JP2005091683A - Reversal development method using amorphous silicon photoreceptor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a reversal development method by which copies and printed matter having good dot reproducibility and sharpness free of texture fogging can be obtained while sufficient image density is maintained even when an amorphous silicon photoreceptor set with the slightly lower surface potential of the photoreceptor is driven at a high speed in a reversal development method by a jumping phenomenon using the above photoreceptor. <P>SOLUTION: In the reversal development method of causing flying development of a magnetic toner on the surface of a developing roller by impressing a developing bias superposed with an AC component on a DC component, a square wave having a frequency from 2 to 3 KHz, amplitude from 1 to 2 KV and a duty ratio from 25 to 45% is used for the AC component and the DC component is the following inequalities: +180V≤Vdc≤+300V, 70V≤V<SB>0</SB>-Vdc≤180V. In the inequalities, Vdc is a DC voltage (V) and V<SB>0</SB>is the main electrostatic charging potential (V) of the amorphous silicon photoreceptor. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a reversal development method using an amorphous silicon photoconductor, and more specifically, using an electrostatic latent image formed on the surface of an amorphous silicon photoconductor as a magnetic toner (one-component magnetic developer). The present invention relates to a reversal development method for developing by all jumping development methods.

In image forming apparatuses using electrostatic photography such as copying machines, printers, facsimiles, etc., the one-component development method using magnetic toner is the limit of the life due to carrier deterioration like the two-component development method using toner and magnetic carrier. Does not occur, and has an advantage of being a method suitable for a long life. Therefore, in recent years, an example in which such a development method is combined with an amorphous silicon photoconductor having a longer life than an organic photoconductor has been reported. For example, a long-life amorphous silicon photoconductor is charged to 250 to 600 V, and the photoconductor is driven at a peripheral speed of 210 or 263 mm / sec, and reversal development is performed by a jumping development method using magnetic toner. A method is known (see Patent Document 1).
JP 2002-148854 A

  By the way, in the reversal development by the jumping method, in order to fly the magnetic toner, a developing bias in which an alternating current component (which gives vibration to the toner) is superimposed is applied to the direct current component. Since the surface potential (main charging potential) of the photoconductor is low, the direct current component must be set low between the dark potential and the light potential of the photoconductor.

  On the other hand, with the increase in the speed of the image forming apparatus, the peripheral speed of the photosensitive member is also increased, and it is not uncommon to form an image while driving the photosensitive member at a peripheral speed of 300 mm / sec or more.

  However, there is a problem in that dot reproducibility is not good when image formation is performed at a peripheral speed of the photosensitive drum of 300 mm / sec or more using a conventionally known developing bias condition as employed in the above-mentioned patent document. There is. About this cause, the present inventors estimate as follows.

  That is, when the drum peripheral speed increases, the laser output for image exposure does not catch up, the rise of the laser beam for one dot is slow, and the peak output is not sufficiently raised. Since the exposed portion potential is not sufficiently lowered, development is performed without obtaining a sufficient effective potential difference for toner development, which is the difference between the developing bias and the exposed portion potential. As a result, it is considered that the image density of one dot is not sufficient and the image quality is poor in reproducibility.

  Accordingly, an object of the present invention is to achieve a sufficient reversal development method by jumping development using an amorphous silicon photoconductor in which the surface potential of the photoconductor is set low, even when the photoconductor is driven at high speed. An object of the present invention is to provide a reversal development method capable of obtaining a high-quality copy or printed matter having good dot reproducibility and no background fog while maintaining the image density.

According to the present invention, by applying a developing bias in which an AC bias component is superimposed on a DC bias component between an amorphous silicon photoreceptor on which an electrostatic latent image is formed by main charging and image exposure and a developing roller, In the reversal development method in which the magnetic toner on the surface of the developing roller is developed by flying,
The AC bias component uses a rectangular wave having a frequency f of 2 to 3 KHz, an amplitude Vp-p of 1 to 2 KV, and a duty ratio of 25 to 45%,
The DC bias component has the following formula:
+ 180V ≦ Vdc ≦ + 300V,
70V ≦ V ₀ −Vdc ≦ 180V
Where Vdc is the DC bias voltage (V),
V ₀ is the main charging potential (V) of the amorphous silicon photoreceptor,
A reversal development method characterized by satisfying the above is provided.

  In the present invention, it is preferable to perform development while driving the photosensitive member at a peripheral speed of 300 mm / sec or more.

