JPH08185009A - Multicolor image forming device - Google Patents

Abstract

PURPOSE: To provide a multicolor image forming device capable of surely preventing color mixture on a toner image being the second and succeeding toner images caused by first toner entering toward a developing device being the second and succeeding developing devices, capable of forming the toner image with sufficiently high density without fogging, and capable of obtaining a good multicolor image for a long time. CONSTITUTION: As to respective developing devices being the second and succeeding developing devices, a developing bias impressed on a developing sleeve is constituted of an alternating voltage, and the alternating voltage has a voltage in a developing direction which moves the toner in a direction from the developing sleeve to a photoreceptor, whose impressed time is T1 and whose peak voltage is Vmax , and a voltage in the returning direction which moves the toner in a direction from the photoreceptor toward the developing sleeve, and whose impressed time is T2 and whose peak voltage is Vmin on one cycle. The time T1-2 required for moving the voltage from the developing direction to the returning direction and the time T2-1 required for moving the voltage from the returning direction to the developing direction satisfy T2-1 >T1-2 .

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multicolor image forming apparatus such as a color copying machine or a color printer.

[0002]

2. Description of the Related Art In recent years, there has been a demand for an electrophotographic or electrostatic recording type image forming apparatus that forms a multicolor image using a plurality of colors.

FIG. 6 shows such a multicolor image forming apparatus.
The image forming process will be described with reference to the two-color image forming apparatus shown in FIG. FIG. 7 shows the surface potential of the photosensitive drum in each step of forming a two-color image.

In FIG. 6, the two-color image forming apparatus has, for example, a photosensitive drum 1 as an electrostatic latent image carrier, and the photosensitive drum 1 is provided with a photoconductive layer such as OPC on its surface and rotates in the direction of arrow A. To be done. The surface of the photosensitive drum 1 is uniformly charged to, for example, −600 V by the first primary charger 2 (FIG. 7).
(A)). Then, the first image exposure 3 is performed using the first image signal. The first image exposure 3 is performed by the first semiconductor laser 12
A laser beam modulated by the first image signal is emitted from the laser beam, is polarized by the rotary polygon mirror 14 rotated by a motor 15, and the polarized laser beam is coupled by a coupling lens 16.
Then, the light is reflected by the folding mirror 17 and raster-scanned on the photosensitive drum 1. By this first image exposure 3, the surface potential of the exposed portion on the photosensitive drum 1 is reduced to, for example, -1.
After being attenuated to 00 V, a first latent image corresponding to the first image signal is formed on the photosensitive drum 1 (FIG. 7 (B)).

Next, the first latent image is developed by the first developing device 4, which is a red two-component developing device, and visualized as a red toner image. The first developing device 4 uses a two-component developer in which negatively charged red toner and a magnetic carrier such as ferrite are mixed for development. During development, the first developing device 4 receives, for example, an AC voltage of 1600 Hz and 1800 Vpp of -500 V.
By applying a developing bias superposed with the DC voltage of, the first latent image is reversely developed (FIG. 7C). The potential on the surface of the obtained first toner image (first toner image potential) rises by about -100 V to about -200 V with respect to the photosensitive drum surface potential of -100 V of the first image portion due to the toner charge.

Then, the photosensitive drum 1 is uniformly charged again by the second primary charger (recharger) 5 to raise the potential of the first toner image. At this time, the non-image portion potential also slightly rises.
The potential of the non-image portion after recharging is −650V, and the potential of the first image portion is approximately −600V (FIG. 7D). A second image exposure 13 is performed on the photosensitive drum 1 to attenuate the surface potential of the exposed portion on the photosensitive drum 1 to, for example, −100 V, and a second latent image corresponding to the second image signal is formed (FIG. 7
(E)). The second image exposure 13 is performed by the second semiconductor laser 13
The laser beam modulated by the second image signal is emitted, is polarized by the polygon mirror 14, and the polarized laser beam is raster-scanned on the photosensitive drum 1 through the coupling lens 16.

After that, the second developing device 7 uses a negatively charged one-component developer made of, for example, black toner, and an AC voltage of, for example, 1600 Hz and 1300 Vpp is -50.
A developing bias in which a direct current voltage of 0 V is superimposed is applied to reversely develop the second latent image (FIG. 7 (F)). As a result, a two-color image of the red toner image and the black toner image is formed on the photosensitive drum 1.

