JP3131244B2 - Multicolor image forming method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used in a copying machine, a facsimile, a printer, and the like, and is used for forming a multicolor image on an electrostatic latent image carrier without color mixing and transferring the image to paper. The present invention relates to a color image forming method.

[0002]

2. Description of the Related Art In recent years, a plurality of colors have been developed by being superimposed on an electrostatic latent image carrier (hereinafter referred to as a latent image carrier) such as a photosensitive drum, and a multi-color image has been transferred to paper once. Various multicolor image forming apparatuses (multicolor image forming methods) have been proposed.

For example, in JP-A-63-60471, there is obtained a multi-color image by providing a plurality of developing devices around the image bearing member, the toner polarity of the downstream side of the developing device from that of the upstream side is reversed, a toner layer image bearing member and a non-contact, an electric field is applied to each separately with the time of development of the downstream side of the developing time and the upstream side, and the toner layer thickness A multi-color image forming method is disclosed in which the upstream side is a two-component developer having a chromatic color toner and the downstream side is a one-component developer of a black toner.

In Japanese Patent Application Laid-Open No. 63-63061, a photosensitive layer having a thickness of 35 to 90 μm and a capacitance of 20 to 170 pF / cm ² selenium or arsenic selenide has a plurality of developing layers around the photosensitive member. A method of obtaining a multicolor image on the same photoreceptor by disposing a dispenser and repeating charging and exposure development a plurality of times is disclosed. In this method, a developing device and a photoreceptor are arranged at an interval of 250 μm or less. This is non-contact DC bias development using a thinner toner. The donor roller that does not contribute to the development is stopped, and the toner on the surface is attached outside the image area to prevent color mixing. The photoreceptor is 0PC of 15 to 50 μm, the development is reverse development, the potential contrast is 400 V or more, the toner layer thickness on the photoreceptor is 5 to 30 μm, and the charging is scorotron.

In Japanese Patent Application Laid-Open No. 63-85578, a plurality of developing devices are arranged on a recording medium in a non-contact state to obtain a multicolor image. The speed is made faster than that of the recording medium by rotating in the same direction, and black developing is applied, a DC biased AC bias is applied, and the developing devices after the second developing device on the downstream side perform constant speed development. A multicolor image forming method in which only a DC bias voltage is applied in color development is disclosed.

Generally, when a multicolor image is formed on a latent image carrier, the developing method for the second and subsequent colors is non-contact development, and the image forming method using a non-magnetic toner is easy because of colorization. A one-component system that is advantageous for cost reduction is preferable. In this method, development is performed with the toner layer and the latent image carrier facing each other in a non-contact manner.

[0007]

However, in the above-described conventional multicolor image forming method, a toner image is formed on the latent image carrier during the development of the second color. According to the publication, when an AC bias voltage is applied as a developing bias, toner reciprocates between the surface of the latent image carrier and the surface of the toner carrier, not only disturbing the first color toner image, but also causing There is a problem that the second color gradually becomes cloudy due to mixing of the color toner.

In order to prevent this problem, Japanese Patent Application Laid-Open No. 63-63061 and Japanese Patent Application Laid-Open No.
In Japanese Patent Application Laid-Open No. 8, the development of the second and subsequent colors is performed by causing the non-magnetic toner to fly by a DC electric field.

However, in this method, the difference between the potential of the image area and the applied bias voltage is made lower than the charging potential to prevent the so-called background stain, in which the toner adheres to the non-image area. As shown in (1), since the potential difference for preventing background contamination gives a force in the reverse flight direction to the toner layer of the first color on the latent image carrier, if the formation of a multicolor image is continued for a long time, However, there is also a problem that the toner of the first color is mixed into the developing devices of the second and subsequent colors.

