JPH052309A - Multicolor image forming method - Google Patents

Abstract

PURPOSE:To stably form an excellent multicolor image without having base soiling and a mixed color over a long period, regardless of the sticking state of the toner of the first color by allowing a toner layer used for noncontact development to comply with specific conditions. CONSTITUTION:In this multicolor image forming method for obtaining a multicolor image in such a manner that an electrostatic latent image on a latent image carrier 1 is sequentially developed with the toner having the same polarity as that of the electrostatic latent image and a different color by developing devices 4 and 7 having respective developing rollers, for the development of the second color or after, a noncontact developing system for soaring up one-component nonmagnetic toner is adopted, and the toner layer on the developing roller, of the second color or after, satisfies dt<2>/epsilont<590[(mum)<2>] (but, the (dt) is the thickness of the toner layer by a laser end measuring machine, the (epsilont) is the volume average dielectric constant of the toner layer, the volume average dielectric constant is epsilont=(1-alpha)+alphaepsilon, when the dielectric constant of the toner is defined as (epsilon), and a toner layer filling ratio is defined as (alpha)). On the other hand, a grain produced in a polymerizing method, has <=10mum volume average diameter.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used in copying machines, facsimiles, printers, etc., and is used for forming a multicolor image on an electrostatic latent image carrier without transferring colors and transferring it to a paper. The present invention relates to a color image forming method.

[0002]

2. Description of the Related Art In recent years, a multi-color image is formed by superposing an electrostatic latent image carrier such as a photoconductor drum (hereinafter referred to as a latent image carrier) to develop a plurality of colors and transferring the image once to a sheet. Various multicolor image forming apparatuses (multicolor image forming methods) have been proposed.

For example, in Japanese Patent Laid-Open No. 63-60471, a plurality of developing devices are provided around a latent image carrier to obtain a multicolor image, and the toner polarity of the developing device is measured downstream. In contrast to the upstream measurement, the toner layer is not in contact with the latent image carrier, and an electric field is applied separately during downstream measurement development and upstream measurement development. A multi-color image forming method is disclosed in which the upstream measurement is a two-component developer having a chromatic color toner and the downstream measurement is a one-component developer having a black toner.

Further, in Japanese Patent 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 is used. A method is disclosed in which a multi-color image is obtained on the same photoconductor by arranging an agent unit and repeating charging exposure development a plurality of times. In this method, the developing device and the photoconductor are arranged at an interval of 250 μm or less. This is a non-contact DC bias development using thin toner, and the donor roller that does not contribute to the development is stopped, and the toner on the surface is adhered outside the image area to prevent color mixture. The photoreceptor is 0 to 50 μm, the development is reversal development, the potential contrast is 400 V or more, the thickness of the toner layer on the photoreceptor is 5 to 30 μm, and the charging is scorotron.

Further, in Japanese Patent 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, and the first developing device on the upstream side is rotated. The speed is made faster than that of the recording medium by rotating in the same direction, and black development is applied to apply a biased AC bias, and the downstream developing devices after the second developing device are subjected to constant speed development. A multicolor image forming method of color development and applying only a DC bias voltage is disclosed.

Generally, in the case of forming a multicolor image on a latent image carrier, non-contact development is used as the developing method for the second and subsequent colors to facilitate colorization, and an image forming method using a non-magnetic toner, particularly downsizing and low cost. A one-component method that is advantageous for cost reduction is preferable. This is a development in which the toner layer and the latent image carrier are opposed to each other in a non-contact manner.

[0007]

However, in the above-mentioned conventional multicolor image forming method, since the toner image is formed on the latent image carrier at the time of developing the second color, for example, the above-mentioned JP-A-63-60471. In the publication, when an AC bias voltage is applied as a developing bias, the toner reciprocates between the surface of the latent image carrier and the surface of the toner carrier, so that not only the toner image of the first color is disturbed but also the developing device of the second color is affected. There is a problem that the second color gradually becomes turbid due to the mixing of the second color toner.

Therefore, in order to prevent this problem, the above-mentioned JP-A-63-63061 and JP-A-63-8557.
In JP-A-8, the second and subsequent developments are carried out by flying non-magnetic toner with a DC electric field.

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

Further, when transferring the toner layer under a DC electric field, a good image cannot be obtained unless the potential difference between the image portion and the applied bias voltage is set sufficiently large. This is because in the fly development with a DC electric field, the toner does not fly unless the electric field exceeds the threshold electric field, that is, the electric field at which the toner starts to fly in the developing area. This threshold electric field is a value that depends on the amount of charge of each toner, van der Waals force, etc., and this variation is large, so that the entire toner layer can fly during development in order to obtain a stable density image. A sufficiently large potential difference is required.