The above is the image forming process called negative recharging. Another method for forming a two-color image on a photosensitive drum is a negative-positive process (Japanese Patent Laid-Open No. 55-137).
No. 538) and a three-valued process (Japanese Patent Laid-Open No. 52-81855).
No.).

The two-color image formed on the photosensitive drum 1 is subjected to a transfer pretreatment using a charger 18 as needed (usually, a DC or AC corona is applied, or this is subjected to light neutralization). (Used together) After the charge amounts of the two color toner images are aligned so as to be easily transferred, they are transferred onto the transfer material 9 supplied to the photosensitive drum 1 by the transfer charger 8. After that, the transfer material 9 is sent to the fixing device 10 to fix the toner image, and a two-color image of red and black is obtained. On the other hand, the photosensitive drum 1 is prepared for the next image formation by removing the transfer residual toner on the photosensitive drum 1 by the cleaning device 11.

[0010]

The above-mentioned second problem
During the second development by the developing device 7, the first toner image is disturbed due to the contact of the second toner as the developer with the first toner image, and the second developing device 7 for the first toner from the first toner image. It is important to prevent internal contamination.

For that purpose, the second developing device 7 sets the gap (SD gap) between the developing sleeve and the photosensitive drum 1 to 100 to 500 μm, and the layer thickness of the toner on the developing sleeve at that time is 50 to 50 μm. It is effective to adopt a non-contact development method, which develops to 200 μm.

It is known that a developing device adopting such a non-contact developing method applies an alternating voltage as a developing bias in order to improve the image quality. An electric field that separates the first toner is generated,
Although the first toner is a small amount, it gradually mixes into the second developing device. As a result, there is a problem that the accumulated first toner is mixed in the second toner image due to long-term use, resulting in color mixing.

The relationship between the potential of the photosensitive drum and the developing bias during the second development is shown in FIG. 7F, and the broken line in the figure shows the alternate bias during the second development. At this time, the electric field for developing the second latent image is given by the potential difference. On the other hand, the peeling bias is given by a bias for peeling off the second toner and by a bias for peeling off the first toner. Here, since>, the first
Toner is easily peeled off from the photosensitive drum.

For such peeling of the first toner, for example, Japanese Patent Application Laid-Open No. 2-77767 discloses a method of reducing the potential difference. This is one in which a so-called duty bias is used for the alternating voltage, and the photosensitive drum potential and the developing bias (shown by broken lines) are as shown in FIG.
(F) ', the bias for peeling off the first toner can be reduced from to, and if the peeling electric field strength of the first toner due to' is set to 2.3 V / μm or less, It is effective in preventing toner from peeling off.

However, even if such a developing bias in which the peeling bias is weakened is used, the first toner is peeled off due to the bias', but is mixed in the second developing device although it is a small amount. There wasn't.

The above-mentioned problems are caused even when the images of three or more colors are superposed on the photosensitive member such as the photosensitive drum.
It occurs during development after development.

An object of the present invention is to reliably prevent color mixture of toner images after the second toner image due to mixing of the first toner into the developing devices after the second developing device, and at the same time, by developing after the second developing device, It is an object of the present invention to provide a multicolor image forming apparatus capable of forming a toner image having a sufficiently high density and no fog and capable of obtaining a good multicolor image for a long period of time.

[0018]

The above object can be achieved by a multicolor image forming apparatus according to the present invention. In summary, the present invention is obtained by sequentially forming at least two or more latent images on an image bearing member and sequentially developing each latent image with a developer of a corresponding color. In a multicolor image forming apparatus for collectively transferring at least two or more color toner images onto a transfer material, after a second developing device for developing a latent image after a second latent image among developing devices for developing the latent image. In each of the developing devices, the developing bias applied to the developer carrying member is composed of an alternating voltage, and in one cycle of the alternating voltage, the toner is moved in a direction from the developer carrying member toward the image carrying member.
A voltage in the developing direction with an application time T ₁ and a peak voltage V _max ,
The toner is moved in the direction from the image bearing member to the developer bearing member, and has an application time T ₂ and a return voltage of peak voltage V _min , which is required to shift from the developing voltage to the returning voltage. When the time is T _1-2 and the time required to shift from the returning voltage to the developing voltage is T _2-1 , T _2-1 > T _1-2 The image forming apparatus.

