JPH05142918A - Image forming device - Google Patents

Abstract

PURPOSE:To prevent the disturbance of images in overlap development, color mixing and fogging by adjusting the duty ratios, peak values and frequencies of the bias voltages of 1st and 2nd developing devices for each of the respective developing devices. CONSTITUTION:The developing bias voltage impressed to a 1st developing device 5 repeats the phase to form the 1st electric field which apply the force toward an image carrying member 1 from a developer carrying member 13 to a toner in a developing section during the time t11 and the phase to form a 2nd electric field in an opposite direction during the time t21. The developing bias voltage impressed to a 2nd developing device 6 repeats the phase to form the 3rd electric field in the same direction as the direction of the 1st electric field in the developing section during the time t12 and the phase to form a 4th electric field of the opposite direction during the time t22. The ratio t12/t22 of the time is smaller than t11/t21 and the sum t12+t22 of the time is smaller than t11+t21. The peak value in the time t12 is larger than within the time t11 and the peak value in the time t22 is smaller than within the time t21.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus for forming toner images of plural colors on an image carrier.

[0002]

2. Description of the Related Art Conventionally, in a multi-color image forming apparatus, a first toner image is formed by a first developing device, and a second developing image is formed on the surface of an image carrier carrying the first toner image. In some, the device acts to form a second toner image. In this case, it is desired to prevent the second developing device from disturbing the first toner image and preventing the toner of the first toner image from mixing into the second developing device.

A developing bias voltage consisting of only a DC voltage component is applied to a developing device on the downstream side in the rotation direction of the photosensitive drum, which is an image carrier, and a visible image is formed without bringing the developer into contact with the outer peripheral surface of the photosensitive drum. Are known to form. However, according to this method, although the effect of preventing color mixture is good, there are some problems in terms of image quality, reproducibility of line and gradation, and uniformity of a solid image.
In particular, these problems tended to be more noticeable in the multicolor overlapping developing section. The reason for this is that when the developer flies from the developing sleeve to the photosensitive drum, there is a threshold electric field. Below this electric field, the latent image on the photosensitive drum is not developed and the above-mentioned image quality deterioration occurs. This is because Since the developing electric field is determined by the potential difference between the latent image potential on the photosensitive drum and the DC voltage applied to the developing sleeve, it is necessary to narrow the developing gap to obtain a sufficient flying electric field. Required high mechanical accuracy. Further, since it is necessary to increase the latent image potential, it is necessary to use a photosensitive drum having a high charging property.

Therefore, a method has been proposed in which, instead of the DC voltage, an AC voltage is applied to the developer carrying member for development.

Use of an AC voltage can alleviate the above-mentioned problems.

However, in the case of developing a plurality of toner images on the photosensitive drum by the AC voltage, a force for transferring the toner from the photosensitive drum to the developer carrying member works, so that color mixing easily occurs. In order to prevent this, for example, there is a method of sequentially changing the peak-to-peak voltage (Vpp) or frequency (f) of the AC voltage for each developing process.

However, these methods are not sufficient for preventing color mixture, and the image quality greatly changes in each developing process, resulting in poor color reproduction in multicolor image formation.

[0008]

Therefore, in accordance with the order of the number of times of development, the time t1 during which an electric field is formed to urge the developer in the direction from the developer carrying member to the photoconductor and the development from the photoconductor. The ratio (duty ratio) t1 / t2 to the time t2 during which an electric field in the direction of urging the developer is applied to the agent carrier is gradually decreased, and the peak of the AC voltage at the phase t1 is obtained. By sequentially increasing the value and sequentially decreasing the peak value of the AC voltage at the t2 phase, a fine image is ensured, and the previous color mixture of toner on the photoconductor at the time of overdevelopment is prevented. However, in the following situations, even if the above method is adopted,
It is not always possible to obtain a good image.

(1) When the charge amount of the toner is reduced in a high humidity environment, etc., the mirroring power of the toner of the developed image formed on the photoconductor to the photoconductor is reduced, and the degree of color mixing is increased. The fog toner increases.

(2) When the diameter of the developing sleeve is reduced in accordance with the thinning and downsizing of the developing device, the surface area of the sleeve is reduced, and the degree of contact between the sleeve surface and the toner is reduced, and the charge amount of the toner is reduced. The problem similar to that of the above (1) occurred due to the decrease of the value.

