JP2000221806A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make an excellent image being free from a transfer omission or the like in a dot shape obtainable by realizing stable multiple transferring by means of reducing re-transferring. SOLUTION: This image forming device is allowed to set rollers 200a and 200b as inverse charging means, in front of the first image forming part therein. Then, the device is allowed to preliminarily charge transfer material P before entering in the transferring part on the first image forming part by holding the same between the rollers 200a and 200b, applying bias on the one hard of roller 200a, letting a constant voltage controlled current flow to the other hand of roller 200b being grounded, and imparting the electric charge in polarity opposite to the transferring time on the rear side of the transfer material P.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming an image on a transfer material using an electrophotographic method to obtain a hard copy,
The present invention relates to an image forming apparatus such as a copying machine, a facsimile, and a printer.

[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 method in which a toner image formed on a photosensitive drum is sequentially transferred onto a transfer material carried on a transfer drum or a transfer belt. Hereinafter, an image forming apparatus using this method will be described.

The image forming process of an image forming apparatus using a transfer belt will be briefly described. First, a first-color photosensitive drum installed along a transfer belt rotates, and the surface thereof is uniformly charged by a charger. I do. Next, a laser beam modulated by the first color of the document, for example, a magenta image signal is irradiated on the photosensitive drum to form an electrostatic latent image of magenta on the photosensitive drum. The electrostatic latent image is developed by a magenta developing device to form a first color magenta toner image on the photosensitive drum.

On the other hand, a transfer material stored in a paper feed cassette is conveyed to a transfer belt by a registration roller or the like. Simultaneously with the transfer of the transfer material, the suction roller is pressed against the front surface of the transfer belt, and is suction-charged by the suction charger from the back surface, so that the transfer material is electrostatically suctioned onto the transfer belt.
The transfer belt rotates in synchronization with the photosensitive drum, and the magenta toner image formed on the photosensitive drum by the transfer charger is transferred to a transfer material carried on the transfer belt.

[0005] Next, the 2 is set along the transfer belt.
A second color cyan toner image is formed on the color photosensitive drum, and the cyan toner image is transferred onto the transfer material conveyed by the transfer belt in a superimposed manner from above the magenta toner image. The same process is repeated for the third and fourth colors, yellow and black, so that magenta, cyan,
A full-color image is obtained by superimposing four yellow and black toner images.

The transfer material onto which the four color toner images have been transferred is separated from the transfer belt and conveyed to a fixing device. The fuser is
The toner image and the transfer material are heated and pressed by a fixing roller and a pressure roller to mix the toner images of each color and fix the transfer material to form a full-color print image. Is discharged.

[0007]

However, in an image forming apparatus using the above-described method, since the transfer of the toner image to the transfer material is repeated, the toner image that has already been transferred to the transfer material has a different color. During the toner image transfer process,
A phenomenon called retransfer, which causes offset on the photosensitive drum, may occur.

For example, when forming a blue image, first, a solid magenta image is transferred onto a transfer material, and then a solid cyan image is transferred onto the transfer material in a multiplex manner. Thereafter, transfer steps such as yellow and black are performed. In the transfer step, there is no toner to be transferred at the corresponding position on the photosensitive drum. In this state, when the toner image on the transfer material surface comes close to or comes into contact with the photosensitive drum surface, the toner image on the transfer material, in this case, is mainly a cyan toner image, and a part of the toner is exposed from the cyan toner image. A phenomenon occurs in which the image is retransferred onto the drum.
In this case, the density of the cyan toner image is reduced at the portion where the cyan toner is re-transferred, and the magenta toner image of the magenta toner image of the layer below the cyan toner image appears, thereby causing a significant deterioration in image quality.

This phenomenon is caused by a change in the charge amount of the toner with a change in the temperature and humidity, and a change in the charge amount per unit area of the transfer material with a change in the type of the transfer material and the temperature and humidity. Then it was remarkable. In particular, in recent years, as the particle size of the toner has been reduced for higher image quality, the charge amount per toner particle has been reduced, but the total charge amount of the toner image has tended to be increased. The transfer current or the transfer voltage required for transferring the image has been increasing. The increase in the transfer current and the transfer voltage also has an adverse effect on the retransfer phenomenon.

SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus capable of realizing stable multiple transfer by reducing retransfer and obtaining a high quality image free from transfer defects such as dot-like transfer omissions. It is.

[0011]

The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention provides:
In an image forming apparatus having an image forming unit for electrostatically forming a toner image on a transfer material, before forming the toner image on the transfer material by the image forming unit, the surface of the transfer material on which the toner image is formed And charging means for controlling the voltage applied to the charging means in accordance with the length of the transfer material in a direction substantially perpendicular to the transport direction of the transfer material. An image forming apparatus characterized in that:

According to the present invention, there is provided humidity detecting means for detecting humidity, and the control means controls a voltage applied to the charging means in accordance with a result of detection by the humidity detecting means. Alternatively, it has a temperature and humidity detecting means for detecting temperature and humidity, and the control means controls a voltage applied to the charging means according to a detection result by the temperature and humidity detecting means. There is provided a determination unit for determining the type of the transfer material, and the control unit controls a voltage applied to the charging unit according to a detection result by the determination unit. There is a constant current power supply connected to the charging means, and the control means controls a current value flowing through the charging means in accordance with a length in a direction substantially orthogonal to a transfer direction of the transfer material.