According to the invention, said T ₁ , T ₂ , T _2-1
The, T _1> T _2-1, and may be a T _2> T _{2- 1,} the rate of voltage change between the _{T 2-1 = | V max -V min} | /
T _2-1 <50 V / μsec, and the potential when the image portion of the latent image where the first latent image is developed is recharged is V _T , and the potential of the developer carrier and the image carrier is When the closest distance is d, | V _T −V _min | / d <2.3 V / μm
And the developing bias can be a duty bias.

[0020]

【Example】

Example 1 The present invention provides a developing bias used in the second developing,
A major feature is that the developing bias having a unique waveform is defined. Since the configuration of the image forming apparatus itself is basically the same as that shown in FIG. 6, the description thereof will be omitted, and the description will be added with reference to FIG. 6 when necessary in the following description.

FIG. 1 is a drawing best showing the features of the present invention. FIG. 1 (A) shows the surface potential of the photosensitive drum during the second development, and FIG. 1 (B) shows the second development. The developing bias at the time is shown.

In FIG. 1A, reference symbol V _D is the potential of the non-image portion at the time of the first development and recharging, and the potential of the non-image portion at the time of the second development. The V _T part is
The image area at the time of the first development has its potential increased to V _T by recharging, and it is the non-image area at the time of the second development.

The developing bias shown in FIG. 1B is an alternating voltage which temporally changes as shown by an arrow in the figure. The V _max is a peak bias for developing the image portion _VL , and is called a skip voltage. V _min is the peak bias of the pullback bias, and is called a return voltage or the like. The one-dot chain line is
The DC component of the effective value of this alternating voltage is referred to as V _DC . During one cycle of the alternating voltage, the time during which V _max continues is T ₁ , V _min
The duration of time T is T ₂ . T _1-2 is the time required to transition from V _max to V _min, T _2-1 is V
It is the time required to shift from _min to V _max .

The present inventor examined the relationship between T _1-2 and T _2-1 and found that T _2-1 > T as shown in FIG. 1 (B).
_{As 1-2} , the time required to change from the return voltage to the skip voltage is made relatively long (that is, the amount of change per unit time when changing from the return voltage to the skip voltage is small). It has been found that the mixing of the toner by pulling back to the second developing device is prevented. Further, by appropriately setting the relationship among T ₁ , T ₂ , T _1-2 , and T _2-1 to secure a sufficient image density and prevent fogging, the above development bias condition for preventing mixing is satisfied. Found to exist.

The reason why mixing can be prevented by setting T _2-1 > T _1-2 is not fully clear, but one is
The second toner having a good response (for example, a high triboelectric charge) corresponding to the change of the electric field obtains a large acceleration in response to the steep rise of the bias and strongly collides when it reaches the photosensitive drum. 1 The toner is repelled and made into a cloud shape (toner floats in a cloud shape),
When the alternating bias is pulled back, it is possible to pull it back to the second developing device.

Therefore, the rising is made gentle and
It is considered that the softening of the collision also reduces the peeling of the first toner, but at this time, simply blunting the entire bias waveform does not provide sufficient density,
I can't even remove the cover.

Therefore, in the present invention, the rising of the bias waveform for the development of the second toner is made gentle so that the second toner does not collide strongly with the first toner on the photosensitive drum. (EN) A second bias waveform capable of preventing fogging while preventing the first toner from entering the second developing device and ensuring a sufficient image density.

The bias of FIG. 1B will be described in time series according to changes. In FIG. 1B, the section I is the rising edge of the bias waveform, and V _max is the bias on the developing side with respect to V _min . By making this section I rise gently, toner with good responsiveness, such as toner with high triboelectric charge, collides with the photosensitive drum softly. The toner is subsequently made to fly in the section II, but if this section is short, naturally the development is insufficient and the density becomes low. Further, in this section, it is considered that the toner having a low electrification charge has mainly reached the photosensitive drum, and therefore the collision is considered to be weak. Next, in the section III, the bias waveform falls, from V _max to V _min , and during the section IV, the toner excessively attached to the _VL portion and the fog toner attached to the non-image area are pulled back. Naturally, if the section IV is short, the fogging is insufficient.

As described above, in the present invention, the rise of the voltage on the side for adhering the second toner to the photosensitive drum as the second developing bias is made gentle. By doing so,
Since the second toner having a high charge of the second toner is prevented from violently colliding with the photosensitive drum, the second toner repels and floats the first toner already attached to the photosensitive drum. Less is. Therefore, it is considered that the pullback of the first toner at the pullback of the second developing bias is also reduced.