In the case where the toner layer is transferred under a DC electric field, a good image cannot be obtained unless the potential difference between the image area and the applied bias voltage is set sufficiently large. This is due to the fact that the toner does not fly unless it becomes equal to or higher than the electric field at which the toner starts flying in the development area, that is, the threshold electric field in the flying development by the DC electric field. The threshold electric field is a value that depends on the charge amount of each toner, van der Waals force, and the like. Since the threshold electric field has large variations, the entire toner layer can fly during development in order to obtain a stable density image. A sufficiently large potential difference is required.

Therefore, the present invention solves the above-mentioned conventional problems and stably forms a good multicolor image for a long period of time regardless of the state of adhesion of the first color toner on the latent image carrier. It is an object of the present invention to provide a multicolor image forming method that can perform the method.

[0012]

SUMMARY OF THE INVENTION The gist of the present invention resides in that an electrostatic latent image on a latent image carrier is provided with respective developing rollers by toners of the same polarity and different colors as the electrostatic latent image. In a multi-color image forming method in which a multi-color image is formed by sequentially developing with a developing device and transferring to a transfer paper at a time to obtain a multi-color image, non-contact in which one-component non-magnetic toner is jetted using DC developing bias for development of two or more colors And the toner layers formed on the developing roller for the second and subsequent colors are represented by the following relational expression (dt).
² / εt) <590 [(μm ² )] (where dt is the thickness of the toner layer measured by a laser measuring device, εt is the volume average relative permittivity of the toner layer, and the volume average relative permittivity εt is Εt = (1) where ε is the relative dielectric constant of α and α is the filling rate of the toner layer.
−α) + αε. ).

[0013]

[Action]

When the condition is satisfied, an increase in the potential of the toner surface layer portion is suppressed (50 V or less), so that it is possible to prevent the background stain due to the low charge amount toner. Further, in order to obtain a sufficient concentration of the toner, it is necessary that the toner layer has two or three layers. This toner is a particle having a volume average particle diameter of 10 μm or less produced by a polymerization method. Therefore, even if the number of the toner layers is two or three, the thickness of the toner layer is only about 20 μm, which satisfies the above-mentioned condition, so that a good multicolor image can be obtained.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows an image forming apparatus according to the multicolor image forming method of the present invention. In the figure, a first charger for charging the surface of the photosensitive drum 1 and a first charger for forming a first toner image are provided in the vicinity of the photosensitive drum 1 as a latent image carrier. A developing device 4, a second charger 5 for recharging the photosensitive drum 1, a second developing device 7 for forming a second electrostatic latent image, and a photosensitive member of the second developing device 7 Cleaning devices 11 are arranged at positions opposite to each other with the drum 1 interposed therebetween. In the case where a negatively chargeable organic photoreceptor is used as the photoreceptor drum 1, a negatively chargeable toner is used as the toner.

The first developing device 4 may use any developing means irrespective of contact or non-contact. In this embodiment, however, the two-component developing device in which the toner is charged by friction with a carrier is used. A contact developing method using a magnetic brush of the agent is used. The second developing device 7 is preferably made of a non-magnetic toner that is easy to colorize, and uses a one-component non-contact method, which is particularly advantageous for miniaturization and cost reduction.

The scanning beams 3 and 6 from the first exposure means and the second exposure means (not shown) form an image on the surface of the photosensitive drum 1.

A transfer belt 8 for transferring transfer paper (not shown) is pressed against the photosensitive drum 1 by being pressed against the back surface of the transfer belt by a transfer charger 9, and the transfer belt 8 is positioned downstream of the transfer belt. Is provided with a fixing device 10 for fixing an image on a transfer sheet.