  According to the present invention, when an amorphous silicon photoconductor and a magnetic toner are used, the reversal development by jumping development is performed by setting the direct current component and the alternating current component in the development bias so as to satisfy the above-described conditions. One-dot reproducibility can be maintained without degrading the quality of density, fog density, etc. For example, such effects can be obtained even when developing while driving the photosensitive member at a peripheral speed of 300 mm / sec or more. It can be effectively secured and is extremely suitable for high-speed development.

  In FIG. 1 showing a schematic configuration of an image forming apparatus for carrying out the reversal developing method of the present invention, a main charging device 2, a developing device 3, A transfer roller 4 and a cleaning device 5 are provided, and an LSU (laser scan unit) (not shown) is provided above the charging device 2 and the developing device 3 to provide a laser for image exposure. Light L is irradiated. The recording paper P passes through a transfer nip N formed between the photosensitive drum 1 and the transfer roller 4, and a fixing device 7 is installed on the downstream side in the transport direction.

  The amorphous silicon photosensitive drum 1 is provided with an amorphous silicon photosensitive layer on a conductive element tube such as aluminum. For example, the photosensitive drum 1 is driven in a direction indicated by an arrow a by driving a predetermined motor. The image forming process is executed while rotating at the peripheral speed.

  As the charging device 2, a corona charger such as corotron or scorotron, or a charging roller is used, and the surface of the amorphous silicon photosensitive drum 1 is uniformly charged in the range of +250 to + 480V by corona charging or contact charging. The This charging potential is lower than other photoconductors such as organic photoconductors.

  After the main charging as described above, the surface of the photosensitive drum 1 is irradiated with laser light L based on image information input from a computer or image information related to a predetermined document read by a scanner or the like. An electrostatic latent image corresponding to the information is formed. That is, the potential of the light irradiating portion is lowered and an electrostatic latent image is formed.

  The developing device 3 includes a developing roller 3b with a built-in magnet disposed opposite to the photosensitive drum 1 at the opening of the developing container 3a, and the magnetic toner carried on the surface of the developing roller is transferred to the electrostatic latent image on the surface of the photosensitive drum 1. And is developed (visualized) as a toner image.

  That is, the magnetic toner accommodated in the developing container 3a is agitated by the agitating member, frictionally charged to the same polarity as the charging polarity of the photosensitive drum 1 (that is, positive polarity), and conveyed to the developing roller 3b side. The developing roller 3b is disposed with a predetermined gap (about 0.3 mm) with respect to the photosensitive drum 1, and the magnetic toner on the surface of the developing roller 3b is formed to have a predetermined layer thickness by the layer thickness regulation by the regulation blade 3c. Is done.

  Further, a predetermined developing bias in which an AC component is superimposed on a DC component is applied from the developing bias power supply 10 between the developing roller 3 b and the photosensitive drum 1, and positively charged toner is electrostatically charged to the photosensitive drum 1. It adheres to the latent image portion (that is, the bright potential portion where the potential is lowered by light irradiation). Such development bias will be described later.

  The magnetic toner to be used may be a publicly known toner, in which a binder such as a colorant such as carbon black, a positive charge control agent, an offset preventive agent such as wax, or a magnetic agent such as magnetite is blended. It is. Such toner has a particle size of about 6 to 12 μm, and powder of hydrophobic silica, alumina, zinc oxide, titanium oxide or the like is usually 0.1 to 3 parts by weight per 100 parts by weight of the toner. In this amount, the surface treatment agent is externally added to improve the fluidity.

  The toner image formed on the surface of the photosensitive drum 1 as described above is transferred onto the recording paper P by the transfer roller 4.

  The transfer roller 4 is conductive and is disposed in contact with the surface of the photosensitive drum 1 with a predetermined pressing force, forms a transfer nip portion N, and rotates in the direction of arrow b (counterclockwise). To do. When the recording paper P passes through the transfer nip N, a transfer bias is applied to the transfer roller 4 from the transfer bias power source 6 so that the toner image on the surface of the photosensitive drum 1 is transferred to the surface of the recording paper P. . This transfer bias has a polarity opposite to the charging polarity of the toner image, and is usually about 200 V to 3 KV, and is controlled by the transfer current. In the case where images are continuously formed, the transfer bias is changed to 0 V or reverse bias between sheets, and toner adhesion to the surface of the transfer roller 4 is prevented. Instead of using the transfer roller 4, a negative corona charger and an AC corona charger can also be used as a transfer device. In this case, the back side of the recording paper P (photosensitive drum 1) is transferred by the negative corona charger. The toner image is transferred to the surface of the recording paper P by electrostatic attraction, and the recording paper P is prevented from being wound around the photosensitive drum 1 by charge removal by an AC corona charger. Is done.