In view of this, 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 adhesion state of the toner of the first color on the latent image carrier. It is an object of the present invention to provide a multicolor image forming method that can be performed.

[0012]

SUMMARY OF THE INVENTION The gist of the present invention is to separate an electrostatic latent image on a latent image carrier from toner particles of different colors having the same polarity as that of the electrostatic latent image. In a multicolor image forming method of sequentially developing with a developing device and transferring to a transfer paper at one time to obtain a multicolor image, the development of two or more colors is a non-contact development system in which a non-magnetic toner of one component flies, The toner layer formed on the developing roller after the second component has the following relational expression: (However, dt is the thickness of the toner layer measured by the laser length measuring device,
εt is the volume average relative permittivity of the toner layer, and this volume average relative permittivity εt is εt = (1−α) + αε, where ε is the relative permittivity of the toner and α is the toner layer filling rate. ) Is met.

[0013]

[Action]

If the conditions are satisfied, the rise 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 from being caused by the low charge amount toner. Further, in order to obtain a sufficient toner concentration, it is necessary for the toner layer to have two or three layers, and 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 toner layers is two or three, the thickness of the toner layer is only about 20 μm and the above conditions are satisfied, 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 photoconductor drum 1 is provided in the vicinity of the periphery of the photoconductor drum 1 which is a latent image carrier, and a first charger for forming a first toner image. Developing device 4, second charger 5 for recharging photosensitive drum 1, second developing device 7 for forming a second electrostatic latent image, and photoconductor of second developing device 7. Cleaning devices 11 are arranged at positions opposite to each other across the drum 1. When an organic photoconductor having a negative charging property is used as the photoconductor drum 1, a toner having a negative charging property is also used.

Any developing means may be used for the first developing device 4 regardless of contact or non-contact, but in this embodiment, two-component development in which toner is charged by friction with a carrier. The contact development method using a magnetic brush of the agent is used. Further, the second developing device 7 is preferably made of non-magnetic toner that is easy to color, and uses a one-component non-contact method that is particularly advantageous for downsizing and cost reduction.

Then, the scanning beams 3 and 6 by the first and second exposing means (not shown) are focused on the surface of the photosensitive drum 1.

Further, a conveyor belt 8 for conveying a transfer sheet (not shown) is arranged so as to be pressed against the photosensitive drum 1 by being pressed against the transfer charger 9 from the back side of the conveyor belt 8 and measured downstream thereof. A fixing device 10 for fixing the image on the transfer paper is arranged in the.

The operation of the image forming apparatus having the above structure 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 that of the surroundings. Then, the scanning beam 6 forms a second electrostatic latent image on the photosensitive drum 1. At this time, it is possible to prevent the toner of the second color and the toner of the first color from overlapping each other on the photosensitive drum 1 and making the color turbid unless the portion where the toner of the first color adheres is exposed.

Next, the second electrostatic latent image is transferred to the second developing device 7
Develop by. Further, since the DC bias voltage is made substantially equal to the charging potential of the photoconductor drum 1 for development, even if the toner is used for a long period of time, color mixture due to reverse flight of the toner of the first color does not occur. At this time, the toner layer used for the development of the second and subsequent colors satisfies the conditions described below, or the one-component non-magnetic toner produced by the polymerization method and having a volume average particle diameter of 10 μm or less is used to cause the background stain. To be prevented.

In this way, a multicolor image is sequentially formed on the photosensitive drum 1 and transferred onto the transfer paper on the conveying path 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 photoconductor drum 1 is wiped off by the cleaning unit 11, and the photoconductor drum 1 is made ready for image formation again.

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.
This drawing shows a developing device using a non-magnetic one-component toner (second
The developing device 7) is shown. The toner 26 of a color different from that of the first color is supplied to the hopper 25 and agitated 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 adhered toner is made into a uniform toner layer by the toner layer forming member 23 and is conveyed to the developing area. The amount of toner adhered to the developing roller 21 at this time is preferably such that a sufficient image density can be obtained even when the developing roller 21 and the photosensitive drum 1 are opposed to each other in a non-contact manner and move at a substantially constant speed. Further, if there is a speed difference between the developing roller 21 and the photosensitive drum 1, the phenomenon of “toner rear end shift” occurs, the density of the solid portion becomes uneven, and the image quality of the color image deteriorates. The toner to be used is required to have a layered structure of 2 to 3 layers. For this reason, it is preferable to use, as the developing roller 21, one in which an aluminum surface is subjected to sandblasting, one in which a minute groove on the surface of the aluminum roller is filled with a dielectric substance, or the like.

