JP2003295586A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain an electrophotographic image forming apparatus using an LED (light emitting diode) head consisting of an LED array and a rod lens array as an exposure light source, and providing high image quality after stable printer characteristic is realized by widening the latitude of the LED head to the deviation of an image forming position and reducing image ram or density fluctuation. <P>SOLUTION: In the electrophotographic image forming apparatus using the LED head consisting of the LED array and the rod lens array as the exposure light source, the surface layer of a photoreceptor drum is hardly shaved by using a drum which uses an a-Si photoreceptor as the photoreceptor drum, and the divergence of focus caused by the shaving of the surface layer of the drum is restrained to the minimum. <P>COPYRIGHT: (C)2004,JPO

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, and more particularly to an electrophotographic image forming apparatus.

[0002]

2. Description of the Related Art Conventionally, in an image forming apparatus such as a digital copying machine or a printer in an electrophotographic system, a semiconductor laser is generally used as an exposure light source. This is because a laser beam emitted from a semiconductor laser is deflected by an optical deflector such as a rotating polygon mirror, and an image is formed on the surface of a photosensitive drum through an fθ lens to optically scan and form an electrostatic latent image. ing.

On the other hand, in recent years, many image forming apparatuses using a light emitting diode array (hereinafter referred to as an LED array) as an exposure light source have appeared. This is LE
The light flux emitted from the D array is imaged on the surface of the photosensitive drum via the rod lens array to form an electrostatic latent image.

FIG. 2 is a schematic view of the periphery of a photosensitive drum of an image forming apparatus using an LED array as an exposure light source.

In the figure, an LED array is arranged in a direction orthogonal to the rotation direction of the photosensitive drum to selectively emit the LEDs, and an image forming optical system arranged in front of the LED issuing section is used. The image from the LED is formed on the surface of the photosensitive drum by rotating the photosensitive drum at the same time.
An electrostatic latent image is formed three-dimensionally.

In such an image forming apparatus using an LED array, a rod lens array is generally used as an imaging optical system, and an LED is used as an exposure optical system.
It is used as an LED head in which an array and two plates are integrated with a rod lens array in which rod lenses are regularly aligned in one or two rows.

FIG. 3 is an illustration of a rod lens. In the figure, when the working distance, which is the distance from the end surface of the rod lens to the object or the image plane, is L0, and the length of the rod lens itself is Z0, the conjugate length Tc of the rod lens is Tc = Z0 + 2L0.

The image transmission characteristics of this rod lens array are evaluated by the quality of the image sent, that is, the resolution. This is represented by MTF (Modulation Trans).
fer Function). This is an index to see how faithfully an original image of a rectangular wave pattern image as shown in FIG. 4 can be reproduced after passing through the rod lens array. From FIG. 4, MTF is defined as follows.

MTF (w) = (I (w) max-I
(W) min) / (I (w) max + I (w) min)
× 100% Here, I (w) max and I (w) min are the maximum and minimum values of the rectangular wave response at the spatial frequency w (lp / mm), respectively. The closer the MTF is to 100%, the more faithfully the image is reproduced.

This MTF is greatly different when measured at the focal position of the rod lens array and when measured at a position deviated from the focal position. Further, since the Tc length is generally around 10 mm, the system is more sensitive to the shift of the focal position than a laser optical system having a relatively long focal length. Therefore, it is devised so that the distance between the photosensitive drum and the LED head does not change.

By constructing an LED head using such an LED array and rod lens array and using it as an exposure light source, it is possible to dispose the exposure light source in the vicinity of the photosensitive drum and to use a semiconductor laser as the exposure light source. It is possible to reduce the size of the image forming apparatus as compared with the system using the.

Further, when focusing on the photosensitive drum, there are generally two types of structures, that is, a single layer structure and a function separation laminated structure. Among them, in recent years, the function-separated laminated type is becoming mainstream. FIG. 5 shows a cross-sectional view of a general type of function-separated laminated structure type drum. As shown in FIG. 5, a carrier transport layer (hereinafter referred to as CTL) is provided on the surface layer of the photosensitive drum.

The image forming position of the LED array is generally set so as to be on the surface of the photosensitive drum.