  The recording paper P on which the toner image is transferred is introduced into the fixing device 7 and the toner image is fixed on the recording paper P. The fixing device 7 includes a fixing roller 7a with a built-in heater and a pressure roller 7b. By passing the recording paper P through a fixing nip between the fixing roller 7a and the pressure roller 7b, the surface of the recording paper P is obtained. The toner image transferred to is heat-pressed and heat-fixed.

  On the other hand, the toner remaining on the surface of the photosensitive drum 1 after the transfer of the toner image is removed and collected by the cleaning blade 5a. The cleaning blade 5a is made of polyurethane rubber or the like, and usually has a rubber hardness (JIS A) of 60 degrees or more, and is pressed against the surface of the photosensitive drum 1 with a pressing force of about 10 to 30 g / cm.

  By the way, in the image forming apparatus as described above, when the photosensitive drum 1 is driven at a high speed (for example, at a peripheral speed of 300 mm / sec or more), in order to expose one dot with a laser, the rise of LSU emission Responsiveness cannot follow the movement of the photoconductor, the potential of one dot area cannot be sufficiently lowered, and the development time in the development area is also shortened, so dot reproducibility is impaired. In this case, it is conceivable to make the latent image of 1 dot level clear by thinning the photosensitive layer of the photosensitive drum 1, but the surface potential is lowered. That is, since the amorphous silicon photoconductor has a low charging potential, performing further low-potential development causes a reduction in image quality such as a reduction in image density and occurrence of fogging. However, in the present invention, by satisfying the following conditions as an AC component and a DC component of the developing bias, a decrease in dot reproducibility accompanying the increase in the speed of the drum causes a decrease in image density and the occurrence of fog. Therefore, it can be effectively avoided.

With reference to FIG. 2 showing the waveform of the developing bias employed in the present invention, as an AC component at the developing bias voltage applied to the charging roller 3b,
Frequency f: 2 to 3 KHz,
Amplitude Vp-p: 1 to 2 KV,
Duty ratio [In FIG. 2, area ratio (A / B) × 100]: 25 to 45%
Use a rectangular wave that satisfies.

  In the jumping development method, it is necessary to apply toner to the developing bias to vibrate the toner so that it can fly easily. When the frequency f is lower than the above range, the toner particles cannot be sufficiently vibrated, and the toner flying to the latent image portion formed on the surface of the photosensitive drum 1 becomes unstable. , Causing density defects and fogging. On the other hand, when the frequency f is higher than the above range, the toner particles are vibrated finely, so that it is difficult to fly and fog is difficult, but the image density is lowered and the dot reproducibility is deteriorated. That is, by setting the frequency f in the above range, it is possible to apply appropriate vibration to normally used toner particles of 6 to 12 μm and to obtain an appropriate image quality without fogging.

  If the amplitude Vp-p of the alternating current component is made larger than the above range, the vibration width of the toner particles is increased and the image density and dot reproducibility are improved, but the developing amount is increased, so that it is easy to cover. Further, the drum 1 is likely to leak. On the other hand, when the amplitude Vp-p is lower than the above range, the toner is less likely to vibrate, and the image density is lowered.

  Further, in order to increase the developing ability, it is sufficient to set the AC bias Vp-p and a DC voltage Vdc described later to a large value, but it is likely to leak to the photoconductor. In the present invention, by setting the duty ratio to 45% or less, a potential difference sufficient for a latent image of one dot portion where the potential on the photosensitive member is not sufficiently lowered under the condition that no leakage to the photosensitive member occurs. The toner can be developed. However, when the duty ratio is less than 25%, the peak voltage of the developing bias becomes higher than the withstand voltage level of the photosensitive drum 1, and leakage to the photosensitive member is likely to occur. That is, in the present invention, by setting the duty ratio to 25% to 45%, it is possible to increase the developing capability and obtain good dot reproducibility without leaking to the photosensitive drum 1.

  Also, by setting the duty ratio to 45% or less, the potential difference for removing fog is reduced, but the development time is set longer, so that the weakly charged toner adhering to the photoreceptor can be returned to the developing roller, The effect of preventing background fogging will be improved.

In the present invention, the rectangular wave as described above is used as the AC component, and the DC bias component is expressed by the following formulas (1) and (2);
+ 180V ≦ Vdc ≦ + 300V (1)
70V ≦ V ₀ −Vdc ≦ 180V (2)
Where Vdc is the DC bias voltage (V),
V ₀ is the main charging potential (V) of the amorphous silicon photoreceptor,
It must be satisfied.

