JP2000194201A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device possible to obtain excellent images on both sides of recording material by satisfactorily transferring the image on the rear of recording material without generating the dot-like transfer void. SOLUTION: This image forming device is allowed to dispose the temp. humidity sensor in the vicinity of a recording material storing cassette in a main body thereof, to obtain an absolute water quantity in the device based on the detection value of the temp. humidity in the device, at the both side image forming time, to obtain the maximum loading permissible area charge density on the rear of paper based thereon, to determine toner quantity for unit area, based on the charge quantity for unit area of the toner judged from the detection value of the temp/humidity sensor, and the surface charge density, and to reduce the toner quantity on the toner image for the rear side formed on a photosensitive drum to the decided toner quantity.

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 recording material by using an electrophotographic method and obtaining 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 units are provided, and toner images of different colors are formed in each image forming unit, and the toner images are sequentially superimposed and transferred on the same recording material, and are transferred onto the recording material. Various image forming apparatuses for forming a color image have been proposed.

[0003] 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 the color electrophotographic recording apparatus will be described with reference to FIG.

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 units Pa, Pb, Pc, and Pd are each a dedicated image carrier, in this example, an 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.

[0007] 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, 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 polyethylene terephthalate resin (PET), polyvinylidene fluoride resin,
It is made of a sheet of a dielectric resin such as a polyurethane resin, 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 drive 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) 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 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 59 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.

The transfer belt 130 used in the image forming apparatus having the above-described configuration is a dielectric sheet such as a PET sheet, a polyvinylidene fluoride sheet, or a polyurethane sheet as described above. 10 ¹³
Those having a density of 10 to 10 ¹⁸ Ωcm are generally used.

A transfer charger 24 (24a to 24d)
If the current that contributes to the transfer (transfer current) is constant at an appropriate current, the image will be stable. Therefore, even when the volume resistance changes due to the type of recording material (thickness, material, etc.), moisture absorption conditions, etc. In general, constant current control is performed so as to obtain a constant current.

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

[0022]

However, in the above-described image forming apparatus, when paper is used as the recording material P and an image is formed on both sides of the recording material, dot-like transfer omissions occur in the image transferred on the second side. However, there has been a problem that image quality is significantly impaired.

Therefore, an object of the present invention is to provide a recording material
It is an object of the present invention to provide an image forming apparatus capable of transferring an image on a surface thereof without causing a dot-like transfer omission, and obtaining a high-quality image on both sides of a recording material.

[0024]

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 for electrostatically transferring a toner image formed on an image carrier to a recording material, the toner image on the image carrier is transferred to the first surface of the recording material, and then the opposite side from the first surface The toner image on the image carrier can be transferred to the second surface of
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.

According to the present invention, 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 in the toner image on the first surface. A transfer unit that electrostatically transfers the toner image on the image carrier to a recording material, and when the toner image is transferred, a current flowing through the transfer unit is a unit of the toner image formed on the image carrier. It is variable according to the weight per area. The weight per unit area of the toner image formed on the image carrier is variable according to the water content of the recording material.

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. I do. The maximum weight per unit area of at least one color toner image of the plurality of color toner images formed on the image carrier is different between the first surface toner image and the second surface toner image. . 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 plurality of color toner images. A plurality of the image carriers are provided to respectively carry a plurality of color toner images.

The image forming apparatus further comprises fixing means for fixing the toner image on the recording material to the recording material after transferring the toner image. After fixing the toner image on the first surface of the recording material, recording the toner image on the image carrier. The material can be transferred to the second surface.

[0028]

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

Embodiment 1 FIG. 1 is a configuration 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 related art, so that detailed description will be omitted.

In the present invention, the transfer belt 130 shown in FIG.
As the dielectric sheet material, 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, polyamide imide, polycarbonate, polyphenylene oxide, polyether sulfone, polysulfone, aromatic polyester, polyether imide and aromatic polyimide are generally used.

In this embodiment, the material of the transfer belt 130 is selected from the viewpoints of mechanical properties, electrical properties, 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.

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

Contact-type transfer charger 24 (24a to 24a)
d) includes a plate-shaped conductive rubber transfer member extending in a direction (thrust direction) orthogonal to the recording material conveyance direction, and this transfer member is connected to the photosensitive drum 3 via a transfer belt 130.
(3a to 3d). The transfer charger 24 charges the recording material P conveyed to the transfer section from the back surface of the recording material P with a polarity opposite to that of the toner (positive in this example), and the toner image on the photosensitive drum 3 is statically applied to the surface of the recording material P. Electrotransferred. In the present embodiment, the current flowing through the transfer charger 24 is controlled at 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. Becomes).