[0011]

According to the present invention, the first developing bias voltage applied to the first developing device is the first electric field that gives the toner a force in the direction from the developer carrier to the image carrier. During the time t11, the phase formed in the first developing portion during the time t11 and the second electric field in the direction opposite to the first electric field are applied during the time t2.
1 is a voltage at which the phase formed in the first developing unit is repeated, and the second developing bias voltage applied to the second developing unit on the downstream side of the first developing unit is equal to the first developing unit. A third electric field in the same direction as the electric field is formed in the second developing portion during the time t12, and a fourth electric field opposite to the third electric field is applied during the time t22. Is a voltage at which the phase formed in the part is repeated, and the ratio of the time (t12 /
t22) is smaller than (t11 / t21), and the sum of the above times (t12 + t22) is smaller than (t11 + t21), which is smaller than the peak value of the first developing bias voltage within the time t11. Development bias voltage time t1
2 is large, and the peak value of the second developing bias voltage within the time t22 is smaller than the peak value of the first developing bias voltage within the time t21.

As a result, even when the charge amount of the toner is reduced, such as in a high humidity environment, the peak value of the electric field in the direction of urging the developer with the force in the direction from the image carrier to the developer carrier is reduced. While reducing the sum of the time during which the electric field is formed and the time during which the electric field is generated in the direction that urges the developer from the developer carrier to the image carrier. Thus, while preventing color mixture and reducing fog, at the same time, the peak value of the electric field in the direction of urging the developer from the developer carrying member to the image carrying member is gradually increased. Therefore, even if the layer thickness of the multi-layer developer image formed on the developer carrying member is large, the development can be surely performed.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, a first embodiment of the present invention will be described with reference to FIGS.

In FIG. 2, reference numeral 1 is an electrophotographic photosensitive drum which is an image carrier. The photosensitive drum 1 rotates in the direction of the arrow. A corona discharger 2 for uniformly charging the surface of the photosensitive drum 1 is arranged above the photosensitive drum 1.
The drum surface charged by the corona discharger 2 is irradiated with the image forming light flux 3 of the copy original D placed on the document table 4 on the downstream side of the corona discharger 2 in the rotation direction of the photosensitive drum 1. To form an electrostatic latent image. The latent images formed on the surface of the drum are of four types, which are arranged side by side in the rotational direction of the drum, facing the photosensitive drum, on the downstream side of the exposure position in the rotational direction of the photosensitive drum 1. It is developed by one or two or more of the developing devices 5, 6, 7, and 8. The obtained toner image is transferred onto the transfer paper 71 by the transfer charger 72. Transfer paper 7
1 is sent to the transfer portion by the roller 73, and the transfer sheet after transfer is discharged to the outside of the image forming apparatus through the toner image fixing device 74. The multi-color toner images sequentially formed on the drum surface by the plurality of developing devices are transferred onto the same transfer paper surface at the same time. After the required number of color images are transferred, the cleaning device 75 removes the residual toner on the drum. In other words, instead of transferring the first toner image to the transfer sheet and then returning the transfer sheet to the transfer section to transfer the second toner image to the transfer section, the transfer sheet is allowed to pass through the transfer section once. Thus, the toner images of a plurality of colors are transferred onto the transfer paper.

Next, referring to FIG. 3, the developing devices 5, 6,
7 and 8 will be described in more detail.

Each of the developing devices 5, 6, 7 and 8 has a non-magnetic developing sleeve 13 or 1 as a developer carrying member in which magnets 9, 10, 11 and 12 as magnetic field generating means are fixedly arranged.
It has 4, 15 and 16, respectively. The developing sleeves are respectively arranged in the developer accommodating containers 5A, 6A, 7A, 8A facing the surface of the photosensitive drum 1 in the developing section, and are rotated counterclockwise respectively to the developing section. Is carried and conveyed. Around the periphery of each developing sleeve, a regulating blade 17, 18, 19, 20 which is a developer regulating plate for regulating the amount (layer thickness) of the developer on the developing sleeve conveyed to the developing section.
And scraping blades 21, 22, 23, 24 for scraping off the developer on the developing sleeve.

A two-component developer containing a mixture of non-magnetic toner and magnetic carrier particles 37 is contained in each developer container. Toner 3 in the developing device 5
8, toner 39 in the developing device 6, toner 40 in the developing device 7, and toner 41 in the developing device 8 are yellow, magenta, cyan, and black, respectively. Toner supply screws 25, 2 for supplying these toners are provided in each container.
6, 27, 28 and a stirring plate 29 for stirring the developer
30, 31, 32 are arranged.

The magnetic carrier particles 37 have an average particle diameter of 30 to 100 μm, preferably 40 to 80 μm and a resistance value of 10 ⁷ Ωcm or more and 10 ¹² Ωcm or less, preferably 10 ⁸ Ωcm or less.
Ωcm or more and 10 ¹⁰ Ωcm or less. Such magnetic carrier particles include ferrite particles (maximum magnetization 60 em).
A very thin resin coating on u / g) can be used.

The resistance value of the magnetic particles was measured by using a sandwich type cell having a measuring electrode area of 4 cm ² and an interelectrode gap of 0.4 cm. One electrode under a pressure of 1 kg and a voltage E between both electrodes. (V) is applied and the resistance value of the magnetic particles is obtained from the current flowing in the circuit.

