JP2784828B2 - Image forming device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus using an electrophotographic method, and more particularly, to a method for forming a latent image on a photosensitive layer from the rear side of an image carrier by exposing the photosensitive layer to a corresponding color. The present invention relates to an image forming apparatus in which a plurality of images are superposedly formed on an image carrier by repeatedly developing with a developer to obtain a color image.

2. Description of the Related Art In recent years, the market for desktop printers has been remarkably developed, and among them, an electrophotographic laser beam printer (LBP) has been rapidly spreading due to its high image quality and high speed.

With advances in technology, miniaturization and price reduction have progressed, and many products for personal use have appeared. Given the growing demand for digital color copiers at present, it can be said that the development of small color LBPs is coming soon. Two-color LBPs have already been commercialized. Although technically it is not difficult to realize a full-color LBP, it can be said that the point of commercializing a color LBP is how to design a system configuration with reduced size and cost.

The technical background required for miniaturizing a color image forming apparatus such as a color LBP is a back exposure method,
It is considered that multiple development and the use of a belt-shaped photoconductor are important. However, although adoption of the above technical items is advantageous for miniaturization, in the case of a configuration combining back exposure and multiple development, a problem different from a normal process occurs.

Although it has been overlooked in the past, image formation by the multiple development method employs a method of forming an image of a plurality of colors on an image carrier by repeatedly exposing and developing an image. In some places, a process occurs in which the light that has once passed through the photosensitive layer of the image carrier is scattered by the image already formed on the image carrier, and returns to the photosensitive layer again. For example, this is particularly remarkable when a yellow image of a yellow toner having a high scattering coefficient is formed on the image carrier, and the exposure of the image after passing through the photosensitive layer of the image carrier from the back side is performed. That is, the toner is scattered on the surface and inside of the toner layer of the previous image on the body and is sent to the photosensitive layer again. As a result, the latent image of the above-mentioned image, which should be sharply formed on the photosensitive layer, sometimes becomes blurred by about several tens of μm. Similar images occur to some extent even when an image such as magenta is formed first on the image carrier.

For this reason, when trying to form an image of another color on an image of yellow or the like by superimposing, a color shift due to blurring of the latent image occurs in the image of another color, and the resolution of the color image And the problem that color reproducibility is likely to be deteriorated.

Therefore, an object of the present invention is to form a latent image by back exposure and develop the same to form a multi-color image on an image carrier and obtain a color image. It is an object of the present invention to provide an image forming apparatus capable of obtaining a color image with improved image resolution and color reproducibility without causing color shift.

Means for Solving the Problems The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention provides a photosensitive layer of an image carrier having a translucent rear surface, by exposing an image from the back side of the image carrier by an exposure unit, and developing the obtained latent image into a corresponding color. In an image forming apparatus that repeatedly develops and visualizes a multi-color image on the image carrier to form a multi-color image on the image carrier, the visualization by developing with an agent is performed in a wavelength range of light used for exposing the image. On the other hand, there is provided an image forming apparatus which forms the plurality of color images from an image of a developer of a color having the lowest reflectance.

Example The present inventors formed a latent image by backside exposure and developed it to form a plurality of color toner images on the image carrier in an overlapping manner.When obtaining a color image, the latent image was blurred. Intensive research has been conducted to develop an image forming apparatus capable of obtaining a color image with improved image resolution and color reproducibility without causing color misregistration.

As a result, image exposure of each color is performed by the back exposure method,
When it is developed by a multi-developing method to form a multi-color image on the image carrier in an overlapping manner, a developer constituting the image of each color scatters the exposure of the image of each color of the next color. The present invention has been completed by noting that it has an important effect on exposure.

That is, in order to reduce the area or frequency ratio of the adverse effect on the next color image due to the light scattering of the exposure of the next image by the image formed on the image carrier, With respect to light used for exposing the image of the formed color, light scattering, that is, an image of a plurality of colors may be formed from the image of the developer having the lowest reflectance.

Therefore, in the present invention, when forming an image of a plurality of colors on the image carrier by the back exposure method and the multiple development method,
By forming a plurality of color images in order from the image of the developer of the color having the lowest scattering coefficient, the area and frequency ratio of the adverse effect occurring in the next image is reduced by half. In the image forming method employing the back exposure method and the multi-developing method, conventionally, from this viewpoint, the order of forming images of a plurality of colors has not been specified, and according to the present invention,
Reduces color misalignment when superimposing multiple color images,
The resolution and color reproducibility of the color image have been improved, and a high-quality color image can be obtained.