As described above, lengthening T _2-1 is as follows.
Although it is effective in preventing the first toner from entering the second developing device,
If T _2-1 is lengthened with the frequency fixed, T ₁ and T
₂ or T _1-2 becomes short. As described above, when T _{1 of} the second developing bias is short, the density is low, and when T ₂ is too short, the fog removal is insufficient, and in both cases, the image quality of the second toner image is impaired. not appropriate.

Therefore, by setting T _2-1 > T _1-2, and preferably by setting T ₁ > T _2-1 and T ₂ > T _{2-1 further} , it is possible to sufficiently develop and fog. It is good to have

On the other hand, if the frequency is lowered, T ₁ and T ₂ can be set sufficiently long even if T _2-1 is lengthened.
In that case, the original effect of the alternating bias is reduced, which is not preferable. Conversely, if the frequency is raised too high, the toner cannot respond to the high frequency, which is not preferable. Therefore, the frequency is preferably about 1.0 to 8.0 kHz.

The relationship between the rising and falling times of the bias waveform shown in FIG. 1B has been described above.
The relationship among the magnitudes of _max , V _min , and _VDC is disclosed in detail, for example, in Japanese Patent Laid-Open No. 2-77767, but basically, the duty bias (Duty) as shown in FIG.
Bias) is preferable and is effective in preventing the first toner from being mixed.

That is, | V _max −V _DC |> | V _min −
When the bias is set to V _DC |, | V _min −V _T | is kept small, and | V _max is prevented while preventing the first toner from being mixed.
-V _L | a large one in which the development of the second toner can be sufficiently performed. In order to obtain the effect of this duty bias, the duty ratio is 0.1 to
It is preferably about 0.4.

The effects of the present invention will be described below based on the results of image forming experiments.

Experiment 1 An OPC photosensitive member having a sensitivity peak in the infrared region was used as the photosensitive drum 1 in FIG. 6, and this was uniformly negatively charged to −600 V, and image exposure 3 was performed by digital exposure with a laser. , A first latent image was formed on the photosensitive drum 1. An image for exposure is read by arranging a color separation filter on a CCD sensor, separating the three colors to determine the color for each pixel of the image, red for the first color and black for the second color. And the color. In FIG. 6, the first developing device 4 is a red two-component developing device,
The second developing device 7 is a black one-component developing device.

The first latent image was developed by the first developing device 4 and visualized as a red toner image. Next, the photosensitive drum 1 was recharged by the recharging device 5, and the first image portion (red toner adhered portion) became about -600V and the non-image portion became about -650V. Then, a second image exposure 6 is performed on the photosensitive drum 1 to form a second latent image, and the second latent image is developed by the second developing device 7 to form a black toner image, and two color images of red and black are formed. .

The developing device 7 used in this embodiment will be further described.

As shown in FIG. 2, the developing device 7 contains magnetic toner (one-component magnetic developer) therein, and supplies the toner to the developing sleeve 72 while stirring the toner by stirring members 71a and 71b. It is like this. The toner supplied to the developing sleeve 72 is the same as the arrow B on the developing sleeve 72.
It is conveyed in the same direction by the rotation in that direction and the magnetic force of the magnet roller 73 that is arranged inside thereof in a non-rotating manner. During the transportation, the toner is regulated by the regulating member 74, applied in a thin layer on the developing sleeve 72, and then reaches the developing area 75 in the vicinity of the closest contact with the photosensitive drum 1.

The magnetic toner is insulative and is charged by friction with the non-magnetic developing sleeve 72. The charging polarity of the toner in the developing device 7 is negative so that the second latent image, which is a negative latent image (negative) on the photosensitive drum 1, is reversely developed. The amount of triboelectric charge of the toner includes the toner prescription and particle size, the addition of a charge control agent, the surface property of the developing sleeve 72, the distance between the regulating member 74 and the developing sleeve 72, and the strength of toner packing (toner density). ) Etc. In this example, a magnetic toner of styrene-acrylic type having an average particle size of 8 μm was used, and silica was externally added and used for imparting fluidity.

The developing sleeve 72 was used by subjecting the surface of a non-magnetic stainless steel sleeve substrate to a blast treatment with about 400 glass beads. The regulating member 74 is made of magnetic stainless steel and has a thickness of 1.2 mm. The distance between the regulating member 74 and the developing sleeve 72 was 200 μm. At this time, the amount of triboelectric charge of the toner is about 15 to 20 μC / g,
The coating amount was about 0.8 to 1.2 mg / cm ² .