The image forming means includes an image carrier for carrying a toner image, a transfer material carrier for carrying a transfer material, and a transfer material carrying the toner image on the image carrier for the transfer material carrier. And a transfer unit for electrostatically transferring the image onto the recording medium. Toner images of a plurality of colors are sequentially and electrostatically transferred from the image carrier to a transfer material carried on the transfer material carrier. Alternatively, the image forming means may include a plurality of image carriers each carrying a plurality of color toner images, a transfer material carrier carrying a transfer material, and a plurality of color toner images on the plurality of image carriers. And a plurality of transfer means for sequentially transferring electrostatically to a transfer material carried on the transfer material carrier.

The charging means includes a first charging member for charging the surface of the transfer material on which the toner image is transferred to a polarity opposite to the normal charging polarity of the toner, and a charging member opposed to the first charging member. And a second charging member that charges the opposite side of the transfer material surface on which the toner image is transferred to the same polarity as the normal charging polarity of the toner. When charging the transfer material, a voltage having the same polarity as the normal charging polarity of the toner is applied to the second charging member, and the first charging member contacts the transfer material and is electrically grounded. When the transfer material is charged, the first transfer member contacts the transfer material, and the second transfer member contacts the transfer material carrier.

The transfer means is provided on a side of the transfer material carrier opposite to a side on which the transfer material is carried, and transfers a toner image from the image carrier to a transfer material carried on the transfer material carrier. A voltage having a polarity opposite to the normal charge polarity of the toner is applied to the transfer unit. A first charging member provided on a side of the transfer material carrier for supporting the transfer material, the first charging member configured to charge a surface of the transfer material on which the toner image is transferred to a polarity opposite to a normal charging polarity of the toner; And a second charging member that is provided to face the first charging member via the transfer material carrier and charges the transfer material carrier to the same polarity as the normal charging polarity of the toner. Before transferring the toner image from the image carrier to the transfer material carried on the transfer material carrier, the charging unit electrostatically attracts the transfer material to the transfer material carrier.

According to the present invention, there is provided an image forming apparatus having an image forming means for electrostatically forming a toner image on a transfer material, wherein the toner image is formed before the toner image is formed on the transfer material by the image forming means. A charging means for pre-charging the surface of the formed transfer material to a polarity opposite to the normal charging polarity of the toner; and the transfer material by the charging means in accordance with a length in a direction substantially orthogonal to a transfer direction of the transfer material. And control means for controlling the amount of charge per unit area applied to the image forming apparatus.

According to the present invention, there is provided a humidity detecting means for detecting humidity, wherein the control means controls the charge per unit area applied to the transfer material by the charging means in accordance with the detection result by the humidity detecting means. Control the amount. Alternatively, it has a temperature and humidity detecting means for detecting temperature and humidity, and the control means controls an amount of electric charge per unit area applied to the transfer material by the charging means according to a detection result by the temperature and humidity detecting means. I do. The control unit controls the amount of charge per unit area applied to the transfer material by the charging unit according to the detection result of the determination unit. There is a constant current power supply connected to the charging means, and the control means controls a current value flowing through the charging means in accordance with a length in a direction substantially orthogonal to a transfer direction of the transfer material.

The image forming means includes an image carrier for carrying a toner image, a transfer material carrier for carrying a transfer material, and a transfer material carrying the toner image on the image carrier on the transfer material carrier. And a transfer unit for electrostatically transferring the image onto the recording medium. Toner images of a plurality of colors are sequentially and electrostatically transferred from the image carrier to a transfer material carried on the transfer material carrier. The image forming means includes: a plurality of image carriers each carrying a plurality of color toner images; a transfer material carrier carrying a transfer material; and a transfer material carrying the plurality of color toner images on the plurality of image carriers. And a plurality of transfer means for sequentially transferring electrostatically to a transfer material carried on the carrier.

The charging means includes a first charging member for charging the surface of the transfer material on which the toner image is transferred to a polarity opposite to the normal charging polarity of the toner, and a charging member opposed to the first charging member. And a second charging member that charges the opposite side of the transfer material surface on which the toner image is transferred to the same polarity as the normal charging polarity of the toner. When charging the transfer material, a voltage having the same polarity as the normal charging polarity of the toner is applied to the second charging member, and the first charging member contacts the transfer material and is electrically grounded. When the transfer material is charged, the first transfer member contacts the transfer material, and the second transfer member contacts the transfer material carrier.

The transfer means is provided on a side of the transfer material carrier opposite to a side on which the transfer material is carried, and transfers a toner image from the image carrier to a transfer material carried on the transfer material carrier. A voltage having a polarity opposite to the normal charge polarity of the toner is applied to the transfer unit. A first charging member provided on a side of the transfer material carrier for supporting the transfer material, the first charging member configured to charge a surface of the transfer material on which the toner image is transferred to a polarity opposite to a normal charging polarity of the toner; And a second charging member that is provided to face the first charging member via the transfer material carrier and charges the transfer material carrier to the same polarity as the normal charging polarity of the toner. Before transferring the toner image from the image carrier to the transfer material carried on the transfer material carrier, the charging unit electrostatically attracts the transfer material to the transfer material carrier.

[0021]

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

Embodiment 1 FIG. 1 is a schematic structural view showing an image forming apparatus to which the present invention can be applied. This image forming apparatus is a color copying machine.

As shown in FIG. 1, first, second, third, and fourth image forming units Pa, Pb, Pc, and Pd are provided side by side in the apparatus. Through the processes of latent image formation, development and transfer, toner images of different colors are formed.

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

The recording material P on which the toner images of each color are transferred is
After the toner image is fixed by heat and pressure in the fixing device 9, the toner image is discharged out of the device as a color recorded image.