In developing the second color, it is necessary to perform the 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 0.05-0.5 mm,
A range of 0.1 to 0.2 mm is suitable. If this gap is less than 0.05mm, the first color and the second color
There is a risk that the toner layers of different colors may come into contact with each other.
When m or more, the line portion of the electrostatic latent image becomes a closed electric field, a developing electric field is not formed between the developing roller 21 and the photoconductor drum 1, and flight development becomes impossible. During 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 due to the charge amount of the toner itself. The potential of the writing portion of the second color is -100V and the potential of the background portion is -80
If the voltage is 0V, the potential of the first toner image portion of the first color is -8.
It is 50 to -900V. This developing bias voltage is 1
The toner of the second color does not fly backward and mix into the second developing device 7 of the second color even over time, and the non-magnetic toner layer satisfactorily flies to the latent image potential portion of the second color, and the photosensitive drum The toner must not adhere to the background part of 1. For example, in the process as shown in FIG. 3, the applied bias voltage for the second color is about −700 V [(f) in the figure], and the difference of 100 V from the background potential is necessary as the background stain prevention 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 adhered state of the toner of the first color changes with time or the environmental change causes a decrease in the charge amount or an increase in the adhered amount. In this case, color mixture due to reverse flight easily occurs.

Therefore, as in the step shown in FIG. 4, the bias background potential of −800 V applied during the development of the second color is set to approximately −800 V [(f) in the figure]. This allows
The reverse flying potential difference of the first color toner is always maintained at 100 V or less, and the development gap exists, so even if there is a change in the development of the first color over time or in the environment (regardless of the adhesion state of the first color, ) Color mixing rarely occurs. Furthermore, the developing 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 flight developing method.

Next, the prevention of background stain in the step shown in FIG. 4 will be described. The increase 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 formula. E (y) is the applied bias voltage, εt is the volume average relative permittivity 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 toner layer. As for thickness, Conservation law of electric charge ρdt + σ = 0 (3) Therefore, the potential V of the toner surface layer is

[0025]

[Equation 1]

The smaller the potential rise, the developing roller 2
It is possible to prevent the background stain due to the low charge amount toner carried on the surface layer of No. 1. Further, in the development, a clearer image is obtained as the electric charge amount of the toner is larger, and the adhesive force to the developing roller 21 is strengthened. The charge amount (Q / M) per unit mass of this toner layer is 5
It is at least μC / g, preferably at least 10 μC / g. This Q / M is measured by the suction method, and is about 3
About 10 mg of toner is sucked into a 0 g Faraday cage and the amount of electric charge is measured to obtain Q / M. Further, the amount of adhesion must be such that a sufficient density can be obtained even when the photosensitive drum and the developing roller 21 move at substantially the same speed, and the toner must be in two or three layers. The adhesion amount (M / V) per unit volume of the toner layer is represented by the bulk specific gravity of the toner used. Generally, the bulk density 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 rising potential V can be suppressed to 50 V or less even though the charge amount in the toner layer on the surface is low Necessary to prevent adhesion. Therefore,
The following relations are 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 amorphous particles, and the volume average particle diameter is 10 μm or less, so that even if the toner is stacked in two or three layers. The toner layer thickness is only about 20 μm. Therefore, the condition of the above formula (11) is inevitably satisfied.

Here, εt = (1−α) + αε (where the relative permittivity of air is 1), where ε is the relative permittivity of the toner and α is the filling rate of the toner layer. The true specific gravity of the toner is m (g / cm ³ ), and the bulk specific gravity of the toner is n (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, and
Black development of the color is performed by contact development with a two-component magnetic brush. Then, the photosensitive drum 1 on which the black toner image is formed is recharged, and the second electrostatic latent image is formed by exposure with the second scanning beam 6. The amount of solid black toner deposited on the photosensitive drum 1 after the recharging is about 0.8 mg / cm ² , and the charge amount Q / M per unit mass of the toner layer is -25 μC / g. At this time, the photoconductor potential of the black toner adhering portion of the photoconductor drum 1 is −880V, the background portion is −800V, and the exposed portion is −10V.
It is 0V. The color development of the second color is performed by the irregular toner having a volume average particle diameter of 7 μm produced by a pulverization method, and the linear velocity of the photosensitive drum 1 and the developing roller 21 is equal to 120 mm / sec. The developing roller 21 carrying the non-magnetic one-component toner layer has an aluminum surface subjected to a sandblast process. This toner layer has a charge amount Q / M per unit mass of −18 μC / g,
The toner layer thickness dt measured by the laser 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.
/ Cm ³ and the bulk specific gravity n are 0.42 g / cm ³ , the toner layer filling rate α is 37.8%. Therefore, the volume average relative permittivity εt of the toner layer is 1.8, and the volume average charge amount ρ is 7.6 C / cm ³ .