The operation of the image forming apparatus having the above configuration will be described. First, the photosensitive drum 1 is charged by the first charger 2.
After the surface is charged, a first electrostatic latent image is formed on the photosensitive drum 1 by the scanning beam 3, and a first toner image is formed by the first developing device 4. Next, the photosensitive drum 1 is rotated in the direction of the arrow, and the photosensitive drum 1 is recharged from above the first toner image of the first color by the second charger 5, and the first exposure is performed by the scanning beam 3. The potential state of the performed portion is made substantially equal to the surroundings. Then, a second electrostatic latent image is formed on the photosensitive drum 1 by the scanning beam 6. At this time, if the portion where the first color toner is adhered is not exposed, it is possible to prevent the second color toner and the first color toner from overlapping on the photosensitive drum 1 to make the color turbid.

Next, the second electrostatic latent image is transferred to the second developing device 7.
And develop. Since the development is performed with the DC bias voltage substantially equal to the charging potential of the photosensitive drum 1, color mixing due to reverse flight of the first color toner does not occur even when used for a long time. At this time, the toner layer used for the development of the second and subsequent colors satisfies the conditions described later, or a one-component non-magnetic toner having a volume average particle diameter of 10 μm or less produced by a polymerization method is used, so that the background stain is reduced. Is prevented.

In this way, a multicolor image is sequentially formed on the photosensitive drum 1 and is transferred to the transfer paper on the conveyor belt 8 by the transfer charger 9 at one time. Then, the toner image on the transfer paper is fixed by the fixing device 10 to complete the image. On the other hand, the untransferred toner on the photosensitive drum 1 is swept off by the cleaning unit 11, and the photosensitive drum 1 is again brought into a state usable for image formation.

Next, the developing method for the second and subsequent colors by the second developing device 7 will be described in detail with reference to FIG.
FIG. 1 shows a developing device using a non-magnetic one-component toner (second developing device).
Of the developing device 7). The toner 26 of a color different from the first color is supplied to the hopper 25 and stirred by the agitator 24. Then, the toner carried to the supply member 22 is frictionally charged with the developing roller 21 and adheres to the developing roller 21. The attached toner is formed into a uniform toner layer by the toner layer forming member 23 and is conveyed to the developing area. At this time, the amount of toner adhered to the developing roller 21 is preferably such that a sufficient image density can be obtained even when the developing roller 21 and the photosensitive drum 1 move in a non-contact manner and move at substantially the same speed. Further, if there is a speed difference between the developing roller 21 and the photosensitive drum 1, a phenomenon called "toner trailing end" occurs, the density of the solid portion becomes non-uniform, and the image quality of a color image deteriorates. It is necessary that the toner to be formed has two or three layers. For this reason, it is preferable to use, as the developing roller 21, a roller in which sand blasting is performed on an aluminum surface or a roller in which a dielectric substance is sealed in minute grooves on the surface of the aluminum roller.

In the development of the second color, it is necessary to perform development without disturbing the first toner image of the first color already formed on the photosensitive drum 1, and the developing roller 21 and the photosensitive drum The gap of 1 is 0.05-0.5mm,
Preferably, the range of 0.1 to 0.2 mm is appropriate. If this gap is 0.05 mm or less, the first color and the second color
0.5m
Above m, the line portion of the electrostatic latent image becomes a closed electric field, and no developing electric field is formed between the developing roller 21 and the photosensitive drum 1, so that flying development becomes impossible. By the way, at the time of development, a power supply 27 applies a development bias voltage. Due to the recharging of the second charger 5 after the development of the first color, the potential of the first toner image portion of the first color rises by about 50 to 100 V from the surroundings depending on the charge amount of the toner itself. The writing portion potential of the second color is -100 V, and the background portion potential is -80.
When the voltage is set to 0 V, the potential of the first toner image portion of the first color is -8.
50 to -900V. This developing bias voltage is 1
The non-magnetic toner layer does not fly back into the second developing device 7 of the second color and mixes with time, and the non-magnetic toner layer satisfactorily flies to the latent image potential portion of the second color. The toner must not adhere to the background. For example, in the process shown in FIG. 3, the applied bias voltage of the second color is about -700 V [FIG. 3 (f)], and a difference of 100 V from the background potential is required as the background contamination preventing bias voltage. At this time, since the difference from the surface potential of the toner layer of the first color is 150 V or more, the state of adhesion of the toner of the first color has decreased with time or due to environmental changes, or the amount of adhesion has increased. In this case, color mixing easily occurs due to reverse flight.