Exposure lamps 111a, 111b, and 11 are provided on the outer periphery of the photosensitive drums 3a, 3b, 3c, and 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 (not shown) and a polygon mirror 117 are further installed above the device.

The laser light emitted from the light source device is rotated and scanned by the polygon mirror 117, the light beam of the scanning light is deflected by the reflection mirror, and the bus direction of each of the photosensitive drums 3a, 3b, 3c, and 3d by the fθ lens. The photosensitive drums 3a, 3b, 3c, 3
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, respectively, as a developer by a supply device (not shown). The developing devices 1a, 1b, 1c, and 1d develop the latent images on the photosensitive drums 3a, 3b, 3c, and 3d, respectively, and visualize the latent images as cyan, magenta, yellow, and black toner images.

The recording material P is stored in a recording material cassette 10. The recording material P is supplied onto the transfer belt 130 through a plurality of conveyance rollers and the registration rollers 12 from there.
These are sequentially sent to the transfer sections facing b, 3c and 3d.

The transfer belt 130 is made of a dielectric resin sheet, and its both ends are overlapped and joined to form an endless shape, or a seamless (seamless) belt is used. .

When the transfer belt 130 is rotated by the driving roller 13 and is confirmed to be at a predetermined position, the recording material P is transferred from the registration roller 12 to the transfer belt 13.
0 and is conveyed to the transfer section of the first image forming section 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 unit Pa at a certain timing based on the signal.

When the transfer charger 24a applies an electric field or electric charge at the lower transfer portion of the photosensitive drum 3a, the first color toner image formed on the photosensitive drum 3a is transferred onto the recording material P. You. Due to this transfer, the recording material P is firmly held on the transfer belt 130 by electrostatic attraction, and is conveyed to the second image forming portion Pb and thereafter.

The transfer charger 24 (24a to 24d) has
A non-contact charger such as corona discharge, or a contact charger using a charging member such as a conductive blade, roller, or brush is used. The non-contact charger has problems such as generation of ozone, and susceptibility to fluctuations in the temperature and humidity environment of the atmosphere due to charging through air, resulting in unstable image formation.
The contact charger has no such problems, and has advantages such as ozonelessness, resistance to environmental changes in temperature and humidity, and high image quality. In this embodiment, a contact-type charger is used as the transfer charger 24.

Image formation and transfer in the second to fourth image forming units Pb to Pd are performed in the same manner as in the first image forming unit Pa.
Next, the recording material P onto which the four color toner images have been transferred is separated from the end of the transfer belt 130 by eliminating the charge by the separation charger 32 at the downstream portion in the transport direction of the transfer belt 130 to attenuate the electrostatic attraction force. . Normally, the drive roller 13 is grounded for stable separation. Since the separation charger 32 removes the charge of the recording material P in a state where the toner image is not fixed, a non-contact charger is used.

Recording material P detached from transfer belt 130
Is transported to the fixing device 9 by the transport unit 62. The fixing device 9 includes a fixing roller 51 and a pressure roller 52, and heat-resistant cleaning members 54 and 55 for cleaning the fixing roller 51 and the pressure roller 52, respectively.
And a heater 56 installed in the rollers 51 and 52,
57, an application roller 52 that applies a release agent oil such as dimethyl silicone oil to the fixing roller 51, an oil reservoir 53, and a thermistor 58 that detects the temperature of the surface of the pressure roller 52 and controls the fixing temperature. It is composed of

The recording material P to which the four color toner images have been transferred is
By the fixing, the color mixture of the toner image and the fixation to the recording material P are performed to form a full-color copy image, and the image is discharged to the paper discharge tray 63.

The photosensitive drums 3a, 3b, 3 after the transfer is completed.
c and 3d are the respective cleaners 4a, 4b, 4c and 4d
Thus, the transfer residual toner is cleaned and removed, and the apparatus is ready for the formation of the next latent image. Transfer belt 1
The toner and other foreign matters remaining on the transfer belt 130 are cleaned on the surface of the transfer belt 130 by a cleaning web (nonwoven fabric) 19.
To make contact and wipe off.

In the present invention, as the dielectric sheet material of the transfer belt 130, PET, polyacetal, polyamide, polyvinyl alcohol, polyether ketone,
Engineering such as polystyrene, polybutylene terephthalate, polymethylpentene, polypropylene, polyethylene, polyphenylene sulfide, polyurethane, silicone resin, polyamideimide, polycarbonate, polyphenylene oxide, polyether sulfone, polysulfone, aromatic polyester, polyetherimide, aromatic polyimide, etc. Plastic film-shaped sheets are commonly used.

In the present embodiment, as a material of the transfer belt 130, from the viewpoints of mechanical characteristics, electrical characteristics, flame retardancy and the like,
A polyimide resin was used. Its volume resistivity is 10 ¹⁶ Ω
cm, the thickness is 100 μm, and it is a seamless type.

Driving means for driving the transfer belt 130;
A detecting means for detecting the width of the transfer material; and a process speed is 100 mm / sec.

The contact-type transfer charger 24 (24a to 24a)
d) has a plate-shaped conductive rubber transfer member extending in a direction (thrust direction) orthogonal to the transfer material transport direction, and this transfer member is connected to the photosensitive drum 3 via the transfer belt 130.
(3a to 3d). The transfer charger 24 charges the transfer material P conveyed to the transfer section from the back surface of the transfer material P with a polarity opposite to that of the toner (positive in this example). Electrotransferred. In the present embodiment, the transfer charger 24 is controlled at a constant current, and the transfer current is set to 10 μA.

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

The drive roller 13 is connected to the main body ground and also functions as a counter electrode of the discharge wire. In this embodiment, the transfer charger 24d of the image forming unit Pd at the last stage
And the distance between the separation charger and the separation charger is 50 mm.
2 was applied with 10 kVpp AC.

