JP3236042B2 - Image forming device - Google Patents

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 a plurality of color toner images on an image carrier.

[0002]

2. Description of the Related Art Conventionally, in a multicolor 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 cases, the device operates 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 from mixing the toner of the first toner image into the second developing device.

[0003] A developing bias voltage comprising only a DC voltage component is applied to a developing device on the downstream side in the rotation direction of a photosensitive drum as an image carrier, and a visible image is formed without bringing a developer into contact with the outer peripheral surface of the photosensitive drum. Are known. However, according to this method, although the effect of preventing color mixture is good, there is a problem that the reproducibility of lines and gradation and the uniformity of a solid image are inferior in terms of image quality.
In particular, these problems tend to be more conspicuous in the multicolor overlapping developing section. The reason is that when the developer flies from the developing sleeve to the photosensitive drum, the threshold electric field exists.Below this electric field, the latent image on the photosensitive drum is not developed, and the above-described image quality deterioration occurs. It is because it becomes. Also, 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 gap between the developments to obtain a sufficient flying electric field. Required high mechanical accuracy. In addition, a photosensitive drum having good chargeability must be used because the latent image potential needs to be increased.

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

[0005] The use of an AC voltage can improve the above-mentioned problems.

However, when a plurality of toner images are developed and formed on the photosensitive drum, the AC voltage exerts a force for transferring the toner from the photosensitive drum to the developer carrying member, so that color mixing is likely to occur. In order to prevent this, for example, there is a method of sequentially changing the peak-to-peak voltage (Vpp) of the AC voltage or the frequency (f) for each development process.

[0007] However, these methods do not sufficiently prevent color mixing, and the image quality greatly changes in each development step, resulting in poor color reproduction in multicolor image formation.

[0008]

Therefore, the time t1 during which an electric field is formed in a direction for urging the developer in the direction from the developer carrying member to the photoreceptor in the order of the number of times of development, and the time from the photoreceptor to developing. The ratio (duty ratio) t1 / t2 to the time t2 during which an electric field in the direction of urging the developer in the direction toward the developer carrier is gradually reduced, and the peak of the AC voltage during the t1 phase is reduced. By successively increasing the value and gradually decreasing the peak value of the AC voltage at the time t2, a fine image is ensured, and color mixing of the toner on the photosensitive member at the time of the previous development during the superposition development is prevented. However, in the following situations, even if the above method is adopted,
It is not always possible to obtain a sufficiently good image.

(1) When the charge amount of the toner is reduced in a high humidity environment or the like, the mirror image of the developed image formed on the photoconductor to the photoconductor is reduced, and the degree of color mixing is increased. Fog toner increases.

(2) When the diameter of the developing sleeve is reduced along with the reduction in thickness and size of the developing device, the surface area of the sleeve is reduced, so that the degree of contact between the sleeve surface and the toner is reduced, and the charge amount of the toner is reduced. , The same problem as the above (1) occurred.

[0011]

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

Thus, even when the charge amount of the toner is reduced, for example, in a high humidity environment, the peak value of the electric field in the direction for urging the developer in the direction from the image carrier to the developer carrier is reduced. And the sum of the time for forming the electric field and the time for forming the electric field in the direction of urging the developer in the direction from the developer carrier to the image carrier is reduced. Therefore, while preventing color mixing and reducing fog, the peak value of the electric field in the direction of urging the developer in the direction from the developer carrier to the image carrier gradually increases. Therefore, even if the layer thickness of the multi-layered developer image formed on the developer carrying member is large, the development is reliably 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 denotes an electrophotographic photosensitive drum serving as an image carrier. The photosensitive drum 1 rotates in the direction of the arrow. Above the photosensitive drum 1, a corona discharger 2 for uniformly charging the surface of the photosensitive drum 1 is provided.
The drum surface charged by the corona discharger 2 is irradiated with the image forming light flux 3 of the original D to be copied placed on the platen 4 downstream of the corona discharger 2 in the rotation direction of the photosensitive drum 1. To form an electrostatic latent image. The latent image formed on the surface of the drum is divided into four types, which are arranged in the rotation direction of the drum opposite to the photosensitive drum further downstream of the exposure position in the rotation direction of the photosensitive drum 1. The image is developed by one or more of the developing devices 5, 6, 7, and 8. The obtained toner image is transferred to a transfer paper 71 by a transfer charger 72. Transfer paper 7
1 is sent to a transfer section by a roller 73, and the transfer paper after the transfer passes through a toner image fixing device 74 and is discharged out of the image forming apparatus. The multicolor toner images sequentially formed on the drum surface by the plurality of developing devices are simultaneously transferred to the same transfer paper surface. After the required number of color images have been transferred, the cleaning device 75 removes residual toner on the drum. In other words, instead of transferring the first toner image to the transfer paper and then returning the transfer paper to the transfer unit and transferring the second toner image to it, the transfer paper is passed once through the transfer unit. Thus, the toner images of a plurality of colors are transferred to the transfer paper.

