and 3d. A transfer belt 130 of a recording material carrier is installed adjacent to each of the photosensitive drums 3a, 3b, 3c, 3d.
The toner images of the respective colors formed on 3b, 3c, and 3d are superposed and transferred onto the recording material P carried on the transfer belt 130 and conveyed.

The recording material P on which the toner images of the respective colors have been transferred is
After fixing the toner image by the fixing device 9 by heating and pressing, it is discharged outside the device as a color recorded image.

Exposure lamps 111a, 111b, 11 are provided on the outer circumferences of the photosensitive drums 3a, 3b, 3c, 3d, respectively.
1c, 111d, drum chargers 2a, 2b, 2c, 2
d, potential sensors 113a, 113b, 113c, 11
3d, developing devices 1a, 1b, 1c, 1d, transfer charger 24
a, 24b, 24c, 24d, and cleaners 4a, 4
b, 4c, and 4d are provided, and a light source device and a polygon mirror 117 (not shown) are installed above the device.

The laser light emitted from the light source device is rotated by the polygon mirror 117 for scanning, the light flux of the scanning light is deflected by the reflection mirror, and the fθ lens is used to generate the direction of the generatrix of the photosensitive drums 3a, 3b, 3c, 3d. By exposing the light to the photosensitive drums 3a, 3b, 3c, 3 by
An electrostatic latent image corresponding to the image signal is formed on d.

The developing devices 1a, 1b, 1c, and 1d are filled with a predetermined amount of cyan, magenta, yellow, and black toners each having a negative charging polarity as a developer by a supply device (not shown). Developer 1a, 1
b, 1c and 1d are photosensitive drums 3a, 3b and 3 respectively.
The latent images on c and 3d are developed and visualized as a cyan toner image, a magenta toner image, a yellow toner image, and a black toner image.

The recording material P is contained in the recording material cassette 10. From there, the recording material P is supplied onto the transfer belt 130 via a plurality of conveying rollers and the registration rollers 12, and is conveyed by the transfer belt 130 to the photosensitive drums 3a, 3a.
b, 3c, and 3d are sequentially sent to the transfer section facing them.

The transfer belt 130 is made of polyethylene terephthalate resin (PET), polyvinylidene fluoride resin,
It is composed of a sheet of a dielectric resin such as polyurethane resin, and both ends thereof are overlapped and joined to each other to form an endless shape, or a seamless belt (seamless) is used.

When it is confirmed that the transfer belt 130 is rotated by the driving roller 13 and is in a predetermined position, the recording material P is transferred from the registration roller 12 to the transfer belt 13.
0, and is conveyed toward the transfer portion of the first image forming portion Pa. At the same time, the image writing signal is turned on, and an image is formed on the photosensitive drum 3a of the first image forming portion Pa at a certain timing with reference to the signal.

Then, the transfer charger 24a applies an electric field or an electric charge at the transfer portion below the photosensitive drum 3a, whereby the first color toner image formed on the photosensitive drum 3a is transferred onto the recording material P. It By this transfer, the recording material P is firmly carried on the transfer belt 130 by the electrostatic attraction force, and is conveyed to the second image forming portion Pb and thereafter.

The transfer charger 24 (24a-24d) includes:
A non-contact charger such as corona discharge or a contact charger using a transfer member such as a conductive blade, roller or brush can be used. The non-contact charger has problems that ozone is generated and that it is charged via air, so that it is vulnerable to temperature and humidity environment changes in the atmosphere and images are not stably formed. The contact charger does not have such a problem, and has advantages such as ozone-less, resistance to temperature and humidity environment variations, and high image quality. In this example, a contact type charger is used as the transfer charger 24.

Image formation and transfer in the second to fourth image forming portions Pb to Pd are performed in the same manner as in the first image forming portion Pa.
Then, the recording material P to which the toner images of four colors have been transferred is removed from the end of the transfer belt 130 by removing the charge by the separation charging device 32 at the downstream portion of the transfer belt 130 in the transport direction to attenuate the electrostatic attraction force. . Usually, the drive roller 13 is grounded for stable separation. The separation charger 32 is a non-contact charger because it discharges the recording material P with the toner image unfixed.

