JP2001042611A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain the best light profile, to reduce unevenness in image and density fluctuation, to realize stable printer characteristics, and then to obtain a high quality images by setting the image-forming position of an image-forming optical system to the light generating carrier generating position of a photoreceptor drum. SOLUTION: An electrostatic latent image is formed by unformly electrifying a photoreceptor drum by a primary electrifier and exposing it according to an inputted image signal by an LED head. The electrostatic latent image is made into an imaged by a developing device and transferred to transfer paper by a transfer means. A toner image transferred to the transfer paper is fixed by a fixing means. Then, luminous flux is formed into image by a CGL, since light radiated to the photoreceptor drum reaches the CGL to generate the carrier without setting the image-forming position of the image-forming optical system on the surface of the photoreceptor drum. Namely, the device is designed so that the light emitting point of an LED is located at 12.2 μm downward from the surface layer of the photoreceptor drum, in order to form the image by the CGL positioned on at 20 μm downward from the surface of the photoreceptor drum.

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 image forming apparatus using an electrophotographic system.

[0002]

2. Description of the Related Art Conventionally, in an image forming apparatus such as a digital copying machine or a printer of an electrophotographic system, a semiconductor laser is generally used as an exposure light source. In this method, a laser beam emitted from a semiconductor laser is deflected by an optical deflector composed of a rotating polygon mirror or the like, and is image-formed on a photosensitive drum surface through an fθ lens to form an electrostatic latent image by optical scanning. I have.

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 come to appear. In this method, a light beam emitted from an LED array is imaged on a photosensitive drum surface via a rod lens array to form an electrostatic latent image.

FIG. 2 is a schematic diagram showing 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 perpendicular to the rotation direction of the photosensitive drum, the LEDs are selectively lit, and the photosensitive drum is arranged via an imaging optical system arranged in front of a light emitting portion of the LED. The image from the LED is formed on the surface, and at the same time, the photosensitive drum is rotated to form a two-dimensional electrostatic latent image.

In such an image forming apparatus using an LED array, a rod lens array is generally used as an image forming optical system, and an LED is used as an exposure optical system.
A rod lens is used as an LED head in which a rod lens array in which one or two rows of rod lenses are regularly and linearly arranged between the array and two plates.

FIG. 3 is an explanatory view of a rod lens. In the same figure, assuming that 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, in other words, from the light emitting point to the imaging point Is represented by TC = Z0 + 2L0.

The image transmission characteristics of the rod lens array are evaluated based on the quality of the transmitted image, that is, the resolution. This is represented by MTF (Modulation Tra).
nsfer Function). This is an index to see how faithfully the original image of the rectangular wave pattern image as shown in FIG. 4, for example, formed after passing through the rod lens array can be reproduced to the original image. MT from FIG.
F is defined as follows.

MTF (w) = [I (w) max−I (w) mi
n] / [I (w) max + I (w) min)] × 100%

Here, I (w) max and I (w) mi
n is the maximum value and the minimum value of the rectangular wave response at the spatial frequency w (lp / mm), respectively. This MTF is 100%
The closer to, the more faithfully the original image is reproduced.

The MTF is greatly different between a case where the measurement is performed at the focal position of the rod lens array and a case where the measurement is performed at a position shifted from the focal position. Further, the TC length of the LED array optical system is generally around 10 mm, so that the LED array optical system is more sensitive to the shift of the focal position than the semiconductor laser optical system having a relatively long focal length. Therefore, the distance between the photosensitive drum and the LED head is not deviated.

By forming an LED head using such an LED array and a rod lens array as an exposure light source, an exposure light source can be disposed near the photosensitive drum, and a semiconductor laser can be used as the exposure light source. It is possible to reduce the size of the image forming apparatus as compared with a system using the image forming apparatus.

When focusing on the photosensitive drum, there are generally two types of structures, a single-layer structure and a function-separated laminated structure. Among them, in recent years, the function separation lamination type is becoming mainstream. The imaging position of the LED array is generally set to be on the surface of the photosensitive drum.

[0013]

However, when the imaging position of the LED head is set on the surface of the photosensitive drum, in the case of the photosensitive drum having the function-separated laminated structure, when light reaches the layer generating the optical carrier, It was slightly away from the image plane.