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

The state of the recording material (paper) used in the above-described image forming apparatus changes significantly depending on the temperature and humidity during storage. The study by the present inventors has revealed that there is a large difference in the electrical characteristics between the paper immediately after opening the package and the paper once heated by the fixing device and the water content is significantly reduced. 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 recording material surface and the potential of the recording material surface. In FIG. 2, graph C shows the result when OHT (made of PET sheet, thickness 100 μm) was used as the recording material. In OHT, the surface potential increased 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 results when the HGA paper store NPI high-quality paper (basis weight 128 g / m ² , the thickness 120 μm) was used once through the fixing device, and the graph B shows the double-sided thick paper for Canon CLC. (Basic weight 105 g / m ² , thickness 10
0 μm) is also the result when it is used once through a fixing device. Looking at this, the paper of graph A is 450 μC / m
^From around ² , the paper of graph B starts from around 500 μC / m ² ,
Each of them loses linearity, and tends to be unseen in paper immediately after opening (Graph D) or OHT (Graph C).

This is because when the moisture in the paper decreases once it passes through the fixing device, when a charge of a certain value or more is supplied to the surface of the paper, electric discharge is caused by minute voids between the fibers in the paper. It seems that the charges on the front and back surfaces of the paper are offset, so that no matter how much charge is supplied, the charges cannot be accumulated on the front and back surfaces of the paper, and the surface potential does not rise.

In the above-described image forming apparatus, when an image is formed on the second side of a sheet passing through such a fixing device, when a certain amount of charge is supplied during the transfer process, the amount of charge held on the sheet is limited. , Discharge in the paper starts, the paper rapidly loses its holding power to the toner, and a point-like image defect occurs on the second surface of the recording material as described in the section of the related art.

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

Generally, in a color image forming apparatus, it is difficult to reproduce black (black) using three colors of cyan, magenta, and yellow, and it is desirable 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 a 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 amount of 1.67 colors, and at present, if the image is once again transferred to the paper that has passed through the fixing device, the above-mentioned dot-like transfer failure occurs.

Therefore, according to the study of the present inventors, when the image is transferred to the paper once 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 on the photosensitive drum is reduced. By doing so, it was found that the above-mentioned point-like transfer failure can be prevented from occurring, and a good image can be obtained. Hereinafter, this method will be described in detail.

Based on FIG. 2, a function Qp = Qp which gives the maximum retained charge amount Qp (C / m ² ) per unit area of the recording material as a variable based on the type of the recording material and the environmental atmosphere in which the recording material is placed. (S, k). Here, s is the type of the recording material, and 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 .

The charge amount Qt per unit weight of the toner
(C / kg) is a function Q that gives the environment atmosphere as a variable.
Let t = Qt (k), and let the UCR upper limit be 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 riding amount) is 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 on 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 front surface of the recording material is negative and the back surface is positive).

In order to prevent the above point-like transfer failure from occurring, the value of M · Qt (k) · n is set to 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) ・ nｎQp (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 largely depends on the type of paper and the operating environment of the image forming apparatus main body, particularly temperature and humidity.

FIG. 3 shows that the paper once passed through the fixing device is subjected to a normal temperature and normal humidity environment (23 ° C., 60% RH) and a normal temperature and low humidity environment (23 ° C.,
5 shows the change in the 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 Science Laboratory.

As shown in FIG. 3, once the paper passes through the fixing device, the water content decreases by about 2 to 3%. afterwards,
Paper left in a room-temperature and normal-humidity environment has its moisture content returned to about the initial 80% due to moisture absorption after 30 seconds. However, paper left in a room-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 the fixing of the first side of the paper to the transfer of the first color on the second side is about 1 hour.
Since it is 5 seconds, it can be seen that the moisture content of the paper on the second side largely depends on the atmosphere of the environment where the paper is placed.

FIG. 4 shows the relationship between the water content of the paper and the maximum surface charge density of the paper. As is clear from FIG. 4, 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.

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 recording 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 apparatus is determined, and then the maximum surface charge density that can be held on the second surface of the recording material is determined. The toner amount of the toner image for the second side formed on No. 3 is to be reduced.

Regarding how much the toner amount is reduced, it is preferable to perform the following procedure based on the above-mentioned equation (1). From the detected values of the temperature and humidity by the temperature and humidity sensor, the moisture content of the paper at the time of transferring the first color on the second side in the double-sided image formation as determined from FIG. From the relationship between the water content and the maximum surface charge density that can be held, the maximum surface charge density that can be loaded on the second side of the paper is determined, and the amount of charge 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, when forming the image on the second side of the two-sided image forming of the paper shown by the graph A in FIG. 2, Qp (s, k) = 5 × 10
^-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 an 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, the present inventors have found that a better effect can be obtained by reducing the transfer current at the same rate as the toner amount. That is, in the present embodiment, since the transfer current is 10 μA for each color, it is sufficient to set the transfer current to 10 × 8.3 × 10 ⁻³ / 1 × 10 ⁻² = 8.3 μA during the transfer of the second surface.