Therefore, as shown in the step shown in FIG. 4, the bias background potential applied at the time of development of the second color is set to about -800 V with respect to -800 V [FIG. This allows
Since the reverse flying potential difference of the first color toner is always kept at 100 V or less and the development gap exists, even if the development of the first color changes with time or in the environment (regardless of the adhesion state of the first color) ) Color mixing rarely occurs. Further, the development potential difference, which is the difference between the applied bias voltage and the writing portion potential, is about 700 V, and a more stable image can be obtained by the DC electric field flying development method.

Next, prevention of background contamination in the step shown in FIG. 4 will be described. The rise in the surface potential formed by the non-magnetic one-component toner layer used for the development of the second and subsequent colors is based on the toner layer concept of FIG. 5 (29 is a toner layer, 30 is a background toner, and 31 is a metal sleeve). It is represented by the following equation. E (y) is the magnitude of the vertical electric field , ε
When t is the volume average relative dielectric constant of the toner layer, ρ is the volume average charge of the toner layer, σ is the mirror image charge per unit area induced on the surface of the developing roller, and dt is the thickness of the toner layer. equation (dE / dy) = (ρ / εt ε 0) ··· (1) boundary conditions E (0) = (σ / εtε 0) ··· (2) charge of the law of conservation of ρdt + σ = 0 From the formulas (1), (2) and (3), E (y) = (ρ / ε ₀ εt) (y−dt) (4) Therefore, the potential V of the toner surface layer portion Is

[0025]

(Equation 1)

As the potential rise is smaller, the developing roller 2
Thus, it is possible to prevent background contamination due to the toner having a low charge amount carried on the surface layer of No. 1. In development, the larger the charge amount of the toner is, the clearer the image is obtained, and the stronger the adhesion to the developing roller 21 is, so that the background stain is less likely to occur. The charge amount per unit mass (Q / M) of the toner layer is 5
It is not less than μC / g, preferably not less than 10 μC / g. This Q / M is based on the measurement by the suction method.
About 10 mg of toner was sucked into a 0 g Faraday cage, and the amount of charge was measured to obtain Q / M. In addition, it is necessary that a sufficient density can be obtained even when the photosensitive drum and the developing roller 21 move at a substantially constant speed, and the toner must be in two or three layers. The amount of adhesion (M / V) of the toner layer per unit volume is represented by the bulk specific gravity of the toner used. Generally, the bulk specific gravity of an irregular toner having a volume average particle diameter of 11 μm is about 0.3,
The range of normal use is 0.3 or more. From these, the range of the volume average charge amount ρ of the toner layer is derived as follows.

Q / M> 5 [μC / g] (6) M / V> 0.3 [g / cm ³ ] (7) From equations (6) and (7), ρ = Q /V>1.5 [C / m ³ ] (8) In addition, the rise potential V can be suppressed to 50 V or less, although the charge amount in the toner layer on the surface is low, but the surface potential of the photosensitive drum 1 may be low. Necessary to prevent adhesion. Therefore,
The following relational expression is derived.

[0028]

[0029]

(Equation 2)

[0030] If the toner is spherical particles formed by a polymerization method, the toner filling rate is higher than that of the irregular particles, and the volume average particle diameter is 10 μm or less. The thickness of the toner layer is only about 20 μm. Therefore, the condition of the above equation (11) is necessarily satisfied.

Here, assuming that the relative permittivity of the toner is ε and the filling rate of the toner layer is α, εt = (1−α) + αε (where the relative permittivity of air is 1). Further, the true specific gravity of the toner is m (g / cm ³ ), and the bulk specific gravity of the toner is n (g / g).
/ Cm ³ ), the toner layer filling rate α = n / m.