The developing sleeves 13, 14, 15, 16
Is an AC power supply (for example, a peak-to-peak voltage of 100V to 3K
Bias power supply 33, 34, 3 composed of V, frequency 100 Hz to 5 KHz) and a DC power supply (for example, 1 KV or less)
5,36 is connected, the DC voltage V _DC voltage oscillates on both sides of the superimposed alternating voltage (0V or positive, or a voltage that oscillates only in the negative side in, a sine wave, square wave,
(Having a waveform such as a triangular wave) is applied to each sleeve.
As a result, an alternating electric field whose direction is alternately reversed is formed in each developing portion.

A minute gap is provided between each developing sleeve and the photosensitive drum (the gap at the closest portion between the drum and each sleeve is, for example, 1 mm or less).

In each developing section, the thickness of the developer layer on the sleeve is smaller than the minimum gap between the sleeve and the photosensitive drum. That is, each regulation blade 17, 18, 19, 20 regulates the thickness of the developer layer on each sleeve in that way.
In any case, each developing device is a so-called non-contact type developing device.

The developing devices 6, 7, and 8 are preferably non-contact type developing devices as described above so as not to scrape off the toner of the toner image formed on the upstream side thereof, but first, the toner The developing device 5 that forms an image may be a contact-type developing device, that is, a developing device that develops a latent image by bringing a developer layer into contact with a drum.

However, in order to obtain a higher quality image without the traces of sweeping by a magnetic brush, the drum is rotated a plurality of times while the toner image is carried, and a different developing device is operated for each rotation to make a plurality of images. In the case of an image forming apparatus that forms color toner images in a superimposed manner, the developing device 5 is also preferably a non-contact type developing device. Therefore, the case where the developing device 5 is also a non-contact type will be described below as an example.

In this embodiment, the photosensitive drum 1 is negatively charged on the entire surface by the corona discharger 2 and then imagewise exposed by the image-forming light beam 3 reflected from the original, and the developing devices 5 to 5 are used.
8 is selected and development is performed.

For example, if the developing devices 5 and 6 are operated, the developing device 5 first develops the electrostatic latent image on the photosensitive drum with yellow toner, and subsequently, the same electrostatic latent image (yellow toner image). The developing device 6 develops magenta toner on the upper surface and sequentially superimposes it. As a result, the visible image after fixing has a color tone (red) different from that of yellow and magenta.

Similarly, an image having an arbitrary color tone is formed on each developing device 5-8.
It can be obtained by superimposing while adjusting the adhesion amount by selecting.

The black image can be reproduced only by the developing device 8.

The toner image thus obtained is collectively transferred and fixed on a transfer sheet by a transfer charger 72 as shown in FIG.

Next, the behavior in the developing section will be described in detail with reference to FIGS. 1 and 4. Since the developing devices have the same configuration, the developing device 5 will be described as an example here.

The photosensitive drum 1 carries electric charges forming a latent image, but in the present embodiment, the electric charges forming an electrostatic latent image have a negative polarity, and the toner is separated from carrier particles so as to perform reversal development. It is negatively charged due to friction.

Further, in this embodiment, the photosensitive drum 1
The developing sleeve 13 and the developing sleeve 13 rotate as indicated by arrows so that they move in the same circumferential direction in the developing section.

An AC electric field is formed in the space between them by the bias power supply 33.

On the other hand, a magnetic pole N of N polarity is located on the upstream side of the closest portion between the photosensitive drum 1 and the developing sleeve 13, and a magnetic pole S of S polarity is located on the downstream side. The magnetic pole S may be arranged on the upstream side of the closest portion and the magnetic pole N may be arranged on the downstream side. In any case, by arranging a pair of magnetic poles adjacent to each other and having different polarities as described above, a strong magnetic field having a component in the tangential direction of the sleeve peripheral surface is formed in the developing portion. As a result, as shown in FIG. 4, the magnetic carrier particles 37 are chained along the sleeve surface. That is, since the chains of the carrier particles lie on the surface of the sleeve, an extremely thin layer of the two-component developer which is not in contact with the drum can be formed in the developing portion.

Since the amount of the developer itself conveyed to the developing section is small, the chains of the magnetic carrier particles are not so dense, and the toner is supplied from the magnetic carrier particles to the drum and adheres to the sleeve surface. The toner is also supplied to the drum through the chains.

Further, since development with an alternating electric field has high development efficiency, a toner image having a sufficient density can be obtained even with a thin developer layer.

As shown in FIG. 1, in the present embodiment, the latent image includes the image portion V _L (light portion potential portion illuminated by light) and the non-image portion V _D (dark portion potential not illuminated by light). Part) is also a negative potential (however, the absolute value of the non-image part potential is larger than the absolute value of the image part potential).

