JP3441992B2 - Image forming device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention forms an image on a recording 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, a plurality of image forming portions are provided, toner images of different colors are formed in the respective image forming portions, and the toner images are sequentially superposed and transferred onto the same recording material and transferred onto the recording material. Various image forming apparatuses for forming color images have been proposed.

Among such image forming apparatuses, a multicolor electrophotographic color electrophotographic recording apparatus using an endless recording material carrier is used for high speed recording. An example of this color electrophotographic recording apparatus will be described with reference to FIG.

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 portions 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 are 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 as a developer by a supply device (not shown). The developing devices 1a, 1b, 1c, 1d develop the latent images on the photosensitive drums 3a, 3b, 3c, 3d, respectively, and visualize them as cyan toner images, magenta toner images, yellow toner images, and black toner images.

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) or 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 a transfer portion on the lower side of 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 charging member such as a conductive blade, roller or brush is 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 59 for applying a release agent oil such as dimethyl silicone oil to the fixing roller 51, an oil reservoir 53 for the fixing roller 51, 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 and 3 after transfer is 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 generally about 10 ¹⁸ Ωcm.

The transfer charger 24 (24a to 24d)
The image stabilizes when the current contributing to the transfer (transfer current) is kept constant at an appropriate current. Therefore, even when the volume resistance changes due to the type of recording material (thickness, material, etc.) and moisture absorption conditions. Generally, constant current control is performed so that a constant current can be obtained.

The transfer voltage is several kV to several tens kV. In an image forming apparatus that performs multiple transfer, a higher transfer voltage is required in each transfer step than in the previous transfer, and particularly in the last transfer, the transfer voltage is considerably higher than in the first transfer.

[0022]

However, in the above-mentioned image forming apparatus, when paper is used as the recording material P and images are formed on both surfaces of the recording material P, dot-shaped transfer omission occurs in the image transferred to the second surface. However, there is a problem that the image quality is significantly impaired.

Therefore, the object of the present invention is to provide a recording material
An object of the present invention is to provide an image forming apparatus capable of satisfactorily transferring an image on a surface without causing dot-shaped transfer omissions and obtaining high-quality images on both surfaces of a recording material.

[0024]

The above object can be achieved by an image forming apparatus according to the present invention. In summary, the first
According to Ming, the toner image formed on the image carrier is transferred.
In an image forming apparatus that electrostatically transfers the toner image on the image carrier to a recording material by means of a means , after transferring the toner image on the image carrier to the first surface of the recording material, the toner image on the image carrier is transferred to the second surface opposite to the first surface. Of the toner image can be transferred, and the maximum value of the weight per unit area of the toner image formed on the image carrier is different between the toner image on the first surface and the toner image on the second surface .
When the toner image is transferred, the current flowing through the transfer means is
Weight per unit area of toner image formed on carrier
Hisage image forming apparatus which is a variable depending on the
Be served. According to one embodiment of the present invention, the image carrier
Maximum weight per unit area of toner image formed on top
The value is the toner on the second surface rather than the toner image on the first surface.
The image is made smaller.

According to the second invention, it is formed on the image carrier.
Image forming device for electrostatically transferring the formed toner image onto a recording material
The toner image on the image carrier on one side of the recording material.
After transferring to the eye, the image bearing is performed on the second surface opposite to the first surface.
It is possible to transfer the toner image on the carrier, and
Maximum weight of toner image formed per unit area
Is the toner image on the first side and the toner image on the second side.
Differently, the image forming apparatus is characterized in that the weight per unit area of the toner image formed on the image carrier is variable according to the amount of water content of the recording material . Starting
According to one embodiment of the present invention, it is formed on the image carrier.
The maximum value of the weight per unit area of the toner image is 1
The toner image on the second side is smaller than the toner image on the eye
According to another embodiment, the toner on the image carrier is
The toner image on the recording material electrostatically.
When the toner image is transferred, the current flowing through the transfer means is
The weight per unit area of the toner image formed on the carrier
It is variable according to

According to one aspect of the present invention, the step of transferring the toner image on the image carrier to the recording material carried on the recording material carrier is repeated to form a plurality of color toner images on the recording material. To do. The maximum value of the weight per unit area of the toner image of at least one color among the toner images of the plurality of colors formed on the image carrier differs between the toner image on the first surface and the toner image on the second surface. . The maximum value of the weight per unit area of the toner image formed on the image carrier is the sum of the maximum values of the weight per unit area of the toner images of the plurality of colors. A plurality of image carriers are provided to carry toner images of a plurality of colors, respectively.