Next, concrete numerical examples will be shown in consideration of the above-mentioned condition (11). First, as a first embodiment, a drum-shaped organic photoconductor is used as a latent image carrier,
The black color development is performed by contact development using a two-component magnetic brush. Then, the photosensitive drum 1 on which the black toner image is formed is recharged, and a second electrostatic latent image is formed by exposure with the second scanning beam 6. After the recharging, the solid toner adhesion amount of the black toner on the photosensitive drum 1 is about 0.8 mg / cm 2, and the charge Q / M per unit mass of the toner layer is −25 μC / g. At this time, the photoconductor potential at the portion of the photoconductor drum 1 where the black toner is attached is -880 V, the background portion is -800 V, and the exposed portion is -10 V.
0V. The color development of the second color is performed using an irregular toner having a volume average particle diameter of 7 μm produced by a pulverizing method, and the linear speed of the photosensitive drum 1 and that of the developing roller 21 are equally 120 mm / sec. The developing roller 21 carrying this non-magnetic one-component toner layer is obtained by subjecting an aluminum surface to a sandblasting process. This toner layer has a charge amount Q / M per unit mass of −18 μC / g,
The toner layer thickness dt measured by a laser length measuring device is 24 μm, the relative dielectric constant ε of the toner is 3.1, and the true specific gravity m of the toner is 1.11 g.
/ Cm3 and bulk specific gravity n are 0.42 g / cm3, so that the toner layer filling rate α is 37.8%. Therefore, the volume average relative permittivity ｔt of the toner layer is 1.8, and the volume average charge ρ is 7.6 C / m 3.

This toner layer satisfies the above condition (11).

[0034] Under the above conditions, the developing roller 21 and the photosensitive drum 1
Is 0.12 mm, and the developing bias voltage is -80.
When development was performed by applying 0 V, a good multicolor image without background contamination was obtained, and even after continuous production of 5000 sheets, a good image was obtained without color mixing and color turbidity. Was done.

Next, a second embodiment of the numerical embodiment will be described. The developing condition of the first color is the same as that of the first embodiment, and the non-magnetic one-component toner used in the development of the second and subsequent colors is performed with an irregular toner having a volume average particle diameter of 11 μm. The charge amount Q / M per unit mass of the toner layer is-
12 μC / g, the toner layer thickness dt measured by a laser measuring device is 33 μm, the relative dielectric constant ε of the toner is 2.7, the true specific gravity m is 1.02 g / cm ³ , and the bulk specific gravity n is 0.31 g / c.
m is ^3. Therefore, the filling rate α of the toner layer is 30.4%, the volume average relative dielectric constant εt is 1.5,
The value of the expression (1) is dt ² / εt = 726 (μm) ² , which does not satisfy the expression (11).

Under the above conditions, the development was performed by applying a developing bias voltage of -800 V with the gap between the developing roller 21 and the photosensitive drum 1 set to 0.12 mm. Then, after lowering the developing bias voltage to -700 V to produce 5000 continuous multicolor images, it was observed that the colors were mixed and the color became cloudy.

Further, a third embodiment of the numerical embodiment will be described. The developing conditions for the first color are the same as those in the first embodiment, and the toner conditions for developing the second and subsequent colors are the second.
Is the same as that of the embodiment. The amount of charge Q / M per unit mass of the toner layer is −9 μC / g, the thickness dt of the toner layer measured by a laser measuring device is 32 μm, and the relative dielectric constant ε of the toner is 3.
7. The true specific gravity m is 1.05 g / cm ³ , and the bulk specific gravity n is 0.
38 g / cm ³ . Therefore, the toner layer filling rate α
Is 36.2%, and the volume average relative permittivity εt is 2.
0, the value dt ² / εt = 512 (μm) ^{2 in the} equation (11)
<590 (μm) ² satisfies the expression (11).