As described above, in the image forming apparatus as shown in FIG. 1, in order to form a color image by multiple transfer in which the toner image is superimposed on the transfer material P and the transfer is repeated, the already transferred toner image is formed. May be transferred onto the photosensitive drum 1 during the next transfer process, a phenomenon called retransfer may occur, and the lower portion of the toner layer located in the lower density portion due to the retransfer of the toner layer on the surface may occur. Colors appear, causing significant degradation of image quality.

The inventor of the present invention has repeatedly studied and studied the above-mentioned retransfer and found out that the retransfer occurs by the following mechanism.

The state of the transfer material (paper) used in the above-described image forming apparatus changes significantly depending on the temperature and humidity during storage. According to the study of the present inventor, there is a great difference in the electrical characteristics between the paper immediately after opening the package and the paper whose humidity has been significantly reduced in a low humidity environment (for example, 23 ° C., 5% RH) after opening. Became clear. That is, it was found that when the amount of water contained in the paper decreased, the amount of charge that could be held on the front and back surfaces was limited.

FIG. 2 is a correlation diagram between the surface charge density externally applied to the transfer material surface and the potential of the transfer material surface. In FIG. 2, a graph C shows a result when an OHT (made of PET sheet, thickness: 100 μm) is used as the transfer material. In the OHT, the surface potential increases linearly with respect to the supplied charge amount. It plays a role as a typical condenser. Graph D shows the results obtained when the paper immediately after opening was used. This graph also shows a small slope with respect to the supplied charge amount, but it rises linearly as in the case where the surface potential is C. It is a target.

On the other hand, the graph A shows the result when the Haga Paper Store NPI fine paper (basis weight 128 g / m ² , thickness 120 μm) was used once through a fixing device to simulate humidity control in a low humidity environment. Graph B shows the results when the double-sided thick paper for Canon CLC (basis weight: 105 g / m ² , thickness: 100 μm) was similarly used once through the fixing device. See, paper Graph A from 450μC / m ² around a paper graph B from 500μC / m ² around, have lost linearity respectively, immediately after opening the paper (graph D) or OH
This tendency is not seen in T (graph C).

This is because the humidity is adjusted in a low-humidity environment, and when the moisture in the paper decreases, when a charge of a certain value or more is supplied to the surface of the paper, a discharge is caused by minute voids between the fibers in the paper. As it happens, the charges on the front and back of the paper are offset, so no matter how much charge is supplied, the charges can not be accumulated on the front and back of the paper,
It seems that the surface potential does not rise.

In the image forming apparatus described above, when an image is formed on paper conditioned in such a low-humidity environment, when a certain amount of charge is supplied during the transfer step, the amount of electric charge held by the paper is limited. As a result, the discharge in the paper starts, the paper rapidly loses the holding power to the toner, and a dot-like image defect as described in the related art section occurs.

According to FIG. 2, a low humidity environment (23 ° C., 5% R
The maximum amount of charge that the paper conditioned in H) can hold is about 50
0 μC / m ² .

In the image forming apparatus, the maximum amount of toner per unit area is 1 mg / cm per color.
^2. Assuming that the charge amount per unit weight of toner is 30 μC / g, every time the transfer step is completed once, 300 μC / m ²
Is applied to the paper. From this, the number of colors of toner that can be transferred to the paper shown by the graphs A and B in FIG. 2 is as follows: 300 × {≦ 500} ≦ 1.67, up to about 1.67 It can be seen that only color components can be transferred.

In general, in a color image forming apparatus, UCR (Under Color R) is used because it is difficult to reproduce black (black) using three colors of cyan, magenta, and yellow, and to reduce the amount of toner.
emoval). UCR is, specifically, a common part (black component) of cyan, magenta, and yellow.
With black, and subtract that amount from cyan, magenta, and yellow. UCR not only can improve black reproducibility, but also can greatly reduce the amount of toner.

Although depending on the model, usually, an image is formed with the maximum total amount of toner of about two colors. In this embodiment, U
CR is adopted, and the total amount of the maximum transfer toner is set to two colors. However, it still exceeds the above-mentioned 1.67 colors, and in the case of development, when transfer is performed on paper having a remarkably low water content, the above-mentioned dot-like transfer failure occurs.
In other words, in order to transfer four colors normally, the maximum amount of retained charge must be at least more than 300 × 2 = 600 μC / m ^2. Transfer failure occurs.

Therefore, according to the study of the present inventor, when transferring onto a transfer material having a significantly reduced water content, for example, paper conditioned in a low-humidity environment, or paper once passed through a fixing device, the transfer material will By applying a charge having a polarity opposite to that of the transfer in advance before the transfer and relatively increasing the maximum retained charge amount of the transfer material, it is possible to prevent the above-mentioned point-like transfer failure from occurring and obtain a good image. I understood. Hereinafter, this method will be described in detail.

Based on FIG. 2, a function Qp = Qp which gives the maximum amount of retained charge Qp (C / m ² ) per unit area of the transfer material as a variable based on the type of the transfer material and the environmental atmosphere in which the transfer material is placed. (S, k). Here, s is the type of transfer material, and k is the absolute moisture content of the environmental atmosphere (g / air 1k).
g). For example, in the transfer material shown by the graph B in FIG. 2, Qp is about 5 × 10 ⁻⁴ C / m ² .