The toner is also negatively charged.

By the way, the direction of the electric field in the developing section is an alternating electric field, and therefore, as shown by arrows a and b in FIG. 4, the electric field is alternately and repeatedly changed between the photosensitive drum and the developing sleeve. In the phase (represented by t11 in FIG. 1) in which the negative component of the bias voltage B1 (the superimposed DC voltage component is V _DC1 ) is applied, the direction of the electric field due to this is the b direction.

Further, since the negative charge is injected from the sleeve into the carrier particles having the above-mentioned electric resistance value, the direction of the electric field is the b direction, so that the carrier particles have an a direction opposite to the b direction. A directional force is applied so that the chains are generally raised in the manner of a drum, and the toner easily separates from the chains and the sleeve surface. Since the toner 38 attached on the surfaces of the developing sleeve 13 and the magnetic particles 37 is negatively charged as described above, it receives a force in the a direction due to the electric field in the b direction formed in this space. The photosensitive drum 1 moves to the bright area potential area.

Further, in the phase in which the positive component of the AC potential B1 is applied to the developing sleeve 13 (represented by t21 in FIG. 1), the direction of the electric field formed in the developing portion (arrow a) is The direction is opposite to the direction (arrow b). Therefore, the chain is brought into contact with the developing sleeve in the contracted state by receiving the force in the b direction by the electric field in the a direction.

On the other hand, the toner 38 on the photosensitive drum 1 is charged in the negative polarity as described above, and therefore receives the force in the direction of arrow b due to the electric field in the direction of a. That is, a part of the toner on the photosensitive drum 1 moves backward to the developing sleeve 13 or the magnetic particles 37 at the phase t21.

Phase t11 and t21 are alternately repeated, the developer repeats the above-described movement, and the development is completed by expanding the gap between the sleeve and the drum as the sleeve rotates, but at that point. The amount of toner corresponding to the potential of the electrostatic latent image remains on the drum. That is, a toner image is formed.

[0044] Since reversal development is performed in the embodiment, the toner adheres to a region of the light potential V _L, the dark potential V _D
Area is a background area where toner is not substantially adhered.

In this embodiment, the second developing device causes the second developing device to superimpose the toner of the second color onto the light potential region to which the first developing device has adhered the first color of toner.

Although the absolute value of the light potential region increases by about 10V to 50V due to the adhesion of the toner of the first color,
Since there is a sufficient potential difference from the dark part potential, a toner image of the second color having a sufficient density can be obtained even when the development is performed by the second developing device as described above.

In any case, the color mixing, which has been a problem of the conventional apparatus, occurs because the toner of the first toner image moves backward from the drum to the sleeve as described above during the development of the second and subsequent colors. A method is used to suppress this reverse movement when developing two or more colors.

The operation of the image forming process of the present invention will be described in detail below.

In FIG. 1, what is indicated by a broken line is an AC bias voltage B2 (superimposed DC voltage component is V _DC2 ) applied to the sleeve at the time of developing the second and subsequent colors. When the negative component of the AC bias voltage B2 is applied to the sleeve 14 of the second developing device, for example, the developing device 6 at the phase t12, the direction of the electric field of the developing portion is the b direction, so that the toner 39 is forced in the direction of the arrow a. In the same manner as described above, the toner 39 is separated from the magnetic carrier particles and the surface of the sleeve and moves to the light potential region of the drum 1. Next, when the positive component of the AC bias voltage B2 is applied to the sleeve 14 at the phase t22, the electric field in the developing section becomes the electric field in the direction of arrow a. However, the peak value V _P22 of the bias voltage B2 at the phase t22 is smaller than the peak value V _P21 of the bias voltage B1 at the phase t21. (In this specification, the magnitude of the peak value means the magnitude of the absolute value). Therefore, the electric field in the developing section at phase t22 (the electric field that gives the toner a force from the drum to the sleeve) is weak, and the phase at the first bias voltage B1 is t.
Compared to the sum of 11 and the phase t21 (t11 + t21), the sum of the phase t12 and the phase t22 at the second bias voltage B2 (t1
2 + t22) is reduced (that is, the frequency is sequentially increased). Therefore, from t21 / t11 (the ratio of the return bias application time for the first color and the skip bias application time),
t22 / t21 (Even though the ratio of the application time of the second color return bias and the application time of the skip bias is large, the return bias time of the second color can be relatively shortened, and the toner forming the first toner image can be obtained. 38 hardly moves to the sleeve 14 of the second developing device, which prevents color mixture.