Next, referring to FIG.
7, 8 will be described in more detail.

Each of the developing devices 5, 6, 7, and 8 has a non-magnetic developing sleeve 13, 1 as a developer carrying member in which magnets 9, 10, 11, 12 as magnetic field generating means are fixedly disposed.
4, 15, and 16, respectively. Each developing sleeve is disposed in a developer storage container 5A, 6A, 7A, 8A opposite to the surface of the photosensitive drum 1 in a developing section. Is carried and conveyed. Around the developing sleeves, regulating blades 17, 18, 19, and 20 serving as developer regulating plates 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 the developer on the developing sleeve.

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

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 to 10 ¹² Ωcm, preferably 10 ⁸ Ωcm.
Ωcm or more and 10 ¹⁰ Ωcm or less. Such magnetic carrier particles include ferrite particles (maximum magnetization 60 em).
u / g) that is extremely thinly resin-coated.

The resistance value of the magnetic particles was measured using a sandwich type cell having a measuring electrode area of 4 cm ² and a gap of 0.4 cm between the electrodes. (V) is applied to obtain the resistance value of the magnetic particles from the current flowing in the circuit.

The developing sleeves 13, 14, 15, 16
Has an AC power supply (for example, a peak-to-peak voltage of 100 V to 3K).
V, a frequency of 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 inverted is formed in each developing unit.

A minute gap is provided between each developing sleeve and the photosensitive drum (the gap at the closest point 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 of the regulating blades 17, 18, 19, and 20 regulates the thickness of the developer layer on each of the sleeves.
In any case, each developing device is a so-called non-contact 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. 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 still higher quality image without any trace of sweeping by the magnetic brush, or by rotating the drum a plurality of times while holding the toner image and operating a different developing device for each rotation, In the case of an image forming apparatus that forms toner images of different colors, the developing device 5 is also preferably a non-contact 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 over its entire surface by a corona discharger 2 and then subjected to image exposure by an image forming light beam 3 reflected from a document.
8 is selected for development.

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

Similarly, an image having an arbitrary color tone is formed in each of the developing devices 5 to 8.
Is obtained by superimposing arbitrarily while adjusting the amount of adhesion.

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

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

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

The photosensitive drum 1 carries a charge constituting a latent image. In this embodiment, the charge constituting an electrostatic latent image has a negative polarity. Is charged negatively by the friction of.

In this embodiment, the photosensitive drum 1
And the developing sleeve 13 rotate as indicated by the arrow so as to move in the same circumferential direction in the developing section.

An AC electric field is formed in a space between these by a 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 part, 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 tangential to the peripheral surface of the sleeve is formed in the developing portion. Thereby, as shown in FIG. 4, the magnetic carrier particles 37 are connected in a chain along the sleeve surface. That is, since the chains of the carrier particles lie on the sleeve surface, an extremely thin layer that does not contact the drum of the two-component developer can be formed in the developing section.

Since the amount of the developer itself transported to the developing unit 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. Waste toner is also supplied to the drum between the chains.

Further, since the development by the AC electric field has a 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 this embodiment, the latent image includes an image portion V _L (a bright portion irradiated with light) and a non-image portion V _D (a dark portion potential not irradiated with light). Is also a negative potential (however, the absolute value of the non-image portion potential is larger than the absolute value of the image portion potential).

The toner is also negatively charged.