Further, the charge amount Qt per unit weight of the toner
Is a function Qt = Qt that gives the environment atmosphere as a variable
(K), and the UCR upper limit is n colors. In this embodiment, Qt = 3 × 10 ⁻² C / kg and n = 2 colors. Further, the weight per unit area of the toner transferred to the transfer material is defined as M (kg / m ² ).

At this time, since the toner charge amount after forming the four color images is M · Qt (k) · n (C / kg), the charge amount per unit area of the front and back surfaces of the transfer material is ± M.・ Qt
(K) · n (in the present embodiment, since the negative toner is used, the front surface of the transfer material is negative and the back surface is positive).

As described above, at present, Qp (s, k) <M · Qt (k) · n, and the amount of charge supplied to the transfer material exceeds the maximum retained charge amount of the transfer material. In order to prevent a point-like transfer failure, the transfer material is reversely charged in advance of the transfer so as to relatively increase the maximum retained charge amount of the transfer material.

That is, the charge amount of the charge opposite to the transfer is represented by Qr
By satisfying the relationship of Qp (s, k) + Qr ≧ M · Qt (k) · n as (C / m ² ), the occurrence of dot-like transfer omission can be prevented. The amount of reverse charge before transfer at this time is
It suffices to satisfy the relationship of Qr = M.Qt (k) .n-Qp (s, k).

In this embodiment, M = 10 ⁻² kg / m ² , Q
When t (k) = 3 × 10 ^-2 C / kg and n = 2, M · Qt
(K) · n = 600 μC / m ² , and Qp
Assuming that (s, k) = 5 × 10 ⁻⁴ C / m ² , Qr = M · Qt (k) · n−Qp (s, k) = 600−500 = 100 (μC / m ² ). That is, the charge amount before transfer is 100 μC / m ²
By doing so, it is possible to prevent the occurrence of dot transfer defects.

The desired reverse charge amount Qr is determined as follows. FIG. 3 shows the relationship between the water content of the paper and the maximum surface charge density that the paper can hold. The water content of the paper was measured using an infrared moisture meter KJT100 manufactured by Kett Science Laboratory.

As shown in FIG. 3, it can be seen that the maximum surface charge density that the paper can hold has a strong relationship with the water content of the paper, and decreases as the water content decreases.

FIG. 4 shows the relationship between the absolute moisture content of the environment where the paper is left and the moisture content of the paper.
It can be seen that the smaller the absolute moisture content of the environment left to stand, the smaller the moisture content of the paper.

Therefore, a temperature / humidity sensor 20 capable of measuring temperature and humidity is provided in the image forming apparatus main body, and as close as possible to the transfer material storage cassette 10, and the temperature / humidity in the apparatus detected by the temperature / humidity sensor 20 is detected. Using the humidity, the absolute water content in the device was calculated, and based on that,
To determine the moisture content of the paper. Further, the maximum surface charge amount Qp (s, k) of the paper that can be held is determined from FIG. 3, and the reverse charge amount Qr applied to the transfer material before transfer is determined from the above equation.

However, in the case of a transfer material such as that shown by the graph B in FIG. 2, there is a limit in applying reverse charging before transfer, and the limit value is the same as Qp (s, k), for example, the maximum value. 500 μC / m ² . Therefore, | Qr |
.Ltoreq.Qp (s, k), and the maximum value of the maximum retained charge of paper is apparently Qp (s, k) + Qr = 2.times.Qp (s, k).

In this embodiment, by applying the reverse charge of the transfer material to the maximum, 300 × {≦ 2 × Qp (s, k) = 100} ≦ 3.
33, and if the total toner amount regulation by the UCR is up to 3.33 colors, dot transfer omission can be prevented without any problem.

FIG. 5 shows an example of performing reverse charging before transfer. In the first image forming unit Pa, in front of a transfer unit including the photosensitive drum 3a and the transfer belt 130, a reverse charging unit 200 including conductive rollers 200a and 200b is installed. As an application roller, a high-voltage power supply E controlled at a constant current is connected thereto, and the other upper roller 200b is used as an opposing roller.
Ground this. The transfer material P before entering the transfer portion
Is sandwiched between the rollers 200a and 200b, a bias is applied to the roller 200a from the power source E, and a current is caused to flow through the roller 200b with constant current control, so that a charge having a polarity opposite to that at the time of transfer is transferred to the back surface of the transfer material P. The transfer material P is reversely charged prior to transfer.

By performing the constant current control, even when the paper type or thickness of the transfer material is changed or the resistance in the vertical direction of the surface of the transfer material is changed variously, the transfer material can be handled without any problem. There is an advantage that a constant reverse charge can be supplied to P. In the present embodiment, Qr is, for example, Qr = 10
In order to obtain 0 μC / m ² , the roller 200a needs
A reverse charging current of 3 μA may be applied to 0b.

However, in general, this type of apparatus can use a small-sized transfer material having a size smaller than the maximum usable recording width (thrust direction). As shown in (a) and (b), the current flowing between the rollers 200a and 200b is the current i of the paper passing portion.
The current is in the non-sheet passing portion becomes significantly larger than p, so that the required reverse charging of the transfer material P cannot be performed. For example, 100m in width
m, the resistance value between the paper passing portion and the non-paper passing portion becomes as high as 10: 1. Most of the current flowing between the rollers 200a and 200b flows through the non-paper passing portion, and a desired amount of The oppositely charged charges cannot be supplied to the paper surface, resulting in a point-like transfer omission during transfer.

Therefore, in the present embodiment, the reverse charging current value is changed according to the width (length in a direction substantially orthogonal to the transfer direction of the transfer material) of the transferred transfer material (power supply E). If the power supply is a constant voltage power supply, the voltage applied to the roller 200a is changed), thereby preventing the reverse charging failure before transfer even for a small-size transfer material, and preventing dot-like transfer omission. The details will be described below.