The recording material P separated from the transfer belt 130
Is transported to the fixing device 9 by the transport unit 62. The fixing device 9 includes a fixing roller 51, a pressure roller 52, and heat resistant cleaning members 54 and 55 for cleaning each of them.
And a heater 56 installed in the rollers 51 and 52,
57, an application roller 50 for applying a release agent oil such as dimethyl silicone oil to the fixing roller 51, an oil sump 53 thereof, and a thermistor 58 for detecting the surface temperature of the pressure roller 52 and controlling the fixing temperature. It consists of

The recording material P on which the four color toner images have been transferred is
The toner images are mixed and fixed to the recording material P by fixing, and a full-color copy image is formed, and the copy image is discharged to the paper discharge tray 63.

Photosensitive drums 3a, 3b, 3 for which transfer has been completed
c and 3d are cleaners 4a, 4b, 4c and 4d, respectively.
Thus, the transfer residual toner is cleaned and removed, and the preparation for the subsequent latent image formation is performed. Transfer belt 1
Toner and other foreign matter remaining on the cleaning belt 30 are transferred onto the surface of the transfer belt 130 by a cleaning web (nonwoven fabric) 19
Abutting against each other and wiping it off.

The transfer belt 130 used in the image forming apparatus having the above structure is a dielectric sheet such as a PET sheet, a polyvinylidene fluoride sheet, a polyurethane sheet, etc., as described above, and their volume resistance is 10 ¹³
It is preferably from 10 to ^{17 17} Ωcm. In addition, the transfer charger 24
Is known to stabilize the image when the current contributing to transfer (transfer current) is kept constant at an appropriate current.
It is preferable to connect a constant current power source and perform constant current control so that a constant current can be obtained even when the volume resistance changes due to the type of recording material (thickness, material, etc.) and moisture absorption conditions. In this case, the transfer voltage takes several kV to several kV.

In the image forming apparatus for carrying out multiple transfer, a transfer voltage larger than that at the previous transfer is required at each transfer stage.
In particular, the transfer voltage for the last (fourth color) transfer is considerably higher than that for the first (first color) transfer.

As the dielectric sheet material of the transfer belt 130, in addition to the above, polyacetal, polyamide, polyvinyl alcohol, polyether ketone, polystyrene, polybutylene terephthalate, polymethylpentene, polypropylene, polyethylene, polyphenylene sulfide, silicone resin, Polyamide imide, polycarbonate, polyphenylene oxide, polyether sulfone, polysulfone, aromatic polyester, polyetherimide, aromatic polyimide, such as engineering plastic film-shaped sheet may be used,
In this example, a polyimide resin was used in terms of mechanical properties, electrical properties, flame retardancy, and the like. It has a volume resistivity of 10 ¹⁶ Ωcm and a thickness of 100 μm, and is a seamless type.

A driving means for driving the transfer belt and a detecting means for detecting the width of the recording material are provided, and the process speed (moving speed of the transfer belt) is 100 mm / sec.

Contact type transfer charger 24 (24a-24)
(d) has a transfer member of a plate-shaped conductive rubber (blade) extending in a direction (thrust direction) orthogonal to the recording material conveyance direction, and this transfer member is provided on the photosensitive drum 3 (via the transfer belt 130). 3a to 3d) are pressed. A voltage having a polarity (plus in this example) opposite to the regular charging polarity of the toner is applied to the transfer charger 24, and the toner image on the photosensitive drum 3 is electrostatically transferred onto the surface of the recording material P. In this embodiment, the current flowing through the transfer charger 24 is controlled to a constant current, and the transfer current is set to 10 μA.

The separation charger 32 is located at the uppermost downstream side of the transfer belt 130, that is, the drive roller 13 of the transfer belt 130.
And is equipped with a discharge wire. The discharge wire is stretched in the thrust direction, and the tension is maintained by providing a spring at one end of the discharge wire. Power is supplied to the discharge wire from a connector on the apparatus main body side through a power supply terminal, a power supply pin, and a spring (not shown).

The drive roller 13 is connected to the earth of the main body and also functions as an opposite electrode of the discharge wire. In this embodiment, the transfer charger 24d of the image forming portion Pd at the final stage is used.
The distance between the separation charging unit and the separation charging unit is 50 mm.
An AC voltage of 10 kVpp was applied to 2.