Further, after the toner image is transferred, there is a fixing means for fixing the toner image on the recording material to the recording material, and after fixing the toner image on the first surface of the recording material, the toner image on the image carrier is recorded. It can be transferred to the second surface of the material.

[0028]

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. The configuration and operation of each part of the image forming apparatus have already been described in the section of the prior art, and thus detailed description thereof will be omitted.

In the present invention, the transfer belt 130 of FIG.
Examples of the dielectric sheet material include PET, polyacetal, polyamide, polyvinyl alcohol, polyether ketone, polystyrene, polybutylene terephthalate,
Polymethylpentene, polypropylene, polyethylene,
Film-shaped sheets of engineering plastics such as polyphenylene sulfide, polyurethane, silicone resin, polyamideimide, polycarbonate, polyphenylene oxide, polyethersulfone, polysulfone, aromatic polyester, polyetherimide, aromatic polyimide are generally used.

In this embodiment, the material of the transfer belt 130 is, in terms of mechanical characteristics, electrical characteristics, flame retardancy, etc.,
A polyimide resin was used. Its volume resistivity is 10 ¹⁶ Ω
cm, thickness 100 μm, seamless type.

A drive means for driving the transfer charger and a detection means for detecting the width of the recording material are provided, and the process speed is 100 mm / sec.

Contact type transfer charger 24 (24a-24)
Reference numeral d) includes a plate-shaped conductive rubber transfer member extending in a direction (thrust direction) orthogonal to the recording material conveyance direction, and the transfer member is provided via the transfer belt 130 to the photosensitive drum 3.
(3a to 3d) are pressed so as to come into contact with them. By the transfer charger 24, the recording material P conveyed to the transfer portion is charged from the back side of the recording material P with a polarity opposite to that of the toner (plus in this example), and the toner image on the photosensitive drum 3 is statically charged on the surface of the recording material P. Electro-transcribed. 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 (in the case of a corona charger, the difference between the current flowing through the corona charger and the current flowing through the shield is the transfer current). Will be).

The separation charger 32 is located at the uppermost downstream side of the transfer belt 130, that is, the driving 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).

Further, the driving 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 portion Pd at the final stage is used.
The distance between the separation charging unit and the separation charging unit is 50 mm.
2 was applied with 10 kVpp AC.

The recording material (paper) used in the image forming apparatus as described above changes its state remarkably depending on the temperature and humidity during storage. The studies made by the present inventors have revealed that there is a large difference in electrical characteristics between the paper immediately after opening the package and the paper once heated by the fixing device and whose water content is significantly reduced. That is, it was found that the paper has a limit to the amount of electric charge that can be retained on the front and back surfaces when the amount of water contained therein decreases.

FIG. 2 is a correlation diagram between the surface charge density externally applied to the surface of the recording material and the potential on the surface of the recording material. In FIG. 2, graph C is the result when OHT (PET sheet, thickness 100 μm) was used as the recording material. In OHT, the surface potential increases linearly with the supplied charge amount, which is ideal. Plays the role of a general condenser. Graph D shows the result when using the paper immediately after opening, which also has a low slope with respect to the supplied charge amount, but has a linear rise as in the case where the surface potential is C, which is ideal. Target.