Then, the MTF deteriorates from the ideal state, and the light profile changes. Here, FIG. 5 shows a light profile on an ideal image plane and a light profile at a point 50 μm away from the image forming position. As is clear from FIG. 5, the light profile spreads at a position shifted from the image forming position, and the exposure intensity gradient at the end of the spot falls.

On the other hand, as shown in FIG. 6, the characteristics of a general photosensitive drum behave in such a manner that it responds sensitively when the light amount is small. In addition, in terms of electrophotographic characteristics, the developing characteristics are generally as shown in FIG. 7, and when the contrast potential is intermediate, the behavior becomes sensitive. That is, the density tends to fluctuate.

Then, at the spot end, the portion having an intermediate potential at the spot end becomes relatively large as compared with the case where the exposure intensity gradient is steep because the exposure intensity gradient is flat. Then, the toner is finally developed and the change in density tends to be large. As a result, the output density characteristics become unstable and image density unevenness or the like easily occurs. In a full-color image forming apparatus, this finally appears as color unevenness, which is a major cause of image quality deterioration.

An object of the present invention is to widen the latitude with respect to defocus of an LED head, fully utilize the performance of a rod lens array, and reduce image unevenness and density fluctuation to achieve high image quality while realizing stable printer characteristics. An object of the present invention is to provide an electrophotographic image forming apparatus using, as an exposure light source, an LED head composed of an LED array and a rod lens array capable of obtaining a complex image.

[0018]

According to the present invention, an electrostatic latent image is formed on a photosensitive drum uniformly charged by charging means by subjecting a light beam generated from an exposure light source to image forming exposure by an image forming optical system. Then, the electrostatic latent image is visualized by a developing unit, the image on the photosensitive drum is transferred onto a transfer body by a transfer unit, and the image on the transfer material is fixed to the transfer material by a fixing unit to form an image. In the image forming apparatus, there is provided an image forming apparatus in which an image forming position of an image forming optical system is set to a photocarrier generating position of a photosensitive drum.

[0019]

The present invention will be described in detail below with reference to specific examples.

(Embodiment 1) FIG. 1 is a schematic sectional view of a main body of an image forming apparatus according to this embodiment. In FIG. 1, an exposure light source is an LED array, and a plurality of LEDs are linearly arranged in the axial direction of a photosensitive drum as an image carrier. As a configuration, an erecting equal-magnification rod lens array as an imaging element is used, and a plurality of rod lenses are linearly arranged in the axial direction of the photosensitive drum.
The light beam emitted from the D array is imaged on the photosensitive drum surface at the same magnification. In this embodiment, an LED array and a rod lens array are integrally formed and used as an LED head.

Next, the image forming process will be briefly described. First, the photosensitive drum 10 is uniformly charged by the primary charger 5, and is exposed thereto by the LED head 6 according to an input image signal to form an electrostatic latent image. The electrostatic latent image is visualized by a developing device 7 and transferred onto transfer paper by a transfer unit 8. The toner image transferred onto the transfer paper by the fixing means 9 is fixed. The number of reproduced gradations per pixel is binary.

In this embodiment, the above-described image forming process is performed simultaneously for four colors, thereby realizing a high speed operation. The first station 1 is Yellow, the second station 2 is Magenta, the third station 3 is Cya
n, the fourth station 4 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 dpi, that is, an element arrangement period of 42.3 μm.
It has become. The electrode size of the light emitting part of the LED is
It is about 20 μm. The LED is used as an exposure light source to form an image on the surface of the photosensitive drum.

Next, a rod lens array as 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. T of this rod lens array
The C length is 9.9 mm.

The TC length of a general rod lens array is 6
The range is from about 0 mm to about 9 mm. Among them, T
A lens having a long C length has a relatively large depth of focus and is relatively resistant to a shift in the imaging position. However, the type used in the present invention has a short TC length, is relatively bright and has a high resolution. This type has a relatively short depth of focus. The present invention is particularly effective when the TC length is 20 mm or less.