In the electrophotographic image forming apparatus as described above, the length of the transfer member of the transfer charger 24 in the thrust direction is usually the maximum size recording so that the maximum size recording material can be covered. It is slightly longer than the width of the material (the length in the same direction as the thrust direction). However, when a small-sized recording material is passed, as described above, the surface charge density of the recording material becomes insufficient, resulting in defective transfer.
This phenomenon will be described in detail below.

FIG. 2 shows the correlation between the surface charge density of paper at the time of transfer and the image density (reflection density) transferred to the paper. It can be seen that as the charge density on the upper surface of the paper increases, the image density on the paper also increases. It can be seen from FIG. 2 that the reflection density of 1.55 or more is necessary and the charge density on paper is at least 24 pC / cm ² in order to prevent image defects.

When the recording material of the maximum size is passed through the structure of this embodiment, the charge density on the paper surface is Transfer Itr = 10 μA,
Thrust width of recording material χ = 30 cm, process speed v
= 10 cm / s, from the above equation, Itr / (χ · v) = 10 / (30 × 10) = 1/30
It becomes [μC / cm ² ], and it can be seen from FIG. 2 that this is the amount of electric charge that sufficiently satisfies the reflection density of 1.55 or more.

However, it has been experimentally clarified that when an image is formed on a recording material of a small size, the charge density on the upper surface of the paper becomes 24 pC / cm ² or less and the density becomes low. Hereinafter, this phenomenon will be described in more detail by calculation.

FIG. 3 shows a cross section of the transfer portion when a small-sized recording material is passed. In FIG. 3, photosensitive drum: photosensitive layer thickness t _D [μm], photosensitive layer volume resistivity ρ _D [Ωc
m], transfer belt: thickness t _B [μm], volume resistivity ρ
_B [Ωcm], recording material: thickness t _P [μm], volume resistivity ρ
_{It has specifications of P} [Ωcm], thrust width χ [cm], and transfer member: thrust width y [cm]. The process speed (moving speed of the transfer belt) is v [cm / s], and the nip width is 1 mm. The peripheral speed of the photosensitive drum is substantially equal to the process speed. In FIG. 3, for convenience, the small size paper is used as the one-sided reference, but the center reference has no problem.

The transfer portion of FIG. 3 can be written in an equivalent circuit as shown in FIG. In FIG. 4, R _D ¹ is the resistance of the portion of the photosensitive drum 3 in contact with the recording material P, R _D ² is the resistance of the portion of the photosensitive drum 3 in direct contact with the transfer belt 130, R _P is the resistance of the recording material P, R _B ¹ is the recording material P of the transfer belt 130.
And R _B ² is the resistance of the portion of the transfer belt 130 in direct contact with the photosensitive drum 3.

During image formation, the transfer current is R _D ¹ → R _P → R _B ¹
Or a path of R _D ² → R _B ² . The current flowing through the recording material can be obtained by obtaining the specific resistance of these paths.

Then, the respective resistances are obtained as follows.

Photosensitive drum: R _D ¹ = ρ _D × (t _D × 10 ⁻⁶ × 10 ² ) / (0.1 × χ) = (t _D / χ) · ρ _D · 10 ⁻³ [Ω] · ^{_{·· (1) R D 2 =}} ρ D × (t D · × 10 -6 × 10 2) / {0.1 × (y-χ)} = {t D / (y-χ)} · ρ D · 10 ⁻³ [Ω] (2) Recording material: R _P = ρ _P × (t _P × 10 ⁻⁶ × 10 ² ) / (0.1 × χ) = (t _P / χ) · ρ _P · 10 ⁻³ [Ω] (3) Transfer belt: R _B ¹ = ρ _B × (t _B × 10 ⁻⁶ × 10 ² ) / (0.1 × χ) = (t _B / χ) ^{_{· ρ B · 10 -3 [Ω}} ] ··· (4) R B 2 = ρ B × (t B × 10 -6 × 10 2) / {0.1 × (y-χ)} = {t B / (Y−χ)} · ρ _B · 10 ⁻³ [Ω] (5) Therefore, from the resistance ratio between the paper passing portion and the non-paper passing portion, the ratio of the currents flowing through each is expressed by the equation (6). Is obtained.