On the other hand, graph A shows the result when HPI paper shop NPI high-quality paper (basis weight 128 g / m ² , thickness 120 μm) was once used through the fixing device, and graph B shows double-sided thick paper for CLC made by Canon. (Basis weight 105 g / m ² , thickness 10
0 μm) is similarly used once through the fixing device. Looking at this, the paper in graph A is 450 μC / m
^From around ^{2 and} the paper of Graph B from around 500 μC / m ² ,
Each of them loses its linearity and tends not to be seen on the paper immediately after opening (graph D) or OHT (graph C).

This is because once the water content in the paper is reduced by passing through the fixing device, when an electric charge of a certain value or more is supplied to the surface of the paper, discharge occurs due to the minute gaps between the fibers in the paper. It seems that the electric charges on the front and back surfaces of the paper cancel each other out, so that no matter how much more electric charge is supplied, the electric charges cannot be accumulated on the front and back surfaces of the paper, and the surface potential does not rise.

Even in the above-mentioned image forming apparatus, when an image is formed on the second side of the paper that has passed through such a fixing device, the amount of charge held on the paper is limited when an electric charge is supplied to some extent during the transfer process. Then, the discharge in the paper starts, the paper suddenly loses the holding power for the toner, and dot-shaped image defects occur on the second surface of the recording material as described in the section of the prior art.

According to FIG. 2, the maximum amount of electric charge that can be held by the paper that has passed through the fixing device is about 500 μC / m ² .

Here, in the image forming apparatus, the maximum amount of toner loaded per unit area is 1 mg / cm per color.
² , assuming that the charge amount per unit weight of toner is 30 μC / g, 300 μC / m ² per transfer process is completed.
Toner charge is applied to the paper. From this, the number of colors of toner that can be transferred to the paper shown in the graphs A and B of FIG. 2 is 300 × χ ≦ 500 ∴χ ≦ 1.67, and the maximum is about 1.67. You can see that only the color components can be transferred.

Generally, in a color image forming apparatus, UCR (Under Color R) is used because it is difficult to reproduce black using three colors of cyan, magenta and yellow, and it is desired to save the toner amount.
Emoval) is adopted. UCR is specifically the common part of cyan, magenta, and yellow (black component)
Is replaced with black, and that amount is subtracted from cyan, magenta, and yellow. The UCR not only improves the black reproducibility, but also significantly reduces the toner amount.

Although it depends on the model, image formation is usually performed with a maximum total amount of toner of about two colors. Also in this embodiment, U
CR is used, and the total amount of maximum transfer toner is for two colors. However, it still exceeds the above 1.67 colors, and under the present circumstances, if the transfer is performed again on the paper which has once passed through the fixing device, the above-mentioned dot-like transfer failure occurs.

Therefore, according to the studies made by the present inventors, when the image is transferred again to the paper that has passed through the fixing device, that is, when the image is transferred to the second surface of the paper, the toner amount of the toner image for the second surface of the photosensitive drum is reduced. It was found that by doing so, it is possible to prevent the occurrence of the above-mentioned dot-shaped transfer failure and obtain a good image. Hereinafter, this method will be described in detail.

Based on FIG. 2, a function Qp = Qp that gives the maximum retained charge amount Qp (C / m ² ) per unit area of the recording material as variables of the type of the recording material and the environmental atmosphere in which the recording material is placed. The value is (s, k). However, s is the type of recording material, k is the absolute moisture content of the environmental atmosphere (g / air 1k
g). For example, in the recording 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 toner
A function Q that gives (C / kg) the environmental atmosphere as a variable
The value of t = Qt (k) is set, and the UCR upper limit value is set to n colors.
In this embodiment, Qt = 3 × 10 ⁻² C / kg and n = 2 colors. Further, the weight of toner per unit area on the photosensitive drum (toner carrying amount) is M (kg / m ² ).

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

In order to prevent the dot-like transfer failure as described above, the value of M · Qt (k) · n is Qp (s,
The value of k) should not be exceeded. That is, the toner amount M on the photosensitive drum may be in the range of M · Qt (k) · n ≦ Qp (s, k) ∴M ≦ Qp (s, k) / Qt (k) / n.