  That is, with respect to the above formula (1), if the DC voltage Vdc applied to the charging roller 3b exceeds + 300V, leakage to the photosensitive drum 1 is likely to occur, and if it is less than + 180V, toner flying occurs. Insufficient image density will result.

Further, with respect to the above formula (2), V ₀ -Vdc is called a blank paper potential (or fog removal potential). When this blank paper potential is set to less than 80 V, the potential difference at which the toner is developed in terms of development characteristics. Therefore, background fog is likely to occur. On the other hand, when the blank paper potential is set higher than 180 V, the reversely charged toner is developed on the photosensitive drum 1 and the background fog is likely to occur. (For normal charged toner, the positively charged drum 1 does not develop toner due to repulsive force, but reversely charged toner is attracted to the drum 1.) When developing toner on an image, positively charged toner particles are affected by the positive potential of the non-exposed area and repulsive, making it difficult to develop, resulting in poor 1-dot reproducibility. Therefore, as shown in the above equation (2), by setting the blank paper potential (V ₀ -Vdc) to 80 V to 180 V, it is possible to obtain good dot reproduction without fogging.

  As described above, according to the present invention, even when the amorphous silicon photosensitive drum 1 is driven at a peripheral speed of 300 mm / sec or more to perform high-speed development, image defects such as a decrease in image density and occurrence of fogging occur. Therefore, dot reproducibility can be ensured.

  Using a digital printer equipped with an amorphous silicon photoconductive drum of the following specifications, as shown in Table 1, various development biases and main charging potentials were changed, and image output was performed. Evaluation was performed.

Photosensitive drum:
Aluminum tube diameter: 90mm
Amorphous silicon photosensitive layer; initial thickness 30μm
Drum width: 240mm
Drum peripheral speed: 350mm / sec
Main charger: Scorotron Developer:
Positively chargeable magnetic toner Surface treatment agent (hydrophobic silica); 1 part by weight per 100 parts by weight of toner Cleaning blade;
Polyurethane rubber hardness (JIS A); 65 degrees Fixing device:
Fixing temperature: 190 ° C
Recording paper: A4 size Transfer current: -8 μA
Transfer roller peripheral speed: 1.02 times the photosensitive drum peripheral speed

The image density, fog and dot reproducibility were evaluated as follows.
Image density (ID); N (Munsell value) = 1.0 Output image for a solid black document is measured with a Tokyo Denshoku white densitometer (TC6MC). ID = 1.3 or higher, less than 1.3 Was impossible.
Fog (FD); FD is calculated by measuring the output image of a blank document with N (Munsell value) 9.5 using the white densitometer and subtracting the white paper portion density (base paper density) of the recording paper from this value. FD = 0.01 or less is acceptable, and the case where it is larger than 0.01 is not acceptable.
Dot reproducibility: An output image for a dot pattern original (corresponding to 600 dpi) with an area ratio of 6.25% is measured by the white densitometer, and is calculated by subtracting the base paper density from that value, and the dot pattern is reproduced. 0.023 or more is acceptable, and less than 0.023 is impossible.

1 is a schematic configuration diagram showing an image forming apparatus used for implementing the present invention. The figure which shows the waveform of the developing bias employ | adopted by this invention.

Explanation of symbols

1: Photosensitive drum 2: Charging device 3: Developing device 4: Transfer roller 5: Cleaning device 5a: Cleaning blade 7: Fixing device 7a: Fixing roller 7b: Pressure roller

Claims (2)

Magnetic toner on the surface of the developing roller is applied by applying a developing bias in which an AC bias component is superimposed on a DC bias component between the amorphous silicon photoreceptor on which an electrostatic latent image is formed by main charging and image exposure and the developing roller. In the reversal development method for flying development,
The AC bias component uses a rectangular wave having a frequency f of 2 to 3 KHz, an amplitude Vp-p of 1 to 2 KV, and a duty ratio of 25 to 45%,
The DC bias component has the following formula:
+ 180V ≦ Vdc ≦ + 300V,
70V ≦ V ₀ −Vdc ≦ 180V
Where Vdc is the DC bias voltage (V),
V ₀ is the main charging potential (V) of the amorphous silicon photoreceptor,
A reversal development method characterized by satisfying the above.   The reversal development method according to claim 1, wherein the development is performed while driving the photosensitive member at a peripheral speed of 300 mm / sec or more.

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