[0059] Current ratio: Paper unit: Hikami unit ^{_{= 1 / (R D 1 +}} R P + R B 1): 1 / (R D 2 + R B 2) ··· (6) From this, the current flowing through the recording material When the transfer current is Itr, is expressed by equation (7).

Paper current = Itr × {1 / (R _D ¹ + R _P + R _B ¹ )} / {1 / (R _D ¹ + R _P + R _B ¹ ) + 1 / (R _D ² + R _B ² )} = { ^{_{(R D 2 + R B 2}} ) / (R D 1 + R D 2 + R B 1 + R B 2 + R P)} × Itr [μA] ··· (7) from which, on paper surface charge density formula (8) Can be expressed as

[0061]     Paper surface charge density = {(R_D ²+ R_B ²) / (R_D ¹+ R_D ²+ R_B ¹+ R_B ²+ R_P)}                     × Itr {1 / (v · χ)} [μA] (8) FIG. 5 shows that the paper width χ [cm] is changed by using the equation (8).
Showed how the charge density on the paper surface changes when
It is a figure. When referring to the values of other parameters, t_D=
20 [μm], ρ_D= 10¹⁴[Ωcm], t_B= 80 [μ
m], ρ_B= 10 ¹³[Ωcm], t_P= 100 [μm],
ρ_P= 10¹⁴[Ωcm], y = 30 [cm], v = 10
[Cm / s], nip width = 1 mm.

Looking at this, the width χ of the recording material passing through is 2
Below 6.8 cm, the charge density on the paper surface is 24 pC /
When the density is less than cm ² , the density is low and transfer failure occurs. Therefore, under these conditions, it is impossible to form an image on a postcard, a longitudinal feed A4 paper or the like having a width χ of 26.8 cm or less, that is, a recording material having a small width, without transfer failure.

Therefore, in the present invention, as shown in FIG.
The maximum size can be obtained by preliminarily accumulating (giving) electric charges to a region B, which is a non-paper-passing part, adjacent to a region A, which is a paper-passing part on which the recording material is carried, of the transfer belt 130 before transfer. At the time of transfer when an image is formed on a recording material having a width equal to or less than a width, it becomes possible to stably supply an appropriate amount of charge to the recording material and perform good transfer. Hereinafter, this will be described in detail.

Since the transfer belt 130 is made of a dielectric material, for example, a laboratory charge supply device 30 as shown in FIG. 7 is used, and the pair of electrodes 31a and 31b prevents the transfer belt 130 from passing through. One electrode 31a with the portion sandwiched therebetween
When a voltage is applied by the high-voltage power supply 32 attached with a voltage to supply a charge to the non-sheet passing portion, the relationship between the polarization current I monitored by the ammeter 33 and the application time t of the constant voltage V is as shown in FIG. Therefore, the more the charges accumulated on the front and back surfaces of the transfer belt 130, the more the impedance of the belt increases.

Therefore, in this embodiment, when an image is formed on a recording material having a width smaller than the maximum size of the recording material, charges are accumulated in advance in the non-sheet passing portion of the transfer belt 130 before the transfer, and the charge is not accumulated. By utilizing the phenomenon that the impedance of the paper passing portion increases, the transfer failure is prevented. In this embodiment,
The polarity of the charge accumulated on the transfer belt 130 is negative on the front surface (recording material carrying side) of the belt (same polarity as the normal charging polarity of the toner) and positive on the back surface (reverse polarity of the normal charging polarity of the toner). is there.

In the equivalent circuit of FIG. 4, the above charge accumulation is equivalent to increasing the value of R _B ² (the resistance of the portion of the transfer belt that is in direct contact with the photosensitive drum). That is,
By increasing R _B ² , the resistance ratio between the paper passing portion and the non-paper passing portion is reduced, so that the difference between the current per unit area flowing through the paper passing portion and the current per unit area flowing through the non-paper passing portion is reduced. to shrink. Therefore, as shown in FIG. 9, it can be seen that as R _B ² increases, the charge density on the upper surface of the paper increases, and the above-described transfer failure does not occur.