The magnet roller 73 is magnetized into four poles, the developing pole located in the developing area 75 faces the photosensitive drum 1, and the magnetic toner on the developing sleeve 72 is erected with a magnetic force of about 800 to 1200 gauss. Let In this state, jumping development is performed by the action of the alternating electric field of the developing bias. The closest gap between the developing sleeve 72 and the photosensitive drum 1 is 300 μm, the rotation of the developing sleeve 72 is in the same direction as the photosensitive drum 1, and the peripheral speed thereof is about 1.5 times the peripheral speed of the photosensitive drum 1. .

As shown in FIG. 1B, the developing bias has a V _max of -1400 V, a V _min of -50 V and a _VDC of -500 V, a frequency of 2.0 kHz and a T ₁ of 1.
15 μsec, T ₂ 280 μsec, T _2-1 70 s
ec and T _1-2 were 35 sec, T _2-1 was about twice as large as T _1-2 , and shorter than T ₁ and T ₂ .

When the second development was performed by applying such a bias, it was possible to prevent the first toner from being peeled off and to keep the image quality of the second toner image excellent.

For confirmation, an image is formed with an image ratio of the first toner image (red toner image) of 6%, and then with a second toner image (black toner image) of 6%. However, it was confirmed that the color toner, which is the first toner, was not mixed in the second developing device even after 10,000 images were formed. On the other hand, when a similar image forming experiment was repeated using a bias of 30 μsec for both T _2-1 and T _1-2 ,
It was observed that some color toner was mixed in the second developing device.

Incidentally, the electric field intensity E _max generated during T ₁ for developing V _L is 4.17 V / μm, V from the figure.
_The electric field strength E _min that pulls back the toner from _L is 0.33 V /
μm, and the electric field strength E _T generated during T _{2 in} the direction of stripping V _T is 1.83 V / μm. Electric field strength E _T
2.3 as described in JP-A-2-77767.
It is set to V / μm or less (FIG. 7 (F) ′ ′).

Experiment 2 In this experiment, the first latent image and the second latent image were formed under the same conditions as in Experiment 1. A developing device having a structure as shown in FIG. 3 was used as the second developing device 7 in FIG. 6, and the second latent image was developed using non-magnetic toner (one-component non-magnetic developer).

As shown in FIG. 3, the developing unit 7 contains non-magnetic toner therein, and supplies the toner to the coating roller 76 'while stirring the toner by the stirring member 71', and supplies the toner to the coating roller 76 '. ′ Is supplied to the developing sleeve 72 ′. The toner is charged by friction with the developing sleeve 72 'and the elastic blade 74' which rotate in the direction of arrow B, and the charged toner is rubbed against the elastic blade 74 'while being attached to the developing sleeve 72' due to the mirror image force. The paper is ejected and conveyed to the developing area 75.

The toner remaining in the developing area without being used for the development is returned to the inside of the developing device 7 by the rotation of the developing sleeve 72 'and scraped off by the friction with the coating roller 76'. The charge polarity of the toner is negative, as in Experiment 1.

The elastic blade 74 'is made of silicone rubber, and the elastic blade 74' is applied to the developing sleeve 72 'at a linear pressure of 1.
Counter contact was performed at 5 to 20 g / cm. As the toner of the developing device 7, a styrene-acryl-based non-magnetic toner having an average particle size of 8 μm colored with carbon black was used, and silica was externally added and used for imparting fluidity.

At this time, the triboelectric charge amount of the toner is about 25-
30 μC / g, coating amount is about 0.4-0.8 mg / cm ²
Met. This toner charge amount is higher than the magnetic toner charge amount of 15 to 20 μC / g in Experiment 1, but this is not a problem because the magnetic toner is attracted to the developing sleeve by the magnetic force, but it is not used in this experiment. This is because the magnetic toner adheres to the developing sleeve only due to the mirroring force, so unless the charging amount is increased to increase the mirroring force, the magnetic toner easily flies to the photosensitive drum and fogging occurs.

The developing device 7 is mounted on the photosensitive drum 1 with a gap of about 280 μm between the developing sleeve 72 'and the photosensitive drum 1.
The developing sleeve 72 'was rotated at a peripheral speed of about 1.5 times in the same direction as the photosensitive drum 1. The developing bias shown in FIG. 4 was applied instead of the developing bias shown in FIG. 1 (B).