 Hereinafter, the present invention will be described in detail with reference to examples.

FIG. 1 is a configuration diagram showing one embodiment of the color image forming apparatus of the present invention.

In FIG. 1, reference numeral 1 denotes a photosensitive belt as an image carrier, and the photosensitive belt 1 is wrapped around upper and lower rollers 2, 3 and an intermediate roller 4, and is installed at an angle. A cleaner 21, a static eliminator 12, and a primary charger 5 are provided from the bottom along the upper track on the outer side of the photoreceptor belt 1, and the first developer 7 and the second developer from the upper along the lower track. Vessel 8, third
A developing device 9 and a fourth developing device 10 are provided, and a transfer charger 14 is provided at a position of the lower roller 3. Further, a fixing device 16 is provided on the extension of the moving direction of the transfer paper 15 sent between the lower roller 3 and the transfer charger 14.

On the other hand, on the inner side of the photoreceptor belt 1, an exposure unit 6 including an exposure lamp 13, an LED 6 a, an optical system 6 b, and a contact slit 6 c for focusing is provided from below along an upper trajectory.
A first contact roller 31, a second contact roller 32, a third contact roller 33, and a fourth contact roller 34 are provided from above along the lower track. The first to fourth contact rollers 31 to 34 contact the photosensitive belt 1 and correspond to the corresponding first to fourth developing devices 7 to
This is for keeping the distance between the developing sleeve 10 and the photoreceptor belt 1 constant.

The photoreceptor belt 1 has a three-layer structure in which a PET (polyester film) layer, an ITO (indium oxide) layer, and an OPC (organic photoconductor) layer are laminated in this order from the inside. Details will be described later. PET of photoreceptor belt 1
Since the layer and the ITO layer are translucent, an image can be exposed from the back side of the photoreceptor belt 1 by the exposure means 6 provided inside the photoreceptor belt 1.

In this image forming apparatus, first, after the primary charger 5 uniformly applies a charge to the OPC layer on the surface of the photoreceptor belt 1,
The OPC layer is exposed to light by the LED 6a of the exposure means 6 through the optical system 6b and the contact slit 6c for focus alignment, and an electrostatic latent image is formed on the OPC layer. Next, first to fourth developing units 7 to 10
The latent image formed on the photoreceptor belt 1 is developed by any one of the above methods, and is visualized as a toner image.

Next, the photosensitive belt 1 repeats the image forming process of the primary charging or less after the remaining charge is removed by the action of the exposure lamp 13 and the charge remover (the charge remover AC charger) 12. During this time, the cleaner 21 is in the OFF state,
Blade 22 does not contact the photoreceptor belt 1.

The formation of the latent image and the development of the latent image are repeated four times as described above, and when an image of four colors is formed on the photoreceptor belt 1, the four-color image is then transferred onto the transfer material 15 by the transfer charger 14. Then, the image transferred to the transfer material is heated or pressed by the fixing device 16 to perform fixing, thereby obtaining a final color image.

The toner remaining on the photoreceptor belt 1 without being transferred is turned on.
The blade 22 of the cleaner 21 in the state comes into contact with the belt 1 and is removed. The charge remaining on the belt 1 is removed by the action of the exposure lamp 13 and the static eliminator 12. The photoreceptor belt 1 is rotated by the feed of rollers including the upper roller 2, and the above steps are repeated as a cycle.

The gist of the present invention corresponds to the wavelength of the latent image forming exposure used by the exposing means 6 (that is, the wavelength of the photosensitive layer of the photoreceptor belt 1) and the light reflection and absorption characteristics of each color developer. In other words, by optimizing the order of forming the images of a plurality of colors, it is possible to reduce or prevent color misalignment when forming toner images of a plurality of colors on the photoreceptor belt 1 in an overlapping manner.

First, it is assumed that a combination of four colors of yellow (Y), magenta (M), cyan (C), and black (B) is used as a color developer, and that light scattering in yellow, magenta, cyan, and black in an exposure wavelength range is performed. Assume that the order is higher as before. 2 (a) and 2 (b) illustrate the principle of the present invention by taking as an example a case where a green (G) image is formed by superimposing yellow and cyan colors.