Table 1 shows the width w of the transfer material to be passed and the paper passing portion /
It is a result of examining the relationship with the resistance ratio u of the non-sheet passing portion.

[0073]

[Table 1] Qr (C / m ² ), process speed v (m
/ S), the total amount of reverse charging of the transfer material is 1 second, that is, Qr · w · v (C). At this time, based on the resistance ratio u in Table 1, the roller 200
By setting the current Ir flowing between a and 200b to be Ir = Qr.w.v. (u + 1), desired reverse charging can be performed even with a small-sized transfer material.

Table 1 shows one example, and it is preferable to store a plurality of tables in the apparatus according to the volume resistivity and thickness of the transfer material. Since the moisture content of the transfer material can be obtained from the installation environment of the apparatus from FIG. 4, the volume resistivity of the paper is determined from the relationship between the moisture content of the paper and the volume resistivity as shown in FIG. The detection may be performed by a manual paper feed unit or a cassette paper feed unit, and a table of necessary sheet passing / non-sheet passing unit resistance ratio u may be read from these numerical values, and Ir may be determined according to the resistance ratio.

In a further preferred embodiment of the present embodiment,
The relational expressions as shown in FIG. 3, FIG. 4, FIG.
The number of transfer materials is stored in the apparatus main body in accordance with the type of the transfer material, and in using the image forming apparatus, the user inputs the type of the transfer material to the apparatus main body in advance, and stores the information in the apparatus main body in advance from the transfer material information. It is apparent that more preferable results can be obtained by determining the stored maximum surface charge density for each transfer material.

In this embodiment, as shown in FIG.
It is assumed that the transfer material P is transported along the edge of the transport belt 130 in the transport direction. However, as shown in FIG. 6A, the transfer material P is always regulated to the center in the thrust direction and transported. May be used.

The paper width w of the transfer material may be automatically detected using a mechanical sensor or an optical sensor, or may be detected by an operator inputting the paper width from an operation unit or the like. For detecting the thickness of the transfer material, some type of detection means may be installed in the apparatus, or the operator may input the thickness of the transfer material from outside the apparatus.

Although the transfer belt has been described as the transfer material carrier, a transfer drum may be used. The image carrier is not limited to an electrophotographic photosensitive member such as a photosensitive drum, and may be a dielectric in electrostatic recording.

The developing device 1 (1a to 1d) may use any type of developing method.

In general, 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.
For the magnetic toner, a one-component non-contact developing method in which the developing sleeve is coated by a magnetic force, is conveyed to a developing unit facing the image carrier by rotation of the developing sleeve, and develops the toner in a non-contact state with the image carrier. There is a one-component contact developing method of developing in contact with an image carrier, and a two-component developing method includes:
Uses a two-component developer in which toner particles and a magnetic carrier are mixed, coats it on the developing sleeve with magnetic force, and transports it to the developing unit by the developing sleeve, where the developer is developed in a non-contact state with the image carrier. Two-component non-contact development method,
There is a two-component contact development method in which development is performed in contact with an image carrier. From the aspects of high image quality and high stability, the two-component contact development method is often used.

In this embodiment, a monochrome printer or a copying machine, which is a color printer or a copying machine, may be used, and a digital or analog printer may be used.

The present invention is constructed as described above, and as shown in FIG.
a and 200b, the transfer material P is sandwiched between the rollers 200a and 200b before the transfer material P enters the transfer portion, and the transfer material is charged to the opposite polarity to that at the time of transfer.
(C / m ² ), process speed v (m / sec), resistance ratio u between the paper passing portion and the non-paper passing portion, and reverse charging current Ir flowing between rollers 200a and 200b, Ir = Qr · w · v · Since (u + 1) is specified, the transfer material P can be reversely charged before transfer without depending on the paper width, regardless of the paper width, and the occurrence of dot-like transfer omission can be prevented even for a small-size transfer material. .

Embodiment 2 In Embodiment 1, the reverse charging current is controlled in accordance with the paper width of the transfer material P, so that the reverse charging can be performed on small-size paper without excess or shortage. However, for example, when a paper having a small width and high resistance is passed, the amount of reverse charging current becomes extremely large, and accordingly, the voltage of the high-voltage power supply E becomes very large, so that the load on the power supply increases extremely. . Also paper width,
In order to store the paper type, the leaving conditions, and the resistance ratio of the paper passing portion / non-paper passing portion in the device, a considerable amount of memory is required, and the device becomes expensive.

Therefore, in this embodiment, the reverse charging width before the transfer can be changed according to the width of the transfer material to be passed, so that the current required to obtain the desired reverse charging amount is minimized. This eliminates the need for excessive current, even for transfer media with a small paper width and high resistance, and eliminates the need to store the resistance ratio between the paper passing part and the non-paper passing part in the device, reducing the amount of memory required. Also made it possible.

In this embodiment, the lower roller 200a shown in FIG.
Instead, a roller 300 as shown in FIG. 8 was used. In the present roller 300, the conductive cylindrical member is notched so as to increase the notch amount in the thrust direction, so that the length of the roller 300 in the thrust direction changes continuously in the circumferential direction.

The roller 300 is rotatable in the circumferential direction by pivotally connecting a center shaft 301 thereof to a mounting portion (not shown). A gear 302 mounted at one end of the center shaft 301 and a driving gear meshed therewith are provided. Gear 30 on the device 304 side
3 and is rotated by the forward / reverse rotation motor of the driving device 304. The rotation amount is controlled by the CPU 305.