Here, the photosensitive drum will be described. The photosensitive drum used in this image forming apparatus is a photosensitive drum of an OPC function separation and lamination type. This sectional view is shown in FIG.
Shown in

An undercoat layer is coated on an aluminum tube as a support of a photosensitive drum, and a carrier generation layer (hereinafter referred to as CGL) containing a carrier generation material has a thickness of 0.1 μm.
m or less (170 mg / cm ² ), and a carrier transporting layer containing a carrier transporting material (hereinafter referred to as CTL) having a thickness of 20 μm. The refractive index of CTL is 1.6.

Conventionally, the image forming position of the image forming optical system is generally set on the surface of the photosensitive drum. However, originally, the light irradiated on the photosensitive drum reaches the CGL to generate carriers. It is desirable that the light flux be imaged by CGL.

Therefore, the distance from the light emitting point of the LED to the CGL is set as shown in FIG. 10 based on the image forming relationship in the rod lens array and the condition of the photosensitive drum.
In the photosensitive drum, the refractive index of CTL is 1.6,
The thickness is 20 μm. When the image forming position is calculated from them, the image is formed 7.8 μm farther than when the refractive index is 1. Therefore, in this embodiment, in order to form an image with CGL 20 μm below the surface of the photosensitive drum, the light emitting point of the LED is designed to be 12.2 μm below the surface of the photosensitive drum.

On the other hand, the distance from the light emitting point to the CGL tends to fluctuate due to various factors. For example, it is affected by the eccentricity of the photosensitive drum and the positioning error of the LED head. The tolerance for this variation is set to ± 50 μm.

FIG. 11 shows the difference in spot profile between the case where the imaging point matches the CGL and the case where the image point is shifted by 70 μm from the CGL.

The difference between the two light intensity distributions is that the characteristic (hereinafter referred to as the EV characteristic) indicating the potential attenuation with respect to the exposure intensity of the photosensitive drum shown in FIG. 12 and the developing bias in the developing means shown in FIG. Through a characteristic (hereinafter referred to as a VD characteristic) indicating a relationship between a DC potential and a latent image potential (hereinafter referred to as a development contrast potential) and a reproduction density.
It appears as a printer output characteristic as shown in FIG. The printer output characteristics are from 0 to 255 on the horizontal axis.
The image output signal values up to and the vertical axis represent the output image density.

The cause will be briefly described. FIG.
When the photosensitive drum is exposed according to the spot profile shown in FIG. 15, the potential distribution of the exposed spot becomes as shown in FIG. 15 from the EV characteristic, and the intermediate potential increases. When such an electrostatic latent image is developed by the developing means exhibiting the VD characteristic shown in FIG. 13, the development area is expanded, so that the printer output characteristic finally becomes as shown in FIG.

Further, the characteristics of the photosensitive drum, the developing characteristics, and the LED
The light output characteristics and the like always include some fluctuation.
Here, the fluctuation amount of the developing bias is considered. FIG. 16 is a schematic diagram showing a change in the development area when the development characteristics fluctuate. FIG. 16 assumes a case where the developing bias changes and the developing contrast increases by 20 V from the normal state.

As can be seen from FIG. 16, when the imaging point is brought to the CGL, the development area fluctuation due to the development contrast fluctuation is smaller than when the imaging point is shifted from the CGL by 70 μm.
Indicates that the amount of concentration fluctuation is small. In other words, it is shown that bringing the image formation point to the CGL is more resistant to fluctuations in characteristics due to a smaller shake due to various shake factors, and a more stable and high image quality is obtained as the final output image. It will be done.

As described above, in an electrophotographic image forming apparatus using an LED head including an LED array and a rod lens array as an exposure light source according to the present invention,
The imaging position of the light beam emitted from the CGL is brought to the CGL of the photosensitive drum, providing high image quality while achieving stable printer characteristics by minimizing unevenness in image formation and density fluctuation, and the performance of the rod lens array. It became possible to fully use.

(Second Embodiment) A schematic sectional view of an image forming apparatus according to a second embodiment is similar to that of the first embodiment. The configuration of the LED is exactly the same as that of the first embodiment.