First, in the present invention, the photoreceptor belt 1 is shown in FIG.
As shown in the figure, a translucent PET layer 1u is used as a base, a conductive and translucent ITO layer 1v is provided thereon, and a photoconductive OPC layer 1w is further formed thereon. I have. The photoreceptor belt 1 has an OPC layer 1w at the boundary between the OPC layer 1w and the ITO layer 1v.
A configuration in which a barrier layer for preventing charge injection into w may be provided may be provided, but since this is not directly related to the operation of the present invention, that point will not be further described.

Now, an image of cyan (C) having a light scattering coefficient lower than that of yellow (Y) with respect to the wavelength of the latent image forming exposure used by the exposure means 6 is first exposed and developed, and FIG. a) forming a cyan image 11c on the photoreceptor belt 1 as shown in FIG.
Thereafter, exposure of a yellow image from the back side of the photoreceptor belt 1 is performed.
When 17y is performed, the exposure 17y of the yellow image passes through the PET layer 1u, the ITO layer 1v, and the OPC layer 1w in order from the bottom of the photoreceptor belt 1, and the remaining unabsorbed light is left on the photoreceptor belt 1. The cyan image 11c reaches the bottom surface of the toner layer. At this time, the reflection and scattered light on the toner layer surface and inside of the cyan image 11c return to the OPC layer 1w very slightly, and the yellow image electrostatic latent image area 18y formed by the above exposure is slightly more It will blur. For this reason, as shown in FIG. 2 (b), when the yellow image 11y is overlaid and formed, the color shift and blur slightly occur in the yellow image 11y.

However, since the scattering coefficient of cyan is relatively small similarly to magenta, the color misregistration of the yellow image and the width of blur are several μm.
m, so that the effect of image quality deterioration does not matter much.

However, in order to form a green image, the yellow image 11y is first exposed and developed to form a yellow image 11y on the photoreceptor belt 1 as shown in FIG.
It is assumed that the cyan image 11c is formed later. Then, since the scattering coefficient of yellow is the highest among the four colors, the light scattering of the exposure 17c of the cyan image 11c is large as shown in FIG.
The latent image 18c of the formed cyan image 11c is much more blurred (about several tens of μm) than in the cases of FIGS. 2 (a) and 2 (b). Therefore, when the cyan image 11c is developed, a cyan image 11c is formed on the yellow image 11y to be slightly larger than the yellow image 11y, and a green image 11g is obtained as a final image after fixing.
Is an image in which the cyan image 11c protrudes by several tens of microns around it. Therefore, the color reproducibility and resolution of the image are extremely deteriorated.

The results shown in FIGS. 3A and 3B are particularly likely to occur when a yellow image having a high scattering coefficient is formed first and an image of another color is formed later. The same is true not only for two-color superposition but also for three-color superposition. For example, in the case of three-color superposition, a magenta image or a cyan image exists on a yellow image at the time of exposure of a third color image, and this is Since the third color image acts as a reflective layer at the time of exposure, the third color image is more strongly scattered with respect to exposure, so that the third color image is more likely to be blurred.

Therefore, in any case, if the ratio of light scattering is lower in the order of yellow, magenta, cyan, and black with respect to the wavelength used for exposing the image, the best image formation order is yellow as shown in FIG. Expose the image from the second time on,
It is formed by developing. As described above, if color misregistration is a problem with only the yellow image, the yellow image may be exposed and developed last, and the other color images may be exposed in an appropriate order. No problem even if developed.

In the conventional developing order, the configuration from the above viewpoint was not made, and in the worst case, the yellow image was formed first, so that the image was formed from a color image having a high scattering coefficient. However, according to the present invention, this can be solved.

In the multiple development of the one-pass system, development must be performed in the order of arrangement of the developing units. In the case of the multiple development of the frame sequential system, even if the arrangement order of the development units is as shown in FIG. The arrangement may be reversed as in the embodiment shown in the drawings, and the arrangement order may be other. In the present invention, a multi-color image is formed from the image of the developer having the lowest light scattering coefficient with respect to the wavelength range used for exposing the image, and therefore, as shown in FIG. 4 or FIG. Accordingly, the development order may be determined by controlling the development bias order over time, and therefore, there is no restriction on the spatial arrangement of the developing devices.

It is conceivable to use only the DC bias as the developing bias in order to avoid mixing of the developer between the developing units.
It does not preclude using the AC bias superimposed on the bias.

The effects of the present invention are irrespective of the development method such as contact type, non-contact type, jumping development method, magnetic brush method, mag toner, non-mag toner, one-component development, two-component development, etc. Will be demonstrated. In order to determine the actual order of image formation, it is easy and sufficient to perform the following.