In the manual paper feed unit or the cassette paper feed unit, the paper width of the transfer material P is constantly detected, and the paper width information is transferred to the CPU 305. The CPU 305 controls the rotation angle of the motor of the driving device 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 corresponding to the paper width of the transfer material P is directed to the transfer belt 130.

As a result, the current required for reverse charging before transfer can be the current value Ir represented by Ir = Qr · w · v with respect to the paper width w, the reverse charging amount Qr, and the process speed v.

The maximum holdable charge amount Qp may be obtained according to the first embodiment.

The roller 300 is rotated to regulate the thrust length so as to be substantially the same as the width of the transfer material P passed by the driving device 304, but it is not necessary to make the length exactly the same. May be slightly longer.

In this embodiment, the roller 300 is the lower roller of the reverse charging roller, but may be the upper roller.

As described above, in this embodiment, since the roller capable of changing the reverse charging area is used, even when a small-size transfer material is passed, the current amount Ir of the reverse charging transmission before the transfer and the image quality can be reduced. It was possible to reduce the amount of memory used in the forming apparatus.

In the above embodiments, the image forming apparatus in which multiple-color toner images are all transferred onto a transfer material has been described. It goes without saying that charging to a polarity opposite to the normal charging polarity of the toner is effective in preventing transfer failure.

For example, in a black and white image forming apparatus, when the charge density of the toner per unit area increases, the transfer material is charged in advance with the opposite polarity to that at the time of transfer for the above-mentioned reason, so that the apparent It is possible to increase the density of the retained charge of the upper transfer material, and it is possible to prevent transfer failure of toner corresponding to the charge that cannot be retained by the transfer material.

The means for charging the transfer material to have a polarity opposite to that at the time of transfer may be incorporated in a process preceding the transfer means such as a pick-up roller for picking up the transfer material from the transfer material storage cassette, a curling roller, and a registration roller. Alternatively, the reverse charging member may be separately provided.

[0096]

As described above, according to the present invention,
Before the transfer material reaches the transfer section, a charge of the opposite polarity to that of the transfer is applied to the transfer material, and the transfer material is pre-charged to the opposite polarity. And a high-quality color image free from transfer failures.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing one embodiment of an image forming apparatus of the present invention.

FIG. 2 is a diagram illustrating a relationship between a surface charge density given to a transfer material and a surface potential of the transfer material.

FIG. 3 is a graph showing the relationship between the water content of the transfer material and the maximum surface charge density that can be held by the transfer material.

FIG. 4 is a graph showing the relationship between the absolute moisture content of an environmental atmosphere and the moisture content of a transfer material.

FIG. 5 is an explanatory diagram showing reverse charging before transfer in one embodiment of the present invention.

FIG. 6 is an explanatory diagram showing a state in which a small-size sheet of a current having a current flowing between rollers of the reverse charging means for performing reverse charging in FIG. 5 is passed.

FIG. 7 is a graph showing the relationship between the water content and the volume resistivity of paper.

FIG. 8 is a perspective view showing a lower roller of reverse charging means and a rotation control system thereof used in another embodiment of the present invention.

[Explanation of symbols]

 3a to 3d Photosensitive drum 12 Registration roller 20 Humidity sensor 24a to 24d Transfer charger 130 Transfer belt 200a, 200b Reverse charging roller 300 Lower roller P Transfer material

Claims (28)