By the way, when the charge amount of the toner is reduced as a whole in a high humidity environment, among others, the toner having an abnormally low charge amount adheres to the photoconductor as fog toner.
As a function of reducing this fog toner, the phase t21,
Of course, the voltage component of t22 greatly contributed, but in the experiment, it was found that the lengths of the phases t11 and t12 also greatly contributed. That is, according to the experiments, it was found that when the charge amount is low overall, the time during which the toner does not fly from the sleeve to the drum contributes to the fog prevention more than the voltage component that returns the toner from the drum to the sleeve.

That is, if the time for which the toner is made to fly from the sleeve to the drum is short, the toner (fogging toner) having an abnormally low charge amount cannot respond at that time, remains in the sleeve, and then enters the developing container as the sleeve rotates. Since it is returned, the fog toner on the drum is reduced.

In the present invention, since the voltage component of the phase t22 is relatively smaller than the voltage component of the phase t21, the fog prevention function is weak from this point, but the length of the phase t12 is set to be relatively shorter than the phase t11. Therefore, the function of preventing fogging is enhanced in this respect, and the effect becomes remarkable in a high humidity environment.

On the other hand, the time of the phase t12 for moving the toner from the sleeve to the drum in the second developing device is shortened in order to improve the function of preventing the above-mentioned fog and color mixture. From this point, the image density is sufficiently high. You won't get it. However, as shown in FIG. 1, since the peak voltage V _P12 is made relatively higher than V _P11 , a sufficient amount of toner can be attached to the image area.

As is apparent from the above, in the case of carrying out the overlapping development of the toners of three colors, the developing device used for the third color,
For example, the relationship between the AC bias voltage applied to the sleeve 15 of the developing device 7 and the AC bias voltage B2 applied to the sleeve 14 of the second developing device is described as follows.
The relationship may be set to be similar to the relationship between 2 and B1.

The phases of the AC bias voltage B3 applied to the sleeve 15 and the AC bias voltage B4 applied to the sleeve 16 which form the electric field in the b direction are t1 respectively.
3, t14, and the phase forming the electric field in the a direction is t
23 and t24, the phase t13 of the AC bias voltage B3
And the peak value at phase t23 are V
Let _P13 and V _P23 be the peak value of the AC bias voltage B4 at the phase t14 and the peak value at the phase t24 be V _P14 and V _P24 , respectively. When developing with four color toner in the apparatus of FIG. The time ratio of each phase of each bias voltage, the time sum of each phase, and each peak voltage may be set as follows.

(T21 / t11) <(t22 / t12)
<(T23 / t13) <(t24 / t14) (t21 + t11)> (t22 + t12)> (t23 +
t13)> (t24 + t14) V _P11 <V _P12 <V _P13 <V _P14 V _P21 > V _P22 > V _P23 > V _P24 (Note that the magnitude relationship of the voltage peak value is the magnitude relationship in absolute value.)

The experimental results by the apparatus of FIG. 3 are shown below.

(Experimental Example 1) First, the peripheral speed of the developing sleeve is 210 mm / sec, and the peripheral speed of the photosensitive drum is 160 m.
It was set to m / sec.

The developing sleeves each have a diameter of 20 mm.
The surface of the stainless steel (SUS316) sleeve of
The number 400 particles were sandblasted with irregular grains, and each magnet was N-pole magnetized with 6 poles.
The poles and the S poles were alternately magnetized as shown in FIG. The gap between each developing sleeve and the tip of each regulating blade was set to 350 μm.

As the regulating blade, non-magnetic stainless steel having a thickness of 1.2 mm was used, and as the magnetic carrier particles, the average particle diameter of the surface coated with a very thin silicone resin was 6
A ferrite having a true density of 5.16 g / cm ³ (maximum magnetization of 60 emu / g) of 0 to 50 μm was used.

As the non-magnetic electrically insulating toner, a toner having an average particle diameter of 8 μm, which is obtained by mixing about 5 parts of pigment with 100 parts of polyester resin, was used. As the pigment, a copper phthalocyanine pigment was used for the blue toner, a disazo pigment was used for the yellow toner, and a monoazo pigment was used for the magenta toner.
As the black toner, a mixture of the above pigments in a ratio of 1: 2: 1 was used. To each of the toners, 0.4% of colloidal silica is externally added to improve the fluidity.

The developer layer formed on the developing sleeve has a thickness of 300 μm in the developing area. Further, the relationship between the weight C of the magnetic carrier and the weight T of the toner {T / (C + T)} × 10
0 is about 8 to 12%. The toner 38, 39 had a charge amount of about −15 μcoul / g.

The developer is erected by the magnetic field at the magnetic pole position in the developing sleeve except the developing portion, and the maximum length is about 0.8 to
A layer made of a magnetic brush with toner adhering to a height of about 1.3 mm was formed. 270 g of magnetic particles and 30 g of toner were mixed and used as the start developing section.