As a more preferred embodiment according to the present embodiment,
The relational expressions as shown in FIGS. 3 and 4 are stored in the apparatus main body in a number corresponding to the type of the recording material, and when the image forming apparatus is used, the user preliminarily stores the type of the recording material in the apparatus main body. Then, from the recording material information, the maximum surface charge density that can be retained for each recording material stored in the apparatus main body in advance is determined, and the weight per unit area of the toner on the photosensitive drum is determined by the above-described method. Obviously, a more favorable result 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 has been transferred. FIG. 5 shows that the toner amount on the photosensitive drum and the density on paper are proportional to each other, and if the toner amount is reduced in order to prevent point-like transfer omission, the density on paper naturally decreases. However, as can be seen from FIG. 5, even if the toner amount is reduced to, for example, 8 × 10 ⁻³ kg / m ² ,
The density on paper is about 1.2, which is sufficient for practical use, so there is no problem.

In the above description, it is needless to say that keeping the toner amount on the second side to the maximum within an allowable range is more effective in eliminating the density difference between the first side and the second side.
However, depending on the condition, the toner amount may exceed the maximum retained charge amount of the paper unless the toner amount is significantly reduced. In this case, although dot-like transfer omission can be suppressed, the density of the image is low, which may cause a practical problem. Therefore, in such a case, in order to secure the minimum density, at least the amount of toner required to achieve a reflection density> 1.0, in this 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. Here, 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 x 10-2 kg / m2.

The amount of toner may be reduced by lowering the developing contrast potential of the latent image or reducing the amount of exposure.

Although the transfer belt has been described as a recording material carrier, a transfer drum may be used. Developing device 1
(1a to 1d) may be any type of developing method.

Generally, the developing method is roughly classified into a one-component developing method and a two-component developing method. In the one-component developing method, a non-magnetic toner is coated on a developing sleeve with a blade or the like,
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.

As described above, in this embodiment, the toner amount on the image carrier of the toner image for the second side of the recording material in the double-sided image formation is represented by 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, a dot-like transfer omission occurs on the second surface. An image can be satisfactorily transferred without causing a high-quality image on both sides of the recording material.

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

Example 2 In Example 1, the amount of toner on the second side of the recording material was reduced. However, even paper that has once passed through the fixing device may not show the electrical characteristics shown in graphs A and B in FIG.

For example, in a high temperature and high humidity environment (30 ° C., 80% R
In H), even if the paper once passes through the fixing device to reduce the water content, the water content returns immediately to the original value due to the high humidity environment, and the electrical characteristics of the paper show the behavior as shown in FIG. 2D. In such an environment, the point-like transfer omission does not occur on the second surface, so that 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 hold the paper on the second side is determined by performing one dummy copy for the double-sided image formation, and the maximum surface charge density on the photosensitive drum 3 during the subsequent double-sided image formation is determined. The amount of toner in the toner image is determined to prevent the occurrence of point-like transfer failure. The details will be described below.

FIG. 6 shows the transfer voltage of each color. In the figure, Vtr is a normal transfer voltage, and each time a color is overlapped (in this embodiment, yellow (Y), magenta (M), cyan (C), black (K)), the transfer voltage Vtr1, Vtr2, Vtr3, and _Vtr4 have increased.
VB is the transition of the transfer voltage under the non-sheet passing state of the recording material,
This also rises to VB1, VB2, VB3, and VB4.
When Vtr and VB are compared, the differences ΔV1, ΔV2, ΔV3, and ΔV4 between Vtr and VB in each color are constant and vary depending on the recording material, but are generally about 200 V to 700 V.

FIG. 7 shows an example of the correlation between Vtr and VB at the time of transfer to the second surface. The difference between Vtr and VB of each color is represented by ΔV1 ′, ΔV2 ′, ΔV3 ′, ΔV
4 '. As is clear from this, ΔV1 ′ and ΔV2 ′ are constant, but the transfer voltage Vtr3 suddenly drops to about VB3 during the transfer of the third color, and the transfer process is proceeding as it is. Also in the fourth color, ΔV4 ′ is almost 0 V, and the transfer process has progressed in a state of Vtr4 = VB4. This is probably because the amount of charge supplied to the paper during the third color transfer exceeds the maximum surface charge density that can be held, and therefore the transfer impedance has dropped sharply. As described above, in such a situation, a dot-like transfer failure occurs and the image quality is remarkably 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. The purpose is to determine the amount of toner on the photosensitive drum. The details will be described below.