Then, the developing roller 21 and the photosensitive drum 1
Is set to 0.12 mm and the developing bias voltage is -800.
When V was applied and development was performed, no background stain was observed, a good multicolor image was obtained, and no color mixing was observed even after continuously forming 5000 sheets, and a clear image was maintained. .

Further, a fourth embodiment of the numerical embodiment will be described. The developing conditions for the first color are the same as in the first embodiment,
The non-magnetic one-component toner used for the development of the second and subsequent colors is substantially spherical particles produced by a polymerization method having a volume average particle diameter of 5 μm. This toner layer had a charge amount per unit mass Q / M of −22 μC / g and a toner layer thickness dt measured by a laser length measuring instrument of 18 μm. The relative dielectric constant ε of this toner is 2.9, and the true specific gravity m of the toner is 1.13 g / c.
m ³ and toner bulk specific gravity n are 0.51 g / cm ³ . Accordingly, the filling rate α of the toner layer is 45.1%, the volume average relative permittivity εt is 1.9, and the value dt ² / ε of the above equation (11) is obtained.
Since t = 170 (μm) ² <590 (μm) ² ,
The condition of the expression (11) is satisfied.

Under the above conditions, development was performed by applying a developing bias voltage of -800 V while setting the gap between the developing roller 21 and the photosensitive drum 1 to 0.12 mm. Obtained and continuously 500
Even after the production of 0 sheets, no color mixing was observed, the sharpness of the color was maintained, and a good multicolor image was obtained stably.

[0041]

As described above, according to the present invention, the development for the second and subsequent colors is non-contact development using a thin layer of non-magnetic toner, and the toner layer used for the non-contact development is dt /
εt <590 (μm) ² (dt: toner layer thickness, εt:
(Volume average relative dielectric constant of the toner layer), so that the toner does not fly backwards and is free from background fouling without being affected by the state of adhesion of the first color toner to the photosensitive drum, and there is no color turbidity. A good multicolor image can be stably obtained without a change over time.

In addition, since the non-magnetic one-component toner used for the development of the second and subsequent colors is a particle having a volume average particle diameter of 10 μm or less formed by a polymerization method, in order to obtain a sufficient density of a toner image, Even when the toner is held in two or three layers on the developing roller, the above expression is satisfied, and similarly, there is an effect that a good multicolor image without color turbidity can be stably obtained.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing a multicolor image forming apparatus according to the present invention.

FIG. 2 is a cross-sectional view illustrating a developing roller in developing the second and subsequent colors.

FIG. 3 is a characteristic diagram of an applied bias voltage in a conventional developing process.

FIG. 4 is a characteristic diagram of an applied bias voltage in a developing step of the present invention.

FIG. 5 is a conceptual diagram of a toner layer formed on a developing roller.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Latent image carrier 4, 7 Developing device 21 Developing roller

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Claims (2)

(57) [Claims] An electrostatic latent image on a latent image carrier is sequentially developed with toner of the same polarity as the electrostatic latent image and of a different color by a developing device having respective developing rollers, and is transferred onto transfer paper at one time. A multi-color image forming method for obtaining a multi-color image, wherein a non-contact developing method in which a non-magnetic toner of one component is caused to fly using a DC developing bias in the development of the second and subsequent colors; The toner layer formed thereon has the following relational expression (dt ² / εt) <590 [(μm ² )] (where dt is the thickness of the toner layer measured by a laser length measuring device, ε
t is the volume average relative permittivity of the toner layer, and the volume average relative permittivity εt is εt = (1−α) + αε where the relative permittivity of the toner is ε and the filling factor of the toner layer is α. A) a multicolor image forming method. 2. A multicolor image forming method according to claim 1, wherein said one-component nonmagnetic toner is a particle having a volume average particle diameter of 10 μm or less produced by a polymerization method.