This developing device was incorporated into the color image forming device shown in FIG. 3, and the minimum gap between the photosensitive drum 1 (made of an organic photosensitive material) and the surface of the developing sleeve 13 was set to 500 μm. The ratio of the peripheral speed of the photosensitive drum to the peripheral speed of the developing sleeve is 1: 1.3. Further, the coating amount M (g / cm ² ) of the developer on the developing sleeve when it was not panicked was set to 35 mg / cm ² . The photosensitive drum used had an outer diameter of 160 mm. An OPC drum is used as the photosensitive drum, and a dark portion potential (non-image portion potential) V _D is −600 V and a light portion potential (image potential) _VL is −2.
The charged latent image potential was 50V. Bias power supply 33 is t2
1 / t11 = 1, frequency f ₁ = 1800 Hz, peak-to-peak value Vpp (voltage between peaks) A rectangular wave AC voltage of 1800 V and a DC voltage of −490 V superposed on each other are applied to the developing sleeve 13 of the developing device 5. ..

The peak-to-peak value Vpp of the developing sleeve 14 of the developing device 6 is the same as the bias voltage applied to the developing sleeve 13, t22 / t12 = 4, and the frequency f ₂ = 20.
A voltage obtained by superimposing a DC voltage of -790 V on a rectangular wave AC voltage of 00 Hz was applied from the bias power source 34. As a result of developing the latent images in order with the developing devices 5 and 6, a clear red image without color unevenness was obtained. Further, even during long-term use of continuous copying, the toner 38 of the developing device 5 was not mixed into the developing device 6, and a clear image was maintained.

(Experimental Example 2) Next, the result of three-color overlapping development using the third developing device 7 in addition to the case of using the two developing devices 5 and 6 of Experimental Example 1 will be shown.

The alternating voltage shown in Experimental Example 1 was applied to the developing sleeve 13 of the developing device 5. The peak-to-peak value Vpp of the developing sleeve 14 of the developing device 6 is the developing sleeve 13.
The same as the bias voltage applied to the rectangular wave AC voltage of t22 / t12 = 3 and frequency f ₂ = 2000 Hz of −690.
An AC voltage on which a DC voltage of V was superimposed was applied from the bias power supply 34.

The peak-to-peak value Vpp of the developing sleeve 15 of the developing device 7 is the same as the bias voltage applied to the developing sleeve 13, and the frequency f ₃ = 2200 at t23 / t13 = 5.
An AC voltage in which a DC voltage of −840 V was superimposed with a rectangular wave of Hz was applied from the bias power source 35.

As a result of the experiment by the above setting, a good image having no color unevenness was obtained as in the case of Experiment 1, and the toners 38 and 39 were hardly mixed into the developing devices 6 and 7.

From the results of various experiments, the t2 / t1 value of the alternating voltage in the case of over-developing was sequentially increased according to the order of the number of times of development, and t22 / t12 = 2 in the second developing.
.About.6, and in the third development, it was found that when the range of t23 / t13 = 3 to 10 was maintained, color mixture was prevented and a good image was obtained.

Experimental Example 3 Furthermore, as a preferable example,
When the triboelectric charge amount of toner in each development is sequentially changed and used, a better image can be obtained, and mixing of different color toners into the developing device can be prevented. The charge amount of the toner is set to a slightly low toner concentration. For example, in the developing device 5 according to the above experimental example, the relationship between the superposition C of magnetic carrier particles and the toner weight T is (T / C + T) × 100. 8%, 10% in the developing device 6 and 12% in the developing device 7, the triboelectric charges of the toners 38, 39 and 40 are −23, −18 and −15 μcoul / g, respectively. It was possible to prevent mixing of different color toners into No. 7 and to prevent fogging even at a toner charge amount of −15 μc / g in the developing device 7 for the third color because the time of t13 is set small. Here, the reason why it is possible to prevent the mixing of different color toners is that the triboelectric charge amount of the toner is set to an absolute value large enough to trace back to the previous developing process. This is because the toner strongly adheres electrostatically to the upper part, and the toner adhered to the drum in the previous developing process does not easily return to the developing sleeve in the latter developing process under an AC electric field in the latter developing process. .. However, the first toner 38 and the second toner 3
9 is electrostatically stronger than the case of Experimental Example 2 and adheres to the carrier particles and the sleeves 13 and 14, so that t11 and t1
In order to give the toner a stronger force toward the drum in the phase of 2, the superimposed DC component of the bias voltage applied to the sleeves 13 and 14 may be set to be larger by 10 to 50 V absolute value than in the case of Experimental Example 2. preferable. For example, sleeve 1
The superimposed DC voltage component of the AC bias voltage applied to 3
520V, and the superimposed DC voltage component of the AC bias voltage applied to the sleeve 14 may be -710V.