To apply a charge of the same polarity as that at the time of transfer in advance to the back surface of the non-sheet passing portion of the transfer belt 130 before the transfer,
On the upstream side of the transfer position of the image forming portion Pa, for example, as shown in FIG.
2, two rollers 34a and 34b sandwiching the non-sheet passing portion of the transfer belt 130 are provided, and the lower roller 34b thereof is provided.
Is grounded, and a voltage having the same polarity as the normal charging polarity of toner may be applied to the upper roller 34a from the high voltage power supply 35.

The upper roller 3 is provided so that the charge can be supplied to the transfer belt 130 corresponding to recording materials of various widths.
4a may be a roller 300 as shown in FIG. 11, for example. The roller 300 is formed by cutting out a conductive cylinder in the thrust direction so that the cutout amount increases.
The length of the roller 300 in the thrust direction is continuously changed in the circumferential direction.

The roller 300 is rotatable in the circumferential direction by pivotally attaching the central shaft 301 to a mounting portion (not shown), and a gear 302 mounted on one end of the central shaft 301 and a driving gear meshed with the gear 302. Gear 30 on device 304 side
3 is rotated by the forward / reverse rotation motor of the drive device 304. The rotation amount is controlled by the CPU 305 which is a control means.

In the manual paper feeding unit or the cassette paper feeding unit, the paper width of the recording material P is always detected, and the paper width information is transferred to the CPU 305. The CPU 305 controls the rotation angle of the motor of the drive unit 304 according to the paper width information. Then, the motor of the driving device 304 rotates to rotate the roller 300 by a predetermined angle, and the peripheral surface of the roller 300 having a length that matches the width of the non-sheet passing portion of the transfer belt 130 is directed toward the transfer belt 130. Contacted.

It is preferable that the roller 300 is in contact with the transfer belt 130 only when an image is formed on a recording material of a small size, and is separated from the transfer belt otherwise.

The charges previously accumulated on the transfer belt 130 are
The transfer belt itself may not disappear until the static elimination process.
Desirable, for that purpose select a material with a long charge relaxation time
Preferably. Therefore, a material with a high volume resistivity
Fee, specifically 1.0 x 10 ¹³~ 1.0 x 10¹⁷Ω cm
Are preferred.

In this embodiment, the lower roller 3 shown in FIG.
4b is grounded, and the upper roller 34a or the roller 3 in FIG.
00, a negative voltage was applied to the upper roller 34,
There is no problem even if a positive voltage is applied to the roller 300 and the lower roller 34b is grounded.

The paper width (length in the thrust direction χ) of the recording material is detected automatically by using a mechanical sensor or an optical sensor, and the operator inputs the paper width from the operation unit or the like. You may change.

Further, although the transfer belt is shown as the recording material carrier in the above, it may be a transfer drum. The image carrier is not limited to an electrophotographic photosensitive member such as a photosensitive drum, but may be a dielectric in electrostatic recording.

The developing device 1 (1a-1d) may be any type of developing method.

Generally, the developing method is roughly classified into a one-component developing method and a two-component developing method. In the one-component developing method, a non-magnetic toner is coated on a developing sleeve with a blade or the like.
Magnetic toner is coated on a developing sleeve by magnetic force, and is conveyed to a developing unit facing the image carrier by the rotation of the developing sleeve, and the toner is developed in a non-contact state with the image carrier, the one-component non-contact developing method, There is a one-component contact developing method that develops in contact with the image carrier, and the two-component developing method is
Using a two-component developer in which toner particles and magnetic carrier are mixed, this is coated on the developing sleeve with magnetic force and conveyed to the developing section by the developing sleeve to develop the developer in a non-contact state with the image carrier. Two-component non-contact developing method,
There is a two-component contact developing method in which development is carried out in contact with the image carrier. The two-component contact developing method is often used from the viewpoint of high image quality and high stability.

In the present embodiment, the value of the required paper top surface charge density is set to 24 pC / cm ² , but the required paper top surface charge density is, for example, the triboelectric charge amount (tribo) of the toner, the type of toner, or The present invention is not limited to this because it changes depending on the installation environment of the image forming apparatus.