[Claims] 1. An image forming apparatus having an image forming means for electrostatically forming a toner image on a transfer material, wherein the toner image is formed before the toner image is formed on the transfer material by the image forming means. A charging unit that pre-charges the surface of the material to a polarity opposite to the normal charging polarity of the toner; and a voltage applied to the charging unit according to the length of the transfer material in a direction substantially orthogonal to the transport direction. An image forming apparatus comprising: a control unit. 2. The image forming apparatus according to claim 1, further comprising humidity detecting means for detecting humidity, wherein said control means controls a voltage applied to said charging means in accordance with a result of detection by said humidity detecting means. 3. An image according to claim 1, further comprising a temperature / humidity detecting means for detecting temperature and humidity, wherein said control means controls a voltage applied to said charging means in accordance with a result of detection by said temperature / humidity detecting means. Forming equipment. 4. The apparatus according to claim 1, further comprising a determination unit configured to determine a type of the transfer material, wherein the control unit controls a voltage applied to the charging unit according to a detection result of the determination unit.
3. The image forming apparatus according to any one of items 3. 5. A control device comprising: a constant current power supply connected to the charging unit; wherein the control unit controls a current value flowing through the charging unit in accordance with a length in a direction substantially orthogonal to a transfer direction of the transfer material. The image forming apparatus according to claim 1, wherein the control is performed. 6. The image forming means includes an image carrier for carrying a toner image, a transfer material carrier for carrying a transfer material, and a toner image on the image carrier carried on the transfer material carrier. Transfer means for electrostatically transferring to a transfer material.
6. The image forming apparatus according to any one of items 5. 7. The image forming apparatus according to claim 6, wherein toner images of a plurality of colors are electrostatically sequentially transferred from the image carrier to a transfer material carried on the transfer material carrier. 8. The image forming means includes: a plurality of image carriers each carrying a plurality of color toner images; a transfer material carrier carrying a transfer material; and a plurality of color toners on the plurality of image carriers. The image forming apparatus according to claim 1, further comprising a plurality of transfer units configured to sequentially transfer an image to a transfer material carried on the transfer material carrier in a sequential manner. 9. A charging device, comprising: a first charging member configured to charge a surface of a transfer material onto which a toner image is transferred to a polarity opposite to a normal charging polarity of the toner; And a second charging member provided on the side opposite to the surface of the transfer material onto which the toner image is transferred, the second charging member being charged to the same polarity as the normal charging polarity of the toner. The image forming apparatus as described in the above. 10. When the transfer material is charged, a voltage having the same polarity as the normal charge polarity of the toner is applied to the second charging member, the first charging member contacts the transfer material, and the second charging member contacts the transfer material. The image forming apparatus according to claim 9, wherein the image forming apparatus is grounded. 11. The image forming apparatus according to claim 9, wherein when charging the transfer material, the first transfer member contacts the transfer material, and the second transfer member contacts the transfer material carrier. 12. The transfer means is provided on a side of the transfer material carrier opposite to a side on which the transfer material is carried, and transfers a toner image from the image carrier to a transfer material carried on the transfer material carrier. 12. The image forming apparatus according to claim 6, wherein a voltage having a polarity opposite to a normal charge polarity of the toner is applied to the transfer unit. 13. The transfer means is provided on a side of the transfer material carrier which supports a transfer material, and charges a surface of the transfer material on which a toner image is transferred to a polarity opposite to a normal charge polarity of the toner. A first charging member, and a second charging member that is provided to face the first charging member via the transfer material carrier and charges the transfer material carrier to the same polarity as the normal charging polarity of the toner. The image forming apparatus according to claim 9, further comprising a member. 14. The method according to claim 1, wherein the charging unit electrostatically attracts the transfer material to the transfer material carrier before transferring the toner image from the image carrier to the transfer material carried on the transfer material carrier. Item 14. The image forming apparatus according to any one of Items 6 to 13. 15. An image forming apparatus having an image forming means for electrostatically forming a toner image on a transfer material, wherein the toner image is formed before the toner image is formed on the transfer material by the image forming means. Charging means for pre-charging the surface of the material to a polarity opposite to the normal charging polarity of the toner; An image forming apparatus comprising: a control unit configured to control a charge amount per unit area. 16. It has humidity detecting means for detecting humidity,
16. The image forming apparatus according to claim 15, wherein the control unit controls an amount of charge per unit area applied to the transfer material by the charging unit according to a detection result by the humidity detecting unit. 17. A temperature / humidity detecting means for detecting temperature and humidity, wherein said control means controls a charge per unit area applied to a transfer material by said charging means in accordance with a detection result by said temperature / humidity detecting means. 16. The image forming apparatus according to claim 15, wherein the amount is controlled. 18. A method according to claim 18, further comprising: a determination unit configured to determine a type of the transfer material, wherein the control unit controls an amount of charge per unit area applied to the transfer material by the charging unit according to a detection result of the determination unit. The image forming apparatus according to claim 15, wherein: 19. A control device comprising: a constant current power supply connected to the charging unit; wherein the control unit changes a current value flowing through the charging unit in accordance with a length of the transfer material in a direction substantially perpendicular to the conveying direction. The image forming apparatus according to claim 15, wherein the image forming apparatus controls the image forming apparatus. 20. The image forming means, comprising: an image carrier for carrying a toner image; a transfer material carrier for carrying a transfer material; and a toner image on the image carrier carried by the transfer material carrier. 2. A transfer means for electrostatically transferring to a transfer material.
20. The image forming apparatus according to any one of items 5 to 19. 21. The image forming apparatus according to claim 20, wherein a plurality of color toner images are sequentially and electrostatically transferred from the image carrier to a transfer material carried on the transfer material carrier. 22. The image forming means, comprising: a plurality of image carriers each carrying a plurality of color toner images; a transfer material carrier carrying a transfer material; and a plurality of color toners on the plurality of image carriers. 16. A plurality of transfer means for sequentially transferring an image electrostatically to a transfer material carried on the transfer material carrier.
20. The image forming apparatus according to any one of the nineteenth aspects. 23. A charging device, comprising: a first charging member for charging a surface of a transfer material on which a toner image is transferred to a polarity opposite to a normal charging polarity of the toner; 23. A second charging member, wherein the second charging member is provided so as to have the same polarity as the normal charging polarity of the toner on the side opposite to the surface of the transfer material on which the toner image is transferred. The image forming apparatus as described in the above. 24. When charging the transfer material, a voltage having the same polarity as the normal charging polarity of the toner is applied to the second charging member, the first charging member contacts the transfer material, and is electrically connected to the second charging member. 24. The image forming apparatus according to claim 23, wherein the image forming apparatus is grounded. 25. The image forming apparatus according to claim 23, wherein when charging the transfer material, the first transfer member contacts the transfer material, and the second transfer member contacts the transfer material carrier. 26. The transfer means is provided on a side of the transfer material carrier opposite to a side on which the transfer material is carried, and transfers a toner image from the image carrier to a transfer material carried on the transfer material carrier. 26. The image forming apparatus according to claim 20, wherein a voltage having a polarity opposite to a normal charge polarity of the toner is applied to the transfer unit. 27. The charging means is provided on a side of the transfer material carrier for supporting a transfer material, and charges a surface of the transfer material on which a toner image is transferred to a polarity opposite to a normal charging polarity of the toner. A first charging member, and a second charging member that is provided to face the first charging member via the transfer material carrier and charges the transfer material carrier to the same polarity as the normal charging polarity of the toner. The image forming apparatus according to claim 23, further comprising a member. 28. The method according to claim 28, wherein the charging unit electrostatically attracts the transfer material to the transfer material carrier before transferring the toner image from the image carrier to the transfer material carried on the transfer material carrier. Item 30. The image forming apparatus according to any one of Items 20 to 26.