Since the direction of the electric field in the developing section is an alternating electric field, the electric field alternately changes between the photosensitive drum and the developing sleeve as shown by arrows a and b in FIG. In a phase (indicated by t11 in FIG. 1) where 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 a negative charge is injected into the carrier particles having the above-described electric resistance value from the sleeve, the direction of the electric field is in the b direction. When a directional force is applied, the chain is raised as a whole in the manner of a drum, and the toner is easily released from the chain and from the surface of the sleeve. Since the toner 38 attached to the surfaces of the developing sleeve 13 and the magnetic particles 37 is negatively charged as described above, the toner 38 receives the force in the direction a by the electric field in the direction b formed in this space. The photosensitive drum 1 moves to a light potential region.

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

On the other hand, since the toner 38 on the photosensitive drum 1 is negatively charged as described above, it receives a force in the direction of the arrow b by the electric field in the direction 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.

The phases t11 and t21 are alternately repeated so that the developer repeats the above-described movement, and the gap between the sleeve and the drum is enlarged by the rotation of the sleeve, whereby the development is completed. An amount of toner corresponding to the potential of the electrostatic latent image remains on the drum. That is, a toner image is formed.

In the embodiment, since the reversal development is performed, the toner adheres to the region of the light portion potential _VL , and the dark portion potential V _D
Is a background area where toner does not substantially adhere.

Further, in this embodiment, the second developing device adheres the toner of the second color in a superimposed manner on the bright portion potential region to which the toner of the first color has been applied by the first developing device.

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

In any case, the color mixture, which is a problem of the conventional apparatus, occurs because the toner of the first toner image moves backward from the drum to the sleeve during the development of the second and subsequent colors as described above. In the development of the second and subsequent colors, a method of suppressing this reverse movement is adopted.

Hereinafter, the operation of the image forming process of the present invention will be described in detail.

In FIG. 1, what is indicated by a broken line is an AC bias voltage B2 (the superimposed DC voltage component is _VDC2 ) applied to the sleeve during the development of 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 sleeve 14 of the developing device 6 at the phase t12, the direction of the electric field of the developing unit is in the direction b, so that the toner 39 exerts a force in the direction of arrow a. As described above, the toner 39 separates from the magnetic carrier particles and the sleeve surface and moves to the light portion potential region of the drum 1 in the same manner as described above. Next, when the positive component of the AC bias voltage B2 is applied to the sleeve 14 at the phase t22, the electric field of the developing unit becomes an electric field in the direction of arrow a. However the peak value V _P22 of the bias voltage B2 in the phase t22 is smaller than the peak value V _P21 in the phase t21 of the bias voltage B1. (In this specification, the magnitude of the peak value means the magnitude of the absolute value.) Accordingly, 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 t at the first bias voltage B1 is low.
11 and the sum of the phase t12 and the phase t22 at the second bias voltage B2 (t1 + t21) (t1 + t21).
2 + t22) is reduced (that is, the frequency is sequentially increased). Therefore, from t21 / t11 (the ratio of the return bias application time of the first color to the skip bias application time),
t22 / t21 (Even though the ratio of the return bias application time of the second color and the skipping bias application time is large, the return bias time of the second color can be relatively shortened, and the toner forming the first toner image can be formed. The reference numeral 38 hardly moves to the sleeve 14 of the second developing device, thereby preventing color mixing.

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

That is, if the time for the toner to fly from the sleeve to the drum is short, the toner (fog toner) having an abnormally low charge amount cannot respond to the time, stays in the sleeve, and then enters the developing container as the sleeve rotates. Because of the return, 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 fogging prevention function is enhanced in this respect, and the effect is remarkable in a high humidity environment.

On the other hand, the time of the phase t12 at which the toner is moved from the sleeve to the drum in the second developing device is shortened in order to improve the function of preventing the fog and the color mixture. No longer available. However since the peak voltage V _P12 as shown in FIG. 1 with relatively larger than V _P11, it is possible to deposit toner of sufficient amount to the image portion.

As is clear from the above, when the three color toners are superimposedly developed, 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 represented by the AC bias voltage B2.
What is necessary is just to set the relationship 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 for forming the electric field in the b direction are represented by t1 respectively.
3, t14, the phase forming the electric field in the a direction is t
23 and t24, and the phase t13 of the AC bias voltage B3.
And the peak value at phase t23 are V
_Assuming that _P13 and _VP23, and the peak value of the AC bias voltage B4 at the phase t14 and the peak value at the phase t24 are _VP14 and _VP24 , respectively, in the case of performing the development with the 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 the 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 between the voltage peak value is the magnitude relationship of the absolute value.)