In this embodiment, the area to be charged before transfer is only the non-sheet passing portion of the transfer belt 130. However, as described above, if the area is charged so that the transfer failure does not occur, before transfer. The charged area of is not limited to this. In this case, the amount of electric charge applied to the non-sheet passing portion is made larger than that of the sheet passing portion.

Further, according to the present invention, the image forming apparatus is
A transfer drum system may also be used in which toner images are sequentially formed on a photoconductor and sequentially transferred to a recording material carried on a transfer drum. In this case, a desired charge may be applied to the non-sheet passing area of the transfer sheet that constitutes the transfer drum.

This is an intermediate transfer system in which toner images are sequentially formed on one or more photosensitive drums, the toner images are sequentially primary-transferred onto an intermediate transfer member, and then secondary-transferred collectively onto a recording material. However, the present invention can be applied when the above-described recording material carrier is used to convey the recording material to the secondary transfer portion.

The present invention can also be applied to the case where a constant voltage power source is connected to the transfer charger to control the voltage applied to the transfer charger by a constant voltage.

An image forming apparatus (for example, a black and white image forming apparatus) that installs one photosensitive drum and transfers a toner image (for example, a black toner image) on the photosensitive drum onto a recording material carried and conveyed by a transfer belt is used. However, the present invention can be applied.

Example 2 The configuration of this example is almost the same as that of Example 1, but the transfer belt 130 is preliminarily set before the transfer depending on the type of recording material and / or the humidity (and temperature) in the apparatus. The difference is that the amount of electrification is controlled.

When an OHT (transparent resin sheet for overhead projector) is used as a recording material and when plain paper is used, the current flowing to the non-sheet passing portion at the time of transfer is different under the same environment (temperature and humidity). When a recording material (OHT) having a relatively high volume resistivity is used, transfer failure may occur.

Therefore, in the present embodiment, the roller 34
a. The transfer failure can be prevented by controlling the approved voltage according to the type of the recording material by the CPU as the control means. Specifically, in the case where OHT is used as the recording material, the voltage applied to the roller 34a is made larger than the case where plain paper is used as the recording material, and the charge amount of the non-sheet passing portion of the transfer belt 130 is increased. I made it bigger.

Further, even with the same kind of recording material, the volume resistivity of the recording material becomes relatively high depending on the environment (noticeable in a low humidity environment), and a transfer failure may occur.

Therefore, in this embodiment, in addition to the control according to the type of the recording material, the voltage applied to the roller 34a is controlled by the CPU according to the humidity (and temperature) in the apparatus, The transfer failure could be prevented.

In Examples 1 and 2 above, the positive charge was applied only to the non-sheet passing portion of the transfer belt before the recording material was carried on the transfer belt, but the present invention is not limited to this.
For example, as shown in FIGS. 16A, 16B, and 16C, the transfer belt 130 may be charged to prevent transfer failure.

Also in FIG. 16, the polarity of the voltage applied to the transfer charger is positive, that is, the polarity of the charge applied from the transfer charger to the transfer belt is positive. That is,
The amount of charge applied to the non-sheet passing portion may be made larger than the amount of charge applied to the sheet passing portion in the positive direction (the polarity of the charge applied from the transfer charger).

As a method of obtaining the charge amount distribution on the transfer belt, a surface potential sensor (EFS-22 manufactured by TDK Corp.) is used.
D, EFS-31D), the potential V of the transfer belt is measured at each point in the thrust direction, and the charge amount at each point is obtained from the electrostatic capacitance C of the transfer belt which is known in advance.

[0092]

As described above, according to the present invention,
With respect to the recording material conveyance direction of the recording material carrier, a charge applying unit is installed on the upstream side of the transfer position, and the same polarity as that at the time of transfer is applied to the front and back surfaces of the non-sheet passing portion where the recording material of the recording material carrier is not located Is applied before the recording material reaches the transfer position, the contact-type transfer means secures a transfer current in the paper passing portion regardless of the size of the recording material, and a visible image on the image carrier is obtained. Can be satisfactorily transferred to a recording material, and a high-quality image without transfer defects can be obtained.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an image forming apparatus to which the present invention is applicable.