Vtr and V of each color on the second side during dummy copy
The difference between B is ΔV1 ′ to ΔV4 ′ as described above,
The charge amounts supplied to the paper at the time of each color transfer are represented by Q1, Q2, Q
3, Q4.

For example, as shown in FIG.
When V3 'becomes substantially zero, the total amount of electric charge supplied to the paper so far is Q1 + Q2. Therefore, the total charge Q1
+ Q2 divided by the paper area gives the maximum surface charge density Q that can be held
p (s, k) can be calculated. Thereafter, as in the first embodiment, the charge amount per unit weight of the toner determined from the detection value of the temperature / humidity sensor may be obtained, and the toner amount per unit area may be determined based on Expression (1).

Thus, the degree of moisture absorption of the paper, which depends on the installation environment of the image forming apparatus and the like, can be grasped without taking a special configuration, and a dot-like transfer failure on the second side in double-sided image formation can be prevented. it can.

Embodiment 3 In this embodiment 3, the characteristics of the second side of the paper, which change with time, in double-sided image formation are directly grasped by providing a sensor in the image forming apparatus, and the occurrence of transfer failure on the second side is determined. It is to prevent it.

As described above, the electrical characteristics of paper greatly change due to the decrease in its water content. Therefore, by directly detecting the water content of the paper, the electrical characteristics of the paper can be accurately grasped and the paper's electrical characteristics can be grasped. The maximum retained charge amount Q can be accurately estimated.

Therefore, in this 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 on 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 side of the paper is calculated from FIG. 4, and the toner amount per unit area on the photosensitive drum 3 is calculated according to the equation (1). To determine. As a result, it was possible to prevent a dot-like transfer failure on the second side of the paper, which is affected by the installation environment of the image forming apparatus, and to obtain a good image.

In the above embodiment, the photosensitive drums 3a to 3d
Although the image forming apparatus has been described in which the toner images of the respective colors are sequentially superimposed and transferred on the recording material P carried on the transfer belt 130, the process of transferring the toner image on one photosensitive drum to the recording material is sequentially repeated. It goes without saying that the present invention can be applied to an image forming apparatus that forms a toner image 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 image for the first side and the second side of the recording material formed on the image carrier is different between the first side and the second side of the recording material. High quality double-sided images were obtained on both the first and second surfaces of the material.

[Brief description of the drawings]

FIG. 1 is a configuration diagram illustrating an image forming apparatus to which the present invention can be applied.

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 amount of toner in a toner image on a photosensitive drum and the reflection density of the toner image on paper to which the toner image has been transferred;

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

FIG. 7 is a diagram schematically showing a transfer voltage for each color at the time of transfer to 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

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Haruhiko Omata 3-30-2 Shimomaruko, Ota-ku, Tokyo Inside Canon Inc. (72) Inventor Masahiro Inoue 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon F term in reference company (reference) 2H030 AB02 AD16 BB13 BB54 2H032 AA05 BA18 BA21 CA02 CA11 CA12 CA14

Claims (9)

[Claims] An image forming apparatus for electrostatically transferring a toner image formed on an image carrier to a recording material, wherein the toner image on the image carrier is transferred to a 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. An image forming apparatus, wherein the toner image on the second surface is different from the toner image on the second surface. 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 in the toner image on the first surface. Forming equipment. 3. A transfer device for electrostatically transferring a toner image on the image carrier to a recording material, wherein a current flowing through the transfer device is formed on the image carrier when the toner image is transferred. 3. The image forming apparatus according to claim 1, wherein said image forming apparatus is variable in accordance with a weight per unit area of the toner image. 4. The image according to claim 1, wherein a weight per unit area of the toner image formed on the image carrier is variable according to a moisture content of a recording material. Forming equipment. 5. The method according to claim 1, wherein a step of transferring the toner image on the image carrier to a recording material carried on the recording material carrier is repeated to form a plurality of color toner images on the recording material. An image forming apparatus according to any one of the above items. 6. The maximum weight per unit area of at least one color toner image of the plurality of color toner images formed on the image carrier is the first surface toner image and the second surface toner image. 6. The image forming apparatus according to claim 5, wherein the image forming apparatus differs from the toner image. 7. 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 plurality of color toner images. 6. The image forming apparatus of item 6. 8. The image forming apparatus according to claim 1, wherein a plurality of said image carriers are provided to respectively carry toner images of a plurality of colors. 9. A fixing device for fixing a toner image on a recording material to a recording material after transferring the toner image, and fixing the toner image on the image carrier after fixing the toner image on the first surface of the recording material. The image forming apparatus according to claim 1, wherein the image can be transferred to the second surface of the recording material.