The results of the experiment using the apparatus shown in FIG. 3 are shown below.

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

Each of the developing sleeves has a diameter of 20 mm.
Surface of stainless steel (SUS316) sleeve of #
No. 400 particles were formed by sandblasting with irregular abrasive grains.
A pole and an S pole alternately magnetized as shown in FIG. 2 were used. 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, an average particle diameter of 6 coated with a very thin silicone resin on the surface was used.
Ferrite having a true density of 5.16 g / cm ³ of 0 to 50 μm (maximum magnetization of 60 emu / g) was used.

As the non-magnetic electrically insulating toner, a toner having an average particle diameter of 8 μm obtained by mixing about 5 parts of a pigment with 100 parts of a 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.
The black toner used was a mixture of the above pigments at a ratio of 1: 2: 1. 0.4% of colloidal silica is externally added to each toner to improve fluidity.

The developer layer formed on the developing sleeve has a thickness of 300 μm in the developing section. The relation 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 charge amounts of the toners 38 and 39 were about -15 μcoul / g.

The developer rises by a magnetic field at a magnetic pole position in the developing sleeve other than the developing section, and has a maximum length of about 0.8 to
A layer formed of a magnetic brush with toner attached thereon was formed to a height of about 1.3 mm. As a start developing unit, 270 g of magnetic particles and 30 g of toner were mixed and used.

This developing device was incorporated in the color image forming apparatus 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 500 μm. The ratio between the peripheral speed of the photosensitive drum and the peripheral speed of the developing sleeve is 1: 1.3. The application amount M (g / cm ² ) of the developer to the developing sleeve at the time of non-earing 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 the dark portion potential (non-image portion potential) V _D is −600 V, and the bright portion potential (image potential) _VL is −2.
The charged latent image potential was 50 V. The bias power supply 33 is t2
1 / t11 = 1, frequency f ₁ = 1800 Hz, peak-to-peak value Vpp (peak-to-peak voltage) A voltage obtained by superimposing a DC voltage of −490 V on a rectangular wave AC voltage of 1800 V is applied to the developing sleeve 13 of the developing device 5. .

The peak-to-peak value Vpp applied to the developing sleeve 14 of the developing device 6 is the same as the bias voltage applied to the developing sleeve 13 and t22 / t12 = 4, and the frequency f ₂ = 20.
A voltage obtained by superimposing a -790 V DC voltage on a 00 Hz rectangular wave AC voltage was applied from the bias power supply 34. As a result of developing the latent images sequentially 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, a clear image was maintained without mixing of the toner 38 of the developing device 5 into the developing device 6.

(Experimental Example 2) Next, the result of three-color superimposition 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 described.

The AC voltage shown in Experimental Example 1 was applied to the developing sleeve 13 of the developing device 5. The peak-to-peak value Vpp is applied to the developing sleeve 13 of the developing device 6.
To the square wave AC voltage of t22 / t12 = 3 and the frequency f ₂ = 2000 Hz, which is the same as the bias voltage applied to −690.
An AC voltage on which a DC voltage of V was superimposed was applied from a 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 obtained by superimposing a DC voltage of −840 V with a rectangular wave of Hz was applied from the bias power supply 35.

As a result of the experiment with the above settings, a good image without color unevenness was obtained as in the case of Experimental Example 1, and the toners 38 and 39 were hardly mixed into the developing devices 6 and 7.

According to various experimental results, the value of t2 / t1 of the AC voltage in the case of the superposition development is sequentially increased in the order of the number of times of development, and t22 / t12 = 2 in the second development.
In the sixth to third developments, it was found that when t23 / t13 was kept in the range of 3 to 10, color mixing was prevented and a good image was obtained.