FIG. 2 is a diagram showing a relationship between a paper top surface charge density and an image density.

FIG. 3 is a cross-sectional view showing a transfer portion at the time of transferring small size paper.

FIG. 4 is a diagram showing an equivalent circuit of the transfer unit in FIG.

FIG. 5 is a diagram showing the relationship between the paper top surface charge density and the paper width χ.

FIG. 6 is an explanatory diagram showing supply of electric charges having the same polarity as during transfer to a non-sheet passing portion of a transfer belt before transfer according to the present invention.

FIG. 7 is a conceptual diagram showing a laboratory-like charge supply device used for supplying charges to a non-sheet passing portion before transfer performed in the present invention.

FIG. 8 is a diagram showing a relationship between a constant voltage application time to a transfer belt and a current.

FIG. 9 is a diagram showing a relationship between a resistance R _B ^{2 of a} portion of the transfer belt which is in direct contact with the photosensitive drum and a charge amount on paper.

FIG. 10 is an explanatory diagram showing a method of supplying electric charges to a non-sheet passing portion before transfer in an embodiment of the present invention.

FIG. 11 is a perspective view showing a preferable modified example of the roller used for supplying electric charge according to the present invention.

12 is a sectional view showing a transfer portion of the image forming apparatus of FIG.

13 is a perspective view showing a target area of a recording material for transfer in a time Δt seconds by the transfer section of FIG.

FIG. 14 is a cross-sectional view showing a transfer portion at the time of transferring small size paper.

FIG. 15 is a diagram showing a surface potential of a photosensitive drum.

FIG. 16 is a diagram showing a modified example of the present invention.

[Explanation of symbols]

3, 3a-3d photosensitive drum 24, 24a to 24d Transfer charger 34a, 34b rollers 35 high voltage power supply 130 transfer belt 300 rollers P recording material

─────────────────────────────────────────────────── ─── Continuation of front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) G03G 15/16 G03G 15/01 114

Claims (28)