Further, for a developing device not used for developing, the developing device is separated from the photosensitive drum, and an electric bias for preventing the toner from flying is applied to the sleeve to prevent unnecessary toner from adhering to the photosensitive drum. You can

Further, a one-component developer may be used as the developer.

The image forming process is not limited to the case of the above-described mono-color process, but the first toner without the transfer and cleaning steps after the first development (in this embodiment, the development by the developing device 5) is completed. The entire surface of the drum 1 carrying the image is charged again by the corona discharger 72, and the second image exposure and development (in this embodiment, the development device 6 is performed) are performed. It is also possible to use a process of performing the charging, exposing and developing processes for the second time, and then collectively transferring the image composed of the four color toners onto the transfer paper.
Further, in such a process, a multicolor image can be formed by using a color separation means or a masking means at the time of exposing each image.

Next, a second embodiment of the present invention will be described with reference to FIG. The same parts as those in the first embodiment are designated by the same reference numerals and the description thereof will be omitted.

In this embodiment, a recharging means and an image exposing means are provided between the developing devices to carry out the recharging and the second image exposing / developing after the first developing.

The first and second exposure beams 45 and 46 are generated by the laser optical system based on drive command signals output from an image signal controller (not shown) corresponding to the first and second image signals, respectively. , The surface of the photosensitive drum 1 is scanned. In this process, the photosensitive drum 1 is uniformly charged by the first charger 55, the first exposure beam 45 is irradiated to form the first latent image, and the one component arranged close to the photosensitive drum 1 is formed. Development is performed by the developing device 51 containing the non-magnetic developer black toner 50.

Then, the surface of the photosensitive drum carrying the first toner image is charged by the second charger 56, and then the second exposure beam 4 is applied to the surface of the photosensitive drum carrying the first toner image.
6 is irradiated to form a second latent image, and the red toner 6 which is a one-component non-magnetic developer and is arranged in proximity to the photosensitive drum 1.
Development is performed by the developing device 61 containing 0. As described above, the steps of charging, image exposure and development are sequentially performed to form toner images of a plurality of colors on the photosensitive drum, and the toner images are collectively transferred to the transfer agent.

The black toner 50 and the red toner 60 in the developing devices 51 and 61 are fur brushes 52 and 61, respectively.
Is supplied to the developing rollers 54 and 64. The fur brushes 52 and 62 not only agitate the toner in the developing devices 51 and 61, but also disturb the toner on the developing roller after development to prevent so-called ghost. Similar effects can be obtained by using an open-cell foam or a single-cell rubber sponge instead of the fur brush.

The rollers 54 and 64 rotate in the directions of the arrows to carry and convey the developer to the developing portions, and the layer thickness of the developer conveyed to the developing portions is regulated by the regulating blades 53 and 63, respectively. The blades 53 and 63 are elastic blades having a two-layer structure in which rubber is attached to a rubber plate, a metal leaf spring, or a metal plate, and are in contact with the rollers 54 and 64 with a light pressure, respectively, and the developer conveyed to the developing unit. Is regulated to be thinner than the gap between the drum 1 and the rollers 54 and 64. Further, the blades 53 and 63 transfer the developer to the developing rollers 54 and 64, respectively.
It is rubbed against and charged with friction.

Developing bias power sources 57 and 65 are connected to the developing rollers 54 and 64, respectively, and an AC electric field is formed between the photosensitive drum 1 and the developing rollers 54 and 64.

(Experimental Example 5) In the apparatus shown in FIG. 5, the gap between the photosensitive drum 1 and the developing rollers 54 and 64 is regulated to about 300 μm, and the toner layer thickness on the developing rollers 54 and 64 is regulated to about 400 μm. Was -20 μcoul / g and the red toner 60 was -15 μcoul / g.

Dark portion potential (non-image portion potential) V _{D of} the first latent image
Is -600 V, the light portion potential (image portion potential) _VL is -250
Set to V, t21 / t11 = 1, frequency 1800H
z, the peak-to-peak value Vpp = 1400V rectangular wave AC voltage, DC voltage -500V superimposed bias voltage,
Power was applied to the developing roller 54 from the power source 57.

The dark portion potential of the second latent image is set to -650.
V, bright part potential is set to -280V, t22 / t12 =
4, the frequency is set to 2000 Hz, and a bias voltage obtained by superimposing a DC voltage of −800 V on the rectangular wave AC voltage having the same peak-to-peak value as the bias voltage applied to the developing roller 54 is supplied from the power source 65 to the developing roller 64. Applied.

Also in this embodiment, a good image having no color unevenness was obtained as in the previous embodiment, and the black toner 50 was not mixed into the developing device 61.

In the above embodiment, in the first developing device,
The toner was transferred from the developer carrying member to the image carrying member by an AC electric field, and then the toner was transferred from the image carrying member to the developer carrying member in reverse. However, this is not essential, and the developer carrying member and the image carrying member are The development may be carried out by bringing them into contact with each other. In this case, the amount of charge of the toner is further increased by the rubbing at the contact portion to further prevent the first developing toner from being mixed into the second developing device.