Explaining the bias waveform of FIG. 4, V
_max and V _DC are the same as those in Experiment 1 (FIG. 1 (B)), and V _min is 100 V lower than that in Experiment 1. In the section I ′, the bias is changed more gradually over V _min and V _max than in Experiment 1. Since the length of this section is set to be about 1.5 times as long as in Experiment 1, the collision of the second toner with the photosensitive drum becomes softer, and the peeling of the first toner is further reduced. Although the length of the section IV ′ is shortened by the length of the section I ′, the frictional charge amount of the second toner is higher than that in Experiment 1 and the response to the AC bias is good, so that the section IV ′ is improved.
Shortening does not reduce the effect on fogging removal. However, since it is not a magnetic toner, there is no magnetic pullback, so this was _handled by lowering V _min .

Further, since the toner has a high triboelectric charge amount and is difficult to separate from the developing sleeve, the closest distance of the developing area 75 is made smaller than in Experiment 1 to substantially increase the electric field strength. Sufficient concentration is obtained. Main experiment 2
Then, T ₁ = T _1-2 is set, but T ₁ is made slightly longer to secure the concentration.

In Experiment 2, E _max = 4.46 V / μm,
E _min = 0.71 V / μm, E _T = 2.32 V / μm
Is. E _T is in slightly larger than the above-mentioned field strength 2.3V / [mu] m (FIG. 7 (F) 'in'). However, since the inclination of T _2-1 is gentle, the first toner is not peeled off.

Embodiment 2 Another embodiment will be described with reference to FIG. FIG. 5 (A)
Is the surface potential of the photosensitive drum at the time of the second development in this embodiment, which is exactly the same as in FIG. FIG. 5B shows the developing bias applied to the second developing device, which does not have the duty ratio as shown in FIG. 1B and FIG. 4. The apparatus is the same as that shown in FIG. 2 The developing device shown in FIG. 3 was used as the developing device 7, but the closest distance (SD gap) between the developing sleeve and the photosensitive drum was reduced to 180 μm.

Therefore, the electric field intensity E _max generated during the development of V _L during T ₁ is 4.17 V / μm, and the electric field E _min for pulling back the toner from V _L is 0.28 V / μm, which is about the same as that described above. Since the second development is performed by the electric field, the density of the second toner image is about the same.

On the other hand, the electric field intensity E _T generated during T ₂ for stripping V _T is 2.78 V / μm, which exceeds the electric field intensity of Experiments 1 and 2 of Example 1, but the first toner is stripped. I confirmed that I could not take it. This is because, as shown in FIG. 5B, the peak-to-peak voltage value of the bias waveform is small, and therefore the change in bias at T _2-1 can be made smaller than that in the first embodiment. , E _T
Although it is larger than in Example 1, it is considered that the amount of peeling could be reduced.

By the way, the change in bias between T _2-1 is
In Experiment 1 of Example 1, it was 19.3 V / μsec, and in Experiment 2 it was about 2/3, which was 13.8 V / μsec, whereas in this example it was 6.7 V / μsec, and the slope was made extremely small. I am able to.

The amount of change in the bias is the peak-to-peak voltage (Vp) which is a parameter of the developing bias.
p), frequency (f), duty ratio, etc., but from the results of the comparative experiment in Experiment 1 of Example 1,
It is considered preferable that the voltage is 50 V / μsec or less.

When the amount of change in bias between T _2-1 and T _2-1 is reduced by narrowing the SD gap and lowering Vpp as in the present embodiment, for example, a bias waveform with a duty applied. Even if it is not, the peeling of the first toner can be prevented, so that the cost of the high voltage transformer of the bias power supply can be reduced. However, SD
If the gap is made too close, the second toner on the sleeve comes into contact with the first toner image on the photosensitive drum, and the possibility of disturbing the first toner image increases, so the SD gap is within a range that can prevent this. It is important to make it narrow.

In each of the above embodiments, the case where the second development is the one-component development system has been described.
It is also applicable to a component developing system in which a similar alternating bias is applied. At this time, since the developer is applied thickly on the developing sleeve of the two-component developing device, it is preferable to widen the SD gap to about 500 to 1000 μm. At this time, the developing bias is adjusted accordingly. Increase the peak-to-peak voltage.