(Experimental Example 3) Further, as a preferable example,
If the amount of triboelectric charge of the toner in each development is changed sequentially, a better image can be obtained, and mixing of a different color toner into the developing device can be prevented. The toner charge amount is set slightly lower than the toner concentration. For example, in the developing device 5 according to the above-described experimental example, the relationship between the superposition C of the magnetic carrier particles and the toner weight T is (T / C + T) × 100, 8%, 10% for the developing device 6 and 12% for the developing device 7, the triboelectric charges of the toners 38, 39, and 40 are respectively -23, -18, and -15 .mu.coul / g. 7 was prevented, and even when the toner charge amount of the developing device 7 for the third color was −15 μc / g, the time t13 was set to be short, so that fogging was also prevented. Here, the reason for preventing the mixing of different color toners is that the frictional charge amount of the toner is set to be larger in absolute value as far back as the previous developing step, so that the toner moved from the developing sleeve to the photosensitive drum is This is because the toner adheres strongly to the drum, and the toner adhered to the drum in the previous developing step does not easily return to the developing sleeve in the subsequent developing step under an AC electric field in the subsequent developing step. . However, the first toner 38 and the second toner 3
9 is more electrostatically attached to the carrier particles and the sleeves 13 and 14 than in the case of the experimental example 2, 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 experimental example 2. preferable. For example, sleeve 1
The superimposed DC voltage component of the AC bias voltage applied to
520 V, and the superimposed DC voltage component of the AC bias voltage applied to the sleeve 14 is preferably -710 V.

In a developing device not used for development, the developing device is separated from the photosensitive drum or an electric bias for preventing toner from flying is applied to the sleeve so that unnecessary toner does not adhere to the photosensitive drum. Can be.

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

The image forming process is not limited to the case of the above-described mono-color process, and the first toner (development by the developing device 5 in this embodiment) after the completion of the first development (the first toner 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 the present embodiment, development by the developing device 6) are performed. It is also possible to use a process in which the charging, exposing, and developing processes are performed a second time, and then the image formed of the four color toners is collectively transferred to the transfer paper.
Further, in such a process, a multicolor image can be formed by using a color separating means or a masking means at the time of each image exposure.

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 denoted by the same reference numerals, and the description is omitted.

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

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

Next, 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 to form a second latent image, and a red toner 6 of a one-component non-magnetic developer disposed close to the photosensitive drum 1.
0 is developed by the developing device 61 containing 0. In this way, the steps of charging, image exposure, and development are sequentially performed to form a plurality of color toner images on the photosensitive drum and collectively transfer them 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.
To the developing rollers 54 and 64. The fur brushes 52 and 62 not only stir the toner in the developing devices 51 and 61, but also disturb the toner on the developing roller after development, thereby preventing a so-called ghost. The same effect can be obtained by using an open-cell or single-cell rubber sponge instead of the fur brush.

The rollers 54 and 64 rotate in the direction of the arrow to carry and transport the developer to each developing unit. The layer thickness of the developer transported to the developing unit is regulated by regulating blades 53 and 63, respectively. The blades 53 and 63 are elastic blades having a two-layer structure in which rubber is adhered to a rubber plate, a metal plate spring, or a metal plate. Is regulated to be smaller than the gap between the drum 1 and the rollers 54, 64. The blades 53 and 63 apply the developer to the developing rollers 54 and 64, respectively.
Is rubbed and triboelectrically charged.

The developing rollers 54 and 64 are connected to developing bias power sources 57 and 65, respectively, to form an AC electric field 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 was regulated to about 300 μm, and the thickness of the toner layer on the developing rollers 54 and 64 was regulated to about 400 μm. Was triboelectrically charged to -20 μcoul / g and the red toner 60 was triboelectrically charged to -15 μcoul / g.

The dark portion potential (non-image portion potential) V _{D of} the first latent image
Is -600 V, and the bright portion potential (image portion potential) _VL is -250.
V, t21 / t11 = 1, frequency 1800H
z, a bias voltage obtained by superimposing a DC voltage −500 V on a rectangular wave AC voltage having a peak-to-peak value Vpp = 1400 V,
The voltage was applied to the developing roller 54 from the power source 57.

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

In this embodiment, as in the previous embodiment, a good image without color unevenness was obtained, 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 carrier by the AC electric field, and then reversely transferred from the image carrying member to the developer carrying member. However, this is not essential, and the developer carrying member and the image carrying member are connected. The development may be performed in contact with the toner. In this case, the rubbing at the contact portion further increases the charge amount of the toner, and further prevents the first developing toner from being mixed into the second developing device.