[0014]

However, for example, consider the case where the image forming position of the LED head is set on the light carrier generating layer (hereinafter referred to as CGL) of the photosensitive drum.

The CTL of the photosensitive drum is worn away by image formation. For example, if it has a thickness of 30 μm,
At the initial stage and near the end of life, the focus position is displaced.

For example, if the refractive index of CTL is 1.6, the focus shift is caused by the difference between the refractive index of air and the refractive index of 1 of air.

Then, the MTF deteriorates from the ideal state and the optical profile changes. Here, FIG. 6 shows an ideal light profile on the imaging plane and a light profile at a point 30 μm away from the imaging position. As is apparent from FIG. 6, the light profile spreads at a position deviated from the image formation position, and the exposure intensity gradient at the end of the spot becomes flat.

On the other hand, as shown in FIG. 7, the characteristics of a general photosensitive drum exhibit a behavior of reacting sensitively when the amount of light is small. Further, in terms of electrophotographic characteristics, the developing characteristics are generally as shown in FIG. 8, and the behavior is sensitive when the contrast potential is intermediate. That is, the fluctuation of the concentration is likely to occur.

Then, since the exposure intensity gradient is laid out at the spot edge, the portion having an intermediate potential at the spot edge becomes relatively large as compared with the case where the exposure intensity gradient is steep. However, after the final development, the change in the density tends to be large. As a result, the output density characteristic becomes unstable and image density unevenness is likely to occur. In a full-color image forming apparatus, this finally appears as color unevenness, which is a major cause of image quality deterioration.

Therefore, in the present invention, in an electrophotographic image forming apparatus using an LED head composed of an LED array and a rod lens array as an exposure light source, the latitude with respect to the image forming position deviation of the LED head is widened, and image ram and density are increased. It is an object of the present invention to provide high image quality while reducing fluctuations and realizing stable printer characteristics.

[0021]

A photosensitive drum uniformly charged by a charging means comprises a light emitting diode array in which a plurality of light emitting diodes are linearly arranged as an exposure light source and a rod lens array as an image forming optical system. An electrostatic latent image is formed by exposing using an LED head as an exposure device, the electrostatic latent image is visualized by a developing unit, and the image on the image carrier is transferred onto a transfer material by a transfer unit, Finally, in the image forming apparatus for forming an image by fixing the image on the transfer material to the transfer material by the fixing means, as a photosensitive drum,
An a-Si photoconductor is used.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS (Embodiment 1) FIG. 1 is a schematic sectional view of a main body of an image forming apparatus according to this embodiment.

In the figure, 1 is an LED as an exposure light source.
It is an array, and is configured by arranging a plurality of LEDs in a straight line in the axial direction of the photosensitive drum as an image carrier.
Reference numeral 2 denotes an erecting equal-magnification rod lens array as an imaging element, which is configured by arranging a plurality of rod lenses linearly with respect to the axial direction of the photosensitive drum. The image is formed on the drum surface at the same size.
In this embodiment, the LED array and the rod lens array are integrated and used as an LED head.

Next, the image forming process will be briefly described.

First, a photosensitive drum is uniformly charged by a primary charger, and an LED latent image is exposed on the photosensitive drum to form an electrostatic latent image. The electrostatic latent image is visualized by a developing device and transferred onto a transfer paper by a transfer means. The toner image transferred onto the transfer paper is fixed by the fixing means.

The number of reproduced gradations per pixel is binary.

In the present embodiment, the above-mentioned image forming process is performed at the same time for four colors to realize high speed operation.

The first station is Yellow, the second station is Magenta, and the third station is Cy.
An, the fourth station forms a Black image.

Here, the LED head in this embodiment will be described in detail.

First, the LED array has an element density of 600.
In other words, the dpi, that is, the array period of the elements is 42.3 μm. In addition, the electrode size of the light emitting part of the LED is about 20μ.
m. An image is formed on the surface of the photosensitive drum by using the LED as an exposure light source.

Next, a rod lens array which is an image forming optical system will be described. The rod lens array is an erecting equal-magnification imaging lens. In this embodiment, the diameter of the rod is 0.6 mm.