(57) [Claims] 1. An image carrier for carrying an image, a recording material carrier for carrying a recording material, and an image on the image carrier electrostatically on the recording material carried on the recording material carrier. For transfer, a first charge applying unit for applying an electric charge of a predetermined polarity on the side of the pre-recording material carrier opposite to the side carrying the recording material, and the recording material are carried on the recording material carrier. An image forming apparatus having a second charge applying unit for applying an electric charge to a side opposite to a side of the recording material carrying member carrying the recording material, the image forming apparatus being substantially orthogonal to a conveying direction of the recording material. A second area outside the first area of the recording material carrier, on which the recording material is carried, by the first charge applying means. Charge amount of
The image forming apparatus, wherein the second charge applying unit applies an electric charge to the recording material carrier so that the second charge applying unit increases the electric charge amount toward the predetermined polarity as compared with the electric charge amount in the first region. 2. The image forming apparatus according to claim 1, further comprising a control unit that controls a voltage applied to the second charge applying unit according to the type of recording material. 3. A recording material detection unit for detecting the type of recording material, wherein the control unit controls the voltage applied to the second charge applying unit according to the detection result of the recording material detection unit. The image forming apparatus according to claim 2. 4. The image forming apparatus according to claim 1, further comprising a control unit that controls the amount of electric charge applied to the recording material carrier by the second electric charge applying unit according to the type of recording material. 5. A recording material detection means for detecting the type of recording material, wherein the control means causes the second charge applying means to apply the recording material to the recording material carrier in accordance with the detection result of the recording material detection means. The image forming apparatus according to claim 4, wherein the amount of applied charges is controlled. 6. The image forming apparatus according to claim 1, further comprising a control unit that controls a voltage applied to the second charge applying unit according to humidity. 7. The image according to claim 6, further comprising humidity detecting means for detecting humidity, wherein the control means controls a voltage applied to the second charge applying means in accordance with a detection result of the humidity detecting means. Forming equipment. 8. The image forming apparatus according to claim 1, further comprising a control unit that controls a voltage applied to the second charge applying unit according to temperature and humidity. 9. A temperature / humidity detecting means for detecting temperature and humidity is provided, and the control means controls a voltage applied to the second charge applying means in accordance with a detection result of the temperature / humidity detecting means. Item 8. The image forming apparatus according to item 8. 10. The image forming apparatus according to claim 1, further comprising control means for controlling an amount of electric charge applied to the recording material carrier by the second electric charge applying means in accordance with humidity. 11. A humidity detecting means for detecting humidity,
The image forming apparatus according to claim 10, wherein the control unit controls the amount of electric charge applied to the recording material carrier by the second electric charge applying unit according to a detection result of the humidity detecting unit. 12. The image forming apparatus according to claim 1, further comprising control means for controlling an amount of electric charge applied to the recording material carrier by the second electric charge applying means in accordance with temperature and humidity. 13. A temperature / humidity detecting means for detecting temperature and humidity is provided, and the control means applies the second charge applying means to the recording material carrier according to a detection result of the temperature / humidity detecting means. 13. Controlling the amount of charge applied.
Image forming device. 14. The image forming apparatus according to claim 1, wherein the charge amount applied to the first region of the recording material carrier by the second charge applying means is zero. 15. The polarity of the charges applied to the first region of the recording material carrier by the second charge applying device is applied to the recording material carrier by the first charge applying device. The image forming apparatus according to claim 1, wherein the polarity of the charges is opposite to that of the charges. 16. The polarity of the charge applied to the second region of the recording material carrier by the second charge applying device is applied to the recording material carrier by the first charge applying device. The image forming apparatus according to claim 15, wherein the polarity of the charges is opposite to that of the charges. 17. The absolute value of the amount of charge applied to the second region of the recording material carrier by the second charge applying means is the absolute value of the charge amount of the recording material carrier by the second charge applying means. The image forming apparatus according to claim 16, wherein the amount of charge applied to the first region is smaller than the absolute value. 18. The polarity of the charge applied to the first region of the recording material carrier by the second charge applying means is applied to the recording material carrier by the first charge applying means. The image forming apparatus according to claim 1, which has the same polarity as the electric charge. 19. The polarity of the charge applied to the second region of the recording material carrier by the second charge applying means is applied to the recording material carrier by the first charge applying means. 19. The image forming apparatus according to claim 18, which has the same polarity as the electric charge. 20. The absolute value of the amount of charge applied to the second area of the recording material carrier by the second charge applying means is the absolute value of the charge amount of the recording material carrier by the second charge applying means. 20. The image forming apparatus according to claim 19, wherein the image forming apparatus is larger than the absolute value of the amount of charge applied to the first area. 21. The polarity of the charge applied to the second region of the recording material carrier by the second charge applying device is applied to the recording material carrier by the first charge applying device. The image forming apparatus according to claim 18, wherein the polarity of the charges is opposite to that of the charges. 22. The second charge imparting means includes a first charge imparting member that imparts an electric charge to a side of the recording material carrying body that carries the recording material, and the first charge imparting member through the recording material carrying body. The image forming apparatus according to claim 1, further comprising a second charge applying member that is provided so as to face the charge applying member and that applies an electric charge to a side of the recording material carrier opposite to a side on which the recording material is carried. 23. A region in which a charge is applied to the recording material carrier by the second charge applying means in a direction substantially orthogonal to the recording material conveying direction is variable according to the length of the recording material. The image forming apparatus according to claim 1. 24. The second electric charge applying means applies electric charges only to the second region of the recording material carrier in a direction substantially orthogonal to a recording material conveying direction.
3. The image forming apparatus of 3. 25. The image forming apparatus according to claim 1, wherein the image carrier has a charging polarity opposite to the predetermined polarity. 26. The volume resistivity of the recording material carrier is 10
The image forming apparatus according to claim 1, which has a density of ¹³ to 10 ¹⁷ Ωcm. 27. The image forming apparatus according to claim 1, wherein images of a plurality of colors are sequentially transferred onto the recording material carried by the recording material carrying body in an overlapping manner. 28. A plurality of the image carriers are provided to respectively carry images of a plurality of colors, and a plurality of images of a plurality of colors are sequentially superposed on the recording material carried by the recording material carrier from the plurality of image carriers. 28. The image forming apparatus according to claim 27, which is transferred by transfer.