In the present embodiment, the photosensitive drum is
As shown in (1), the photoconductor has a single-layer structure in which a carrier generating material and a carrier transporting material are mixed in the same layer.
Therefore, in this embodiment, the design is made so that the image forming position of the lot lens array and the surface layer of the photosensitive drum coincide. By doing so, even the photoconductor having a single-layer structure can be exposed with the sharpest spot profile at the carrier generation position. Therefore, a stable and high-quality image is always obtained.

[0039]

As described above, according to the present invention, the rod lens array is used as the image forming element of the exposure optical system, and the image forming position is set as the position where the optical carrier is generated on the photosensitive drum. As a result, it is possible to obtain the best optical profile, and as a result, it is possible to provide an image forming apparatus capable of obtaining high-quality images while realizing stable printer characteristics by reducing image unevenness and density fluctuation. Was.

[Brief description of the drawings]

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

FIG. 2 is a schematic diagram 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 an MTF.

FIG. 5 is a diagram showing an exposure intensity distribution of an LED.

FIG. 6 is a diagram showing a characteristic (EV characteristic) showing a potential attenuation amount with respect to an exposure intensity of a general photosensitive drum.

FIG. 7 is a view showing a characteristic (VD characteristic) showing a relationship between a reproduction potential and a difference between a DC potential of a developing bias and a latent image potential in a general developing unit.

FIG. 8 is a diagram showing a configuration of a rod lens array.

FIG. 9 is a schematic sectional view of a photosensitive drum according to the first embodiment.

FIG. 10 illustrates an SLA (SelfocLe) according to the first embodiment.
FIG. 5 is a diagram illustrating a relationship between an image forming position of the nsAray) and the photosensitive drum.

FIG. 11 is a diagram illustrating a difference between a spot profile when the CGL has an image forming position and a spot profile when the CGL is defocused by 70 μm in the first embodiment.

FIG. 12 is a diagram illustrating an EV characteristic of the photosensitive drum according to the first exemplary embodiment.

FIG. 13 is a diagram illustrating a VD characteristic of a developing unit according to the first exemplary embodiment.

FIG. 14 is a diagram illustrating a difference in printer characteristics between the case where the CGL has an image forming position and the case where defocusing is performed by 70 μm in the first embodiment.

FIG. 15 is a diagram showing a potential distribution in an exposure spot in Example 1.

FIG. 16 is a schematic diagram illustrating a change in a developing area when a developing bias is changed in the first embodiment.

FIG. 17 is a schematic sectional view of a photosensitive drum according to a second embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Yellow station 2 Magenta station 3 Cyan station 4 Black station 5 Primary charging device 6 LED head 7 Developing device 8 Transfer means 9 Fixing means 10 Photosensitive drum 12 Conveying belt

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Tomohisa Itagaki 3-30-2 Shimomaruko, Ota-ku, Tokyo F-term in Canon Inc. (reference) 2C162 AE05 AE12 AE21 AE28 AE37 AE47 AE54 AE74 AF05 FA04 FA17 FA44 FA50 FA59 2H076 AB42 AB51 EA01 EA04 EA05

Claims (6)

[Claims] An image forming optical system forms an electrostatic latent image on a photosensitive drum uniformly charged by a charging unit by using a light beam generated from an exposure light source to form an electrostatic latent image. In an image forming apparatus, a latent image is visualized, an image on a photosensitive drum is transferred onto a transfer body by a transfer unit, and an image on the transfer material is fixed to the transfer material by a fixing unit to form an image. An image forming apparatus, wherein an image forming position of the system is set to a position where an optical carrier is generated on a photosensitive drum. 2. The image forming apparatus according to claim 1, wherein the exposure light source is a light emitting diode array in which a plurality of light emitting diodes are arranged on a straight line. 3. An exposure light source forms an image of light emitted from a light emitting diode by a rod lens array.
Or the image forming apparatus according to 2. 4. The image forming apparatus according to claim 1, wherein the photosensitive drum has a function-separated laminated structure. 5. The photosensitive drum has a single-layer structure.
3. The image forming apparatus according to any one of 3. 6. The image forming apparatus according to claim 1, wherein the optical imaging element of the exposure optical system has an optical path length from the light emitting point to the image forming point of 20 mm or less.