Here, the maximum retained charge amount Qp (s, k) per unit area of the recording material greatly depends on the paper type and the operating environment of the image forming apparatus main body, especially the temperature and humidity.

In FIG. 3, the paper that has passed through the fixing device once is subjected to normal temperature and normal humidity environment (23 ° C., 60% RH), normal temperature and low humidity environment (23 ° C.,
5 shows changes in water content when left to stand at 5% RH).
The water content of the paper was measured using an infrared moisture meter KJT100 manufactured by Kett Scientific Research Institute Co., Ltd.

As shown in FIG. 3, once the paper passes through the fixing device, the water content is reduced by about 2 to 3%. afterwards,
Paper left in normal temperature and normal humidity environment returns to about 80% of its initial moisture content after 30 seconds due to moisture absorption, but paper left in normal temperature and low humidity environment tends to absorb moisture even after 60 seconds. I can't.

In this embodiment, the time required from the end of fixing the first side of the paper to the transfer of the first color of the second side is about 1
Since it is 5 seconds, it can be seen that the moisture content of the paper on the second side greatly depends on the atmosphere of the environment where the paper is placed.

FIG. 4 shows the relationship between the water content of paper and the maximum surface charge density of paper that can be retained. As is clear from FIG. 4, the maximum surface charge density that can be held by the paper has a strong relationship with the water content of the paper, and it is understood that the maximum water charge density decreases as the water content decreases.

Therefore, a temperature / humidity sensor 20 capable of measuring temperature and humidity is provided in the image forming apparatus main body and as close to the recording material storage cassette 10 as possible, and the temperature / inside temperature of the apparatus detected by the temperature / humidity sensor 20 is measured. The humidity is used to determine the absolute amount of water in the apparatus, and the maximum surface charge density that can be held on the second surface of the recording material is calculated, and based on this, the amount of toner transferred to the second surface of the recording material, that is, the photosensitive drum. It is decided to reduce the toner amount of the toner image for the second surface formed on the third surface.

Regarding the extent to which the toner amount is reduced, it is preferable to use the following procedure based on the above equation (1). From the detected values of temperature and humidity by the temperature and humidity sensor, the moisture content of the paper at the time of transferring the first color of the second surface in the double-sided image formation as shown in FIG. 3 is calculated, and the paper content as shown in FIG. From the relationship between the water content and the maximum surface charge density that can be retained, the maximum surface charge density that can be loaded on the second side of the paper is calculated, and the charge amount per unit area of the toner determined from the detection value of the temperature and humidity sensor From the surface charge density of the paper, the toner amount per unit area may be determined based on Equation (1).

Specific examples of numerical values will be shown. In the present embodiment, Qp (s, k) = 5 × 10 when forming the image on the second side in the double-sided image formation on the paper shown by the graph A in FIG.
^-4 C / m ² , Qt (k) = 3 × 10 ^-2 C / kg, n = 2
Therefore, M ≦ 8.3 × 10 ⁻³ kg / m ² . Therefore,
When forming the image on the second side, the amount of toner per color is set to 8.3.
It may be reduced to × 10 ⁻³ kg / m ² .

At this time, it was revealed by the study of the present inventors that a better effect can be obtained by reducing the transfer current at the same rate as the reduction of the toner amount. That is, in the present embodiment, the transfer current is 10 μA for each color, so that 10 × 8.3 × 10 ⁻³ / 1 × 10 ⁻² = 8.3 μA may be set when the second surface is transferred.

As a more preferred embodiment according to this embodiment,
The relational expressions as shown in FIGS. 3 and 4 are stored in the apparatus main body in the number corresponding to the type of recording material, and the user can specify the type of recording material in advance when using the image forming apparatus. The maximum surface charge density that can be held for each recording material stored in advance in the main body of the apparatus is calculated from the recording material information, and the weight per unit area of the toner on the photosensitive drum is determined by the above method. By doing so, it is apparent that more preferable results can be obtained.