This toner layer satisfies the condition of the above formula (11).

[0034] Under the above conditions, the developing roller 21 and the photosensitive drum 1
And the gap between them is 0.12 mm, and the developing bias voltage is -80.
When 0 V was applied and development was carried out, a good multicolor image free of background stains could be obtained, and a good image was obtained without color mixing even after 5000 sheets were continuously produced. Was given.

Next, a second example of the numerical examples will be described. The developing conditions for the first color are the same as those in the first embodiment, and the non-magnetic one-component toner used for developing the second and subsequent colors is an irregular toner having a volume average particle diameter of 11 μm. The charge amount Q / M per unit mass of this toner layer is −
12 μC / g, toner layer thickness dt measured by a laser length measuring device is 33 μm, relative permittivity ε of toner is 2.7, true specific gravity m is 1.02 g / cm ³ , and bulk specific gravity n is 0.31 g / c.
m ³ . Therefore, the toner layer filling factor α is 30.4%, and the volume average relative permittivity εt is 1.5.
The value of the equation (1) is dt ² / εt = 726 (μm) ² and does not satisfy the equation (11).

Under the above conditions, when the developing bias voltage of -800V was applied with the gap between the developing roller 21 and the photosensitive drum 1 set to 0.12 mm, the background stain was observed. Then, after the development bias voltage was lowered to -700 V and 5,000 continuous multicolor images were produced, it was observed that the mixed colors were turbid.

Further, the third embodiment of the numerical embodiments will be described. The developing conditions for the first color are the same as in the first embodiment, and the toner conditions used for developing the second and subsequent colors are the second.
The same as in the embodiment of FIG. The charge amount Q / M per unit mass of the toner layer is -9 μC / g, the toner layer thickness dt measured by the laser length measuring device is 32 μm, and the relative dielectric constant ε of the toner is 3.
7, true specific gravity m is 1.05 g / cm ³ , and bulk specific gravity n is 0.
It is 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 of the equation (11) dt ² / εt = 512 (μm) ²
Since it is <590 (μm) ² , the formula (11) is satisfied.

Then, the developing roller 21 and the photosensitive drum 1
Bias voltage -800
When V was applied and development was performed, a background stain was not seen, a good multicolor image was obtained, and no color mixture was seen even after continuously producing 5000 sheets, and a clear image was kept. .

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

Under the above-mentioned conditions, when a developing bias voltage of -800 V was applied with the gap between the developing roller 21 and the photosensitive drum 1 set to 0.12 mm, development was carried out. Obtained and continuously 500
Even after preparing 0 sheets, no color mixture was observed, color vividness was maintained, and a stable and good multicolor image was obtained.

[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:
Since the condition of the volume average relative dielectric constant of the toner layer) is satisfied, the reverse flight of the toner and the background stain are prevented without being influenced by the adhesion state of the toner of the first color to the photosensitive drum, and there is no color turbidity. A good multicolor image can be stably obtained without aging.

Further, 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 produced by the polymerization method, in order to obtain a sufficient density of the toner image, Even if the toner is held in two or three layers on the developing roller, the above formula is satisfied, and similarly, a good multicolor image free from color turbidity can be stably obtained.

[Brief description of 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 showing a developing roller in the development of 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 the developing process of the present invention.

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

[Explanation of symbols]

1 latent image carrier 4,7 Developing device 21 developing roller

Claims (2)

[Claims] 1. An electrostatic latent image on a latent image carrier is sequentially developed by a developing device having respective developing rollers with toners of different colors having the same polarity as that of the electrostatic latent image, and transferred onto a transfer paper at once. In a multi-color image forming method for obtaining a multi-color image by transfer, the development of two or more colors is a non-contact development method in which a non-magnetic toner of one component flies, and is formed on the developing roller of the second component or later. The toner layer has the following relational expression (However, dt is the thickness of the toner layer measured by the laser length measuring device,
εt is the volume average relative permittivity of the toner layer, and this volume average relative permittivity εt is εt = (1−α) + αε, where ε is the relative permittivity of the toner and α is the toner layer filling rate. ) Is satisfied, a multicolor image forming method. 2. The multicolor image forming method according to claim 1, wherein the one-component non-magnetic toner is particles produced by a polymerization method and having a volume average particle diameter of 10 μm or less.