In the above-described embodiments, the peak-to-peak voltage Vpp of each bias voltage is the same, but even if Vpp for the second color is made smaller than that for the development for the first color, the same effect on color mixture and fog is obtained. Is obtained.

In the above embodiment, the reversal development in which the toner adheres to the exposed portion (bright portion potential portion) on the surface of the photosensitive drum is described, but the regular development in which the toner adheres to the non-exposed portion (dark portion potential portion) is described. Can also be applied to. In the case of regular development, toner charged to the opposite polarity to the latent image polarity is used.

Further, although the negative electrostatic latent image is developed in the above embodiments, the present invention is applicable to an image forming apparatus for forming a positive electrostatic latent image and performing reversal development or regular development on the electrostatic latent image. Can also be applied.

[0090]

As described above, according to the present invention,
Since the duty, peak value, and frequency of the bias voltage in the first developing device and the second developing device are adjusted for each developing device, image disturbance, color mixture, and fog in overdeveloping are prevented. Furthermore, it was possible to prevent the mixing of toner of other colors in each developing device in a high humidity environment.
Therefore, it is possible to provide a high-speed ultra-small color copying machine which simplifies maintenance and is inexpensive.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a bias voltage according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram of an example of an image forming apparatus to which the present invention can be applied.

FIG. 3 is an explanatory view of a main part of the apparatus shown in FIG.

FIG. 4 is an explanatory diagram of a developing unit.

FIG. 5 is an explanatory diagram of a main part of another embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Photosensitive drum 5 1st developing device 6 2nd developing device 7 3rd developing device 8 4th developing device

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI technical display location G03G 15/09 Z

Claims (8)

[Claims] 1. An image carrier that sequentially moves through a first developing unit and a second developing unit, and a first member disposed opposite to the image carrier along a moving direction of the image carrier. An image forming apparatus including a first developing device and a second developing device, wherein the first developing device includes a first developer containing a toner of a first color, and the first developing unit. The second developing device has a first developer carrying member that conveys to the image carrying member to form a first toner image on the image carrying member, and the second developing device includes a first developer containing a second color toner. A second developer carrying member that conveys to the second developing unit and forms a second toner image on the image carrying member carrying the first toner image; A bias voltage applying means for applying a first developing bias voltage to the carrier and applying a second developing bias voltage to the second developer carrier. In the image forming apparatus according to the first aspect, the first developing bias voltage forms a first electric field in the first developing section for a time t11, which gives a first electric field to the toner in a direction from the developer bearing member toward the image bearing member. And a phase for forming a second electric field in the opposite direction to the first electric field in the first developing section for time t21 are repeated, and the second developing bias voltage is A third electric field in the same direction as the first electric field is formed in the second developing portion for a time t12, and a fourth electric field opposite to the third electric field is generated for a time t22. It is a voltage at which the phase formed in the second developing section is repeated, and the time ratio (t12 / t22) is (t11 / t21).
The sum of the above times (t12 + t22) is smaller than (t
11 + t21), the peak value of the second developing bias voltage within the time t12 is larger than the peak value of the second developing bias voltage within the time t11, and the first developing bias voltage The image forming apparatus is characterized in that the peak value of the second developing bias voltage within the time t22 is smaller than the peak value within the time t21. 2. The image forming apparatus according to claim 1, wherein the first color toner and the second color toner have the same charge polarity. 3. The image forming apparatus according to claim 1, wherein the peak-to-peak voltage of the first developing bay ice voltage and the peak-to-peak voltage of the second developing bay ice voltage are set to be the same. .. 4. The image forming apparatus according to claim 1, wherein the peak-to-peak voltage of the second developing bias voltage is set smaller than the peak-to-peak voltage of the first developing bias voltage. .. 5. The second developing device comprises a second developing section,
5. A means for restricting the second developer so that the layer thickness of the second developer is thinner than the minimum gap between the second developer carrier and the image carrier. The image forming apparatus described. 6. The second developing device is provided with a second developer containing magnetic carrier particles and a magnetic field generating means fixed in the second developer carrying member. Is
It has two adjacent magnetic poles having mutually different polarities, the first magnetic pole is located upstream of the closest position between the image carrier and the second developer carrier, and the second magnetic pole is The image forming apparatus according to claim 5, wherein the image forming apparatus is located downstream of the contact position. 7. The image forming apparatus according to claim 6, wherein the charge amount of the second color toner is smaller than the charge amount of the first color toner. 8. The image forming apparatus according to claim 1, further comprising a transfer unit that transfers the first toner image and the second toner image onto the same transfer material surface at the same time. ..