FIG. 5 is a graph showing the relationship between the toner amount of the toner image on the photosensitive drum and the reflection density of the toner image on the paper onto which the toner image is transferred. From FIG. 5, the toner amount on the photosensitive drum and the on-paper density are in a proportional relationship, and if the toner amount is reduced in order not to cause dot transfer omission, the on-paper density naturally decreases. However, as can be seen from FIG. 5, even if the toner amount is reduced to 8 × 10 ⁻³ kg / m ² , for example,
There is no problem because the density on paper is about 1.2, which is sufficient for practical use.

Needless to say, in the above, it is effective to keep the toner amount on the second side to the maximum in the allowable range in order to eliminate the density difference between the first side and the second side.
However, depending on the conditions, the maximum held charge amount of paper may be exceeded unless the toner amount is significantly reduced. At this time, although the dot-shaped transfer omission can be surely suppressed, the image has a low density and may pose a problem in practical use. Therefore, in such a case, in order to secure the minimum density, the amount of toner required to achieve at least the reflection density> 1.0, in the present embodiment, 6. Do not lower it below 10 ⁻³ kg / m ² (60% of the initial value).

That is, in the equation (1), if Qp (s, k) / Qt (k) / n <6 × 10 ⁻³ , then M = Mi × 0.6. However, Mi is the initial value of the toner amount per unit area of the toner image on the photosensitive drum, and in this embodiment, Mi =
It is 1 × 10 −2 kg / m 2.

To reduce the toner amount, a method such as reducing the development contrast potential of the latent image or reducing the exposure amount may be used.

Although the transfer belt is shown as the recording material carrier in the above, a transfer drum may be used. Also developing device 1
As (1a to 1d), any type of developing method may be used.

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.

As described above, in this embodiment, the toner amount of the toner image for the second surface of the recording material on the image carrier in the double-sided image formation is Mi × 0.6 ≦ M ≦ Qp (s, k). / Qt (k) / n However, when Qp (s, k) / Qt (k) / n <Mi × 6, by setting M = Mi × 0.6, dot-like transfer omission occurs on the second surface. It is possible to obtain a high-quality image on both sides of the recording material by satisfactorily transferring the image.

Here, as a method of measuring the reception of the toner image on the photosensitive drum per unit area, a predetermined method (the area is known) is used.
If a test toner image is formed on the photosensitive drum, the density of the test toner image is detected by a sensor (light receiving unit), and the density value is converted to a weight value having a certain correlation, the value is obtained. be able to.

Example 2 In Example 1, the amount of toner on the second surface of the recording material was reduced. However, even the paper that has once passed through the fixing device may not show the electric characteristics as shown by the graphs A and B in FIG. 2 but may show the ideal electric characteristics as shown by the graph D depending on the conditions.

For example, high temperature and high humidity environment (30 ° C., 80% R
In H), even if the water content of the paper once passes through the fixing device, the water content immediately returns to the original value due to the high humidity environment, and the electrical characteristics of the paper show the behavior as shown by D in FIG. In such an environment, since dot-shaped transfer omission does not occur on the second surface, it is not necessary to reduce the toner amount. However, it is clear from FIG. 5 that it is important to stabilize the toner amount in order to secure the density.

Therefore, in the present embodiment, the maximum surface charge density that can be held on the second side of the paper is calculated by performing one dummy copy for double-sided image formation, and then on the photosensitive drum 3 during the double-sided image formation thereafter. The amount of toner in the toner image is determined to prevent the occurrence of dot-shaped transfer defects. The details will be described below.

FIG. 6 shows the transfer voltage for each color. In the figure, Vtr is a normal transfer voltage, and each time the colors are overlapped (in this embodiment, yellow (Y), magenta (M), cyan (C), and black (K)), the transfer voltage Vtr1, It is rising to Vtr2 , Vtr3, and Vtr4 . V
B is a transition of the transfer voltage of the recording material under non-sheet passing, which also rises to VB1, VB2, VB3, and VB4. V
Comparing tr and VB, the differences ΔV1, ΔV2, ΔV3, and ΔV4 between Vtr and VB for each color are constant and are usually about 200V to 700V, although they differ depending on the recording material.