In the above embodiment, the peak-to-peak voltage Vpp of each bias voltage was the same. However, even if the Vpp of the second color was reduced as compared with the development of the first color, the same effect on color mixture and fog was obtained. Is obtained.

In the above embodiment, the reversal development in which toner is adhered to the exposed portion (light portion potential portion) on the surface of the photosensitive drum is described. However, the normal development in which toner is adhered to the non-exposed portion (dark portion potential portion). Also applicable to In the case of regular development, toner charged to the polarity opposite to the polarity of the latent image is used.

Further, in the above embodiment, the negative electrostatic latent image is developed. However, in the present invention, a positive electrostatic latent image is formed, and this is applied to an image forming apparatus which performs reverse development or regular development. 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, it is possible to prevent disturbance, color mixing and fogging of the image in the overlap development. Further, the mixing of toners of other colors in each developing device in a high-humidity environment or the like could be prevented.
Therefore, it is possible to provide a high-speed microminiature color copier that can simplify maintenance and is inexpensive.

[Brief description of the 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 of FIG. 2;

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

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

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Photosensitive drum 5 First developing device 6 Second developing device 7 Third developing device 8 Fourth developing device

──────────────────────────────────────────────────続 き Continuation of the front page (51) Int.Cl. ⁷ Identification symbol FI G03G 15/09 G03G 15/09 Z (56) References JP-A-3-206473 (JP, A) JP-A-3-175462 ( JP, A) JP-A-3-67278 (JP, A) JP-A-2-186370 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G03G 13/01 G03G 15/01 -15/01 117 G03G 15/06-15/06 102

Claims (8)

(57) [Claims] An image carrier that moves sequentially through a first developing unit and a second developing unit; and a second image carrier that is disposed to face the image carrier along a moving direction of the image carrier. An image forming apparatus comprising one developing device and a second developing device, wherein the first developing device includes a first developer including a toner of a first color and a first developing unit. Has a first developer carrier that conveys to the image carrier to form a first toner image, the second developing device includes a second developer including a second color toner. A second developer carrier that is transported to the second developing unit and forms a second toner image on the image carrier on which the first toner image is carried; the first developer A bias voltage applying unit that applies a first developing bias voltage to the carrier and applies a second developing bias voltage to the second developer carrier; In the image forming apparatus, the first developing bias voltage forms a first electric field in the first developing section for applying a force in a direction from the developer carrier to the image carrier to the toner for a time t11. And a phase in which a second electric field opposite to the first electric field is formed in the first developing unit for a time t21, and the second developing bias voltage is: A phase in which a third electric field in the same direction as the first electric field is formed in the second developing unit for a time t12, and a fourth electric field opposite to the third electric field is formed for a time t22. The phase formed in the second developing section is a voltage that is repeated, and the time ratio (t12 / t22) is (t11 / t21).
And the sum of the times (t12 + t22) is (t
11 + t21), wherein the peak value of the second developing bias voltage during the time t12 is larger than the peak value of the first developing bias voltage during the time t11; An image forming apparatus, wherein the peak value of the second developing bias voltage in time t22 is smaller than the peak value of voltage in time t21. 2. The image forming apparatus according to claim 1, wherein the first color toner and the second color toner have the same charging polarity. 3. The image forming apparatus according to claim 1, wherein a peak-to-peak voltage of the first developing Bais voltage and a peak-to-peak voltage of the second developing Bais voltage are set to be the same. . 4. 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.
An image forming apparatus according to claim 1. 5. A second developing device, wherein:
5. The image forming apparatus according to claim 1, further comprising means for regulating the thickness of the second developer to be smaller than the minimum gap between the second developer carrier and the image carrier. An image forming apparatus according to any one of the above. 6. A second developing device comprising: a second developer containing magnetic carrier particles; and a magnetic field generating means fixed in a second developer carrier. Is
It has two adjacent magnetic poles having 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 1, wherein the charge amount of the second color toner is smaller than the charge amount of the first color toner. 8. The image according to claim 1, further comprising a transfer unit that transfers the first toner image and the second toner image to the same transfer material surface at the same time. Forming equipment.