The two-color image forming process has been described by using the so-called negative / negative recharging method, but the present invention uses another image forming process, for example, a negative / positive process, in which two types of toner having different polarities are used. It can also be applied to processes and the like. At this time, the transition time of the voltage is lengthened so that the inclination of the bias in the direction in which the voltage changes in the direction in which the second toner flies to the photosensitive drum is loosened among the rising and falling of the bias waveform. Good.

Further, the multi-color image is not limited to a two-color image, and it is also possible to superimpose three or more colors on a photosensitive member such as a photosensitive drum or photosensitive belt by development, and it is clear that the present invention can be similarly applied. Is.

[0065]

As described above, the present invention relates to the rising time and the falling time of the waveform of the developing bias during the second developing, of which the voltage changes in the direction in which the second toner flies to the photosensitive drum. Since the longer time is used and the change in the voltage during that time is made smaller, the collision of the second toner with the photosensitive drum becomes weaker, and the second toner becomes the first toner on the photosensitive drum formed by the first development. It is possible to prevent the first toner from being repelled from the toner image, and it is possible to prevent the first toner from entering the second developing device.

Further, by appropriately setting the time during which the developing-side bias (bias for causing the second toner to fly to the photosensitive drum) of the bias waveform is applied and the time for the pullback direction, At the time of development, fogging can be performed while preventing the first toner from being peeled off, so that a second toner image having a sufficiently high density and no fogging can be formed.

Therefore, according to the present invention, a good two-color image can be obtained over a long period of time. Similar effects can be obtained even when three or more colors are overlaid.

[Brief description of drawings]

FIG. 1 is a diagram showing a surface potential of a photosensitive drum during second development in Experiments 1 and 2 and a developing bias during second development in Experiment 1 of Example 1 of the present invention.

FIG. 2 is a cross-sectional view showing a second developing device used in Experiment 1.

FIG. 3 is a cross-sectional view showing a second developing device used in Experiment 2.

FIG. 4 is a diagram showing a developing bias at the time of second developing in Experiment 2.

FIG. 5 is a diagram showing a surface potential of a photosensitive drum and a developing bias at the time of second developing in Embodiment 2 of the present invention.

FIG. 6 is a schematic configuration diagram showing a two-color image forming apparatus.

FIG. 7 is a diagram showing a surface potential of a photosensitive drum in each step of forming a two-color image.

[Explanation of symbols]

 1 Photosensitive Drum 2 Charging Device 4 First Developing Device 5 Recharging Device 7 Second Developing Device 72 Developing Sleeve 73 Magnet Roller 72 'Developing Sleeve 76' Coating Roller

Claims (5)

[Claims] 1. At least two or more latent images are sequentially formed on an image bearing member, and each time each latent image is formed, the latent image is sequentially developed with a developer of a corresponding color, and at least the obtained image is obtained. In a multi-color image forming apparatus for collectively transferring two or more color toner images onto a transfer material, a development after a second developing device for developing a latent image after a second latent image among the developing devices for developing the latent image. In each of the containers, the developing bias applied to the developer carrying member is composed of alternating voltages, and in one cycle of the alternating voltage,
Toner is moved in the direction from the developer bearing member to the image bearing member, the voltage is applied in the developing direction at the application time T ₁ , the peak voltage V _max , and the toner is moved in the direction from the image bearing member to the developer bearing member. With the application time T ₂ and the peak voltage V _{min in} the return direction voltage, the time required to shift from the developing direction voltage to the returning direction voltage is T _1-2 , from the returning direction voltage to the developing direction voltage. The time required to make the transition is T _2-1
In the multicolor image forming apparatus, T _2-1 > T _1-2 . 2. The multicolor image forming apparatus according to claim ₁ , wherein the T ₁ , T ₂ , and T _2-1 are T ₁ > T _2-1 and T ₂ > T _2-1 . 3. The voltage change rate between the T _2-1 and the voltage change rate is | V _max −V _min | / T _2-1 <50 V / μ
The multicolor image forming apparatus according to claim 1, wherein the multicolor image forming apparatus is sec. 4. When the potential of the image portion of the latent image developed from the first latent image upon being recharged is V _T , and the closest distance between the developer carrying member and the image carrying member is d. _{, | V T -V min | /} d <2.3V / μm and a multi-color image forming apparatus according to claim 1, 2 or 3. 5. The multicolor image forming apparatus according to claim 1, wherein the developing bias is a duty bias.