On the other hand, FIG. 7 shows an example of the correlation between Vtr and VB at the time of transfer to the second surface, and the difference between Vtr and VB of each color is ΔV1 ′, ΔV2 ′, ΔV3 ′, ΔV.
4 '. As is clear from this, although ΔV1 ′ and ΔV2 ′ are constant, the transfer voltage Vtr3 suddenly drops to about VB3 during the transfer of the third color, and the transfer process proceeds in that state. The ΔV4 ′ of the fourth color is almost 0V, and the transfer process has progressed in the state of Vtr4 = VB4. It is considered that this is because the amount of charges supplied to the paper during the transfer of the third color exceeded the maximum surface charge density that can be held, and thus the transfer impedance dropped sharply. As described above, in such a situation, dot-shaped transfer defects occur, and the image quality is significantly impaired.

Therefore, in this embodiment, one double-sided copy is performed as a dummy copy before the double-sided copy, and the maximum surface charge density that can be held on the second side of the paper is grasped, and then the double-sided image formation is performed. The purpose is to determine the amount of toner on the photosensitive drum. This will be described in detail below.

Vtr and V of each color on the second side during dummy copy
The difference of B is set to ΔV1 ′ to ΔV4 ′ as described above,
Q1, Q2, Q
3 and Q4.

For example, as shown in FIG. 7, Δ in the third color
When V3 'becomes almost 0, the total amount of charges supplied to the paper by then is Q1 + Q2. Therefore, the total charge amount Q1
The maximum surface charge density Q that can be held by dividing + Q2 by the paper area
p (s, k) can be calculated. After that, as in the first embodiment, the amount of charge per unit weight of toner determined from the detection value of the temperature / humidity sensor may be obtained, and the amount of toner per unit area may be determined based on Equation (1).

As a result, the degree of moisture absorption of the paper, which is influenced by the installation environment of the image forming apparatus, can be grasped without taking a special constitution, and the dot transfer defect on the second side in the double-sided image formation can be prevented. it can.

Embodiment 3 In Embodiment 3, the characteristics of the second surface of the paper, which changes moment by moment in the double-sided image formation, are directly grasped by providing a sensor in the image forming apparatus, and the occurrence of transfer failure of the second surface is confirmed. It is to prevent it.

As described above, since the electric characteristics of the paper change greatly due to the decrease of the water content, the electric characteristics of the paper can be accurately grasped by directly detecting the water content of the paper, and The maximum retained charge amount Q can be accurately estimated.

Therefore, in the present embodiment, an infrared moisture meter is provided in the apparatus immediately before the transfer of the first color, and the moisture content of the paper is measured immediately before the transfer of the first color, particularly immediately before the transfer of the first color of the second side. I made it possible.

Then, based on the detection value of the moisture meter, the maximum surface charge density that can be held on the second surface of the paper is calculated according to FIG. 4, and the toner amount per unit area on the photosensitive drum 3 is calculated according to the equation (1). To decide. As a result, it was possible to prevent the dot-like transfer failure on the second surface of the paper, which depends on the installation environment of the image forming apparatus, and to obtain a good image.

In the above embodiments, the photosensitive drums 3a to 3d are used.
The image forming apparatus has been described in which the toner images of the respective colors are sequentially superimposed and transferred onto the recording material P carried on the transfer belt 130. However, the steps of transferring the toner images on one photosensitive drum to the recording material are sequentially repeated, It goes without saying that the present invention can also be applied to an image forming apparatus that forms toner images of a plurality of colors on a recording material.

[0082]

As described above, according to the present invention,
The maximum value of the toner weight per unit area of the toner images for the first surface and the second surface of the recording material formed on the image carrier is made different between the first surface and the second surface of the recording material , and Tona
-During image transfer, the electric current flowing through the transfer means is formed on the image carrier.
Variable according to the weight per unit area of the formed toner image
Of the toner image formed on the image carrier.
The weight per unit area can be changed according to the water content of the recording material.
Because of the strange structure, it is possible to obtain high-quality double-sided images on both the first and second sides of the recording material.