The rods are arranged in two rows as shown in FIG.

The Tc length of this rod lens array is 9.9.
mm.

Now, the photosensitive drum will be described.

The photosensitive drum used in this image forming apparatus is an a-Si type photosensitive drum. This sectional view is shown in FIG.

A carrier injection blocking layer having a thickness of 3 μm is formed on the aluminum tube of the photosensitive drum, and a photosensitive layer having a thickness of 25 μm is formed thereon.

From these various conditions, in the present embodiment, the distance from the light emitting point of the LED to the photosensitive layer is set as shown in FIG. 11 from the image forming relationship in the rod lens array and the condition of the photosensitive drum. .

The service life of the photosensitive drum is 5 in terms of A4.
It is, 000,000 pages. At this time, the abrasion amount of the a-Si layer is a very small value of about 1300 angstroms. That is, it can be said from this that the focus deviation of the imaging optical system due to the abrasion of the photosensitive drum hardly occurs.

On the other hand, if the photosensitive drum is OPC,
In the case of peeling, the scraping of about 10 to 30 μm may occur in general, depending on the number of lifespan of the photosensitive drum and the configuration. For example, if it is cut by 30 μm, CGL
The focus position at is shifted by about 12 μm.

From this point of view, the photosensitive drum is a-Si.
By doing so, it is possible to make a configuration in which abrasion is unlikely to occur, that is, a focus deviation due to abrasion is less likely to occur.

On the other hand, the distance from the light emitting point to the photosensitive layer easily changes due to various factors. For example, it is affected by the eccentricity of the photosensitive drum, the positioning error of the LED head, and the like. The allowable width for this variation is ± 50 μm.

With respect to this ± 50 μm, if the error due to abrasion is set to 10 and several μm, it is a small value, but ± 50 μm.
μm is a stack of various errors,
Eliminating the deviation of the minute is very effective in terms of the main body configuration.

Here, for example, FIG. 12 shows the difference in spot profile between when the image is in focus and when it is deviated by 70 μm.

The difference between the light intensity distributions of the two is a characteristic (hereinafter referred to as an EV characteristic) showing a potential attenuation amount with respect to the exposure intensity of the photosensitive drum shown in FIG. 13 and a developing bias in the developing means shown in FIG. Through a characteristic (hereinafter, V-D characteristic) showing the relationship between the reproduced density and the difference between the DC potential and the latent image potential (hereinafter, referred to as development contrast potential).
The printer output characteristics appear as shown in FIG. The printer output characteristics are 0 to 225 on the horizontal axis.
The image output signal value up to and the output image density is plotted on the vertical axis.

The cause will be briefly described. 12
When the photosensitive drum is exposed according to the spot profile shown in FIG. 16, the potential distribution of the exposure spot becomes as shown in FIG. 16 due to the EV characteristic, and the intermediate potential increases. When such an electrostatic latent image is developed by the developing means having the V-D characteristic shown in FIG. 14, the developing area is widened, so that the printer output characteristic finally becomes as shown in FIG.

Further, the photosensitive drum characteristic, the developing characteristic, the LED
The optical output characteristics of s, etc. always include some fluctuation. Here, the shake amount of the developing bias is considered.
FIG. 17 is a schematic diagram showing changes in the development area when the development characteristics are varied. FIG. 17 assumes a case where the developing bias changes and the developing contrast increases by 20 V from the normal time.

FIG. 17 shows that when the image is in focus, the variation in the developing area due to the variation in the developing contrast is larger and the amount of the density variation is larger than when the image is deviated by 70 μm. In other words, it can be understood that the out-of-focus greatly affects the image quality deterioration.

As a result, by using a-Si for the photosensitive drum and making it difficult to scrape, it becomes difficult to produce a focus deviation, and as a result, the output image can be kept more stable.

From the above, according to the present invention, in an electrophotographic image forming apparatus using an LED head composed of an LED array and a rod lens array as an exposure light source, stable printer characteristics are realized by reducing image unevenness and density fluctuation. It is possible to provide high image quality in.

(Second Embodiment) A schematic sectional view of an image forming apparatus according to the second embodiment is similar to that of the first embodiment.

In this embodiment, the layer structure of the a-Si photosensitive drum is as shown in FIG.

A carrier injection blocking layer having a thickness of 3 μm is coated on the aluminum tube of the photosensitive drum, and a photosensitive layer having a thickness of 40 μm is coated thereon. In the photosensitive layer, the photocarrier generation layer is 10 μm on the support side. The refractive index of the carrier transport layer is about 3.4.

Further, a surface protective layer having a thickness of 0.6 μm is provided on the photosensitive layer.

From these various conditions, in this embodiment, the distance from the light emitting point of the LED to the photosensitive layer is set as shown in FIG. 19 based on the image forming relationship in the rod lens array and the conditions of the photosensitive drum. .

With such a structure, it is possible to form an image on the a-Si photosensitive drum in a state in which it is always in focus from the initial stage to the photosensitive drum life.

[0056]

According to the present invention, in the image forming apparatus using the rod lens array as the image forming element of the exposure optical system, by using the a-Si photosensitive member as the photosensitive drum, the surface layer of the photosensitive drum is As a result, it is less likely to be scraped, and as a result, out-of-focus is less likely to occur, and it is possible to obtain a good optical profile at all times during the life of the photosensitive drum. It is possible to provide high image quality.

[Brief description of drawings]

FIG. 1 is a schematic view of a main body of an image forming apparatus according to the present invention.

FIG. 2 is a schematic view of a main part of an image forming apparatus using a conventional LED array.

FIG. 3 is an explanatory diagram of a rod lens.

FIG. 4 is a conceptual diagram of MTF.

FIG. 5 is a sectional view of a general functionally separated laminated structure type drum.

FIG. 6 is a diagram showing an exposure intensity distribution of LEDs.

FIG. 7 is a diagram showing an EV characteristic of a general photosensitive drum.

FIG. 8 is a diagram showing a VD characteristic of a general developing unit.

FIG. 9 is a diagram showing a configuration of a rod lens array.

FIG. 10 is a schematic cross-sectional view of the photosensitive drum according to the first exemplary embodiment.

FIG. 11 is a diagram showing a relationship between the image forming position of the SLA and the drum in the first embodiment.

FIG. 12 is a diagram showing a difference in spot profile between the case where the photosensitive layer has an image formation position and the case where defocusing is performed by 70 μm in the first embodiment.

FIG. 13 is a diagram showing an EV characteristic of the photosensitive drum in the first embodiment.

FIG. 14 is a diagram showing VD characteristics of the developing unit in the first embodiment.

FIG. 15 is a diagram showing a difference in printer characteristics between the case where the photosensitive layer has an image formation position and the case where defocusing is performed by 70 μm in the first embodiment.

FIG. 16 is a diagram showing a potential distribution in an exposure spot in the first embodiment.

FIG. 17 is a schematic diagram showing changes in the developing area when the developing bias in Embodiment 1 is varied.

FIG. 18 is a diagram showing a layer structure of a photosensitive drum according to the second embodiment.

FIG. 19 is a diagram showing a distance from a light emitting point of an LED to a photosensitive layer based on an image forming relationship in a rod lens array and conditions of a photosensitive drum.

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Claims (4)

[Claims] 1. An LED head comprising a light emitting diode array having a plurality of light emitting diodes linearly arranged as an exposure light source and a rod lens array as an image forming optical system is exposed on a photosensitive drum uniformly charged by a charging means. An electrostatic latent image is formed by exposing as an apparatus, the electrostatic latent image is visualized by a developing unit, an image on an image carrier is transferred onto a transfer material by a transfer unit, and finally a fixing unit. An image forming apparatus for forming an image by fixing an image on a transfer material to the transfer material by using an a-Si photoconductor as a photosensitive drum. 2. The image forming apparatus according to claim 1, wherein the optical image forming element of the exposure optical system has a performance of an optical path length from a light emitting point to an image forming point of 20 mm or less. 3. The image forming apparatus according to claim 1, wherein the number of reproduced gradations per pixel is 8 or less. 4. The image forming apparatus according to claim 1, wherein the photosensitive drum has a function separation laminated structure.