[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 surface charge density given to a recording material and a surface potential of the recording material.

FIG. 3 is a diagram showing a change in water content of paper after fixing.

FIG. 4 is a graph showing the relationship between the water content of paper and the maximum surface charge density that can be held.

FIG. 5 is a graph showing the relationship between the toner amount of the toner image on the photosensitive drum and the reflection density of the toner image on the paper onto which the toner image is transferred.

FIG. 6 is a diagram schematically showing a transfer voltage for each color during transfer onto a recording material.

FIG. 7 is a diagram schematically showing a transfer voltage for each color at the time of transfer onto the second surface of the recording material.

[Explanation of symbols]

1a to 1d developing device 3a to 3d photosensitive drum 20 temperature and humidity sensor 24a to 24d Transfer charger 130 transfer belt P recording material

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masahiro Inoue 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (56) Reference JP-A-7-160078 (JP, A) JP-A-5 −257355 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) G03G 15/16 G03G 15/01 114

Claims (10)

(57) [Claims] 1. A toner image formed on an image carrier is transferred.
In an image forming apparatus for electrostatically transferring to a recording material by a copying means , after transferring the toner image on the image carrier to the first surface of the recording material,
The toner image on the image carrier can be transferred to the second surface opposite to the first surface, and the maximum value of the weight per unit area of the toner image formed on the image carrier is the first surface. Unlike in the toner image and the toner image of the second side, when the toner image transfer, the current flowing through the transfer means, the image
Weight per unit area of toner image formed on carrier
The image forming apparatus is characterized by being variable according to . 2. The image according to claim 1, wherein the maximum value of the weight per unit area of the toner image formed on the image carrier is smaller in the toner image on the second surface than on the toner image on the first surface. Forming equipment. 3. A toner image formed on an image carrier is recorded.
In an image forming apparatus that electrostatically transfers to a material, after transferring the toner image on the image carrier to the first surface of the recording material,
The toner on the image bearing member is provided on the second surface opposite to the first surface.
-It is possible to transfer the image, and per unit area of the toner image formed on the image carrier
The maximum weight of the toner image is the toner image on the first side and the toner image on the second side.
Differs between the toner image, the weight per unit area of the toner image formed on the image bearing member is variable depending on the water content of the recording medium
An image forming apparatus characterized by. 4. A toner image formed on the image carrier.
The maximum value of the weight per unit area is the toner on the first side.
The toner image on the second surface is smaller than the image.
Image forming apparatus. 5. A toner image on the image carrier is statically recorded on a recording material.
When a toner image is transferred, the transfer means for electrically transferring the
The current flowing through the transfer means is formed on the image carrier.
It is variable depending on the weight per unit area of the toner image.
The image forming apparatus according to claim 3 or 4. 6. Repeat step of transferring to a recording material borne the toner image on the image bearing member onto a recording material bearing member, according to claim 1 to 5 for forming a toner image of a plurality of colors on a recording medium The image forming apparatus according to any one of items. 7. The maximum value of the weight per unit area of the toner image of at least one color among the toner images of the plurality of colors formed on the image carrier is the maximum value of the toner image of the first surface and the toner image of the second surface. The image forming apparatus according to claim 6 , which is different from the toner image. 8. The maximum value of the weight per unit area of the toner image formed on the image carrier is the sum of the maximum values of the weight per unit area of the toner images of the plurality of colors.
6 or 7 image forming apparatus. Wherein said image bearing member is an image forming apparatus according to any one of claims 1-8 which is more provided for bearing a toner image of a plurality of colors, respectively. 10. A fixing device for fixing the toner image on the recording material to the recording material after the toner image is transferred, and after fixing the toner image on the first surface of the recording material, the toner image on the image carrier is fixed. the image forming apparatus according to any one of the paragraphs can be transferred to the second surface of the recording material according to claim 1-9.