That is, when an image of the next color is exposed from the back side of the photoreceptor belt 1 to the photosensitive layer of the photoreceptor belt 1 on which the image is formed, the amount of light reflected on the back side of the exposure is measured. Keep it. The amount of light diffused in a parallel direction (lateral direction) parallel to the surface of the photoreceptor belt 1 during exposure that causes image bleeding is
Since it is considered that the amount of reflected light including the diffusion in the vertical direction (thickness direction) of the photoreceptor belt 1 is substantially proportional to the amount of reflected light, the image formation from the developer image of the color determined to be small in the amount of reflected light in the above measurement of the amount of diffused light do it. Although the amount of reflected light has an exponential relationship with the thickness of the toner layer of the image, the above measurement is convenient because it also includes a factor corresponding to the thickness of the toner layer. As a method of measuring the amount of reflected light, it is of course possible to directly measure the scattering in the parallel direction.

According to the present invention, the same effects can be obtained regardless of whether the exposure light source is in the visible light range or the infrared light range. According to the results of experiments performed by the present inventors, LED light sources (wavelength 6
80 nm) and an infrared laser light source (wavelength 780 nm), relative yellow and magenta, cyan, and black values of the amount of scattered and reflected light were obtained as shown in Table 1.

Therefore, when the image was formed in the order of small reflected light amount, that is, in the order of black, cyan, magenta, and yellow, the bleeding and blurring of the image were improved as compared with the case where the image was formed in the completely reverse order. 20 μm maximum for the latter
The image blur of the width occurred, but in the former, the image blur
It was suppressed to 10 μm or less.

In the above description, it is preferable to form images in the order of black, cyan, magenta, and yellow. However, the order of colors for forming images is not limited to the unique order determined by this experiment.

What is the ratio of the light diffusion is the dispersion state of the binder and the pigment of the developer, the light reflectance, the light absorption, the light refractive index, the particle size distribution,
This is because it greatly varies depending on the thickness of the developing layer and the like. Therefore, the above-mentioned reflected light amount was directly measured for each system of the image forming apparatus. It is necessary to determine the order of the colors to be imaged. In particular, magenta and cyan are close to each other in the amount of reflected light, and depending on the material of these developers, there is a great possibility that the order of magnitude of the amount of reflected light is switched.

In the above embodiments, an LED was used as an example of the exposure light source, but using a single-wavelength light source such as a laser is more effective than using a light source with a wider wavelength range. Is more apparent, and the effect of the present invention, that is, the effect of preventing color shift, becomes clearer. Of course, even with a light source having a wide wavelength range, such as a white light source such as a halogen lamp, the color shift of the image is similarly formed by forming an image in the image order of the developer having a lower scattering coefficient with respect to the wavelength range to be used. And blurring can be prevented.

In the above description, the photoreceptor belt 1 of a belt-shaped photoreceptor has been described as an example of the image carrier, but the present invention may be applied to a case where a photoreceptor drum of a commonly used drum-shaped photoreceptor is used. It is clear that the same effect can be obtained. Conventionally, as a method of improving color shift and blur prevention of an image, there is a method of electrically controlling an exposure signal so that a large or small dot is formed corresponding to color superposition. It takes a lot of time. In the present invention, a solution from a more fundamental aspect is made without such a difficulty.

As shown in FIG. 7, the OPC layer 1
There is also a method in which an insulating light absorbing layer 1x is provided on w to prevent the influence of light scattering upon exposure of an image of the next color by an image formed on the photoreceptor belt 1. According to this, the above-mentioned problem can be completely solved. If an insulating light reflecting layer is provided in place of the light absorbing layer, multiple light scattering may be caused inside the photoreceptor belt 1 and conversely, image blur may be worsened. 1x is preferred.

In this case, external light from the outer surface of the photoreceptor belt 1 is
Since it does not reach 1w, there is also an advantage that the photodegradation of the photosensitive layer 1w due to external light is prevented. Therefore, it can be used as a display device in which an image formed on the photosensitive belt 1 is used as a display. However, in this case, since a process step called a so-called NP method is required, the structure of the image forming apparatus is somewhat complicated as in the embodiment shown in FIG.

That is, in the image forming apparatus shown in FIG. 8, primary charging is performed by the primary charger 5, exposure is performed by the exposing unit 13a, and then static elimination is performed by the static eliminator (AC eliminator for static elimination) 5a. The point that an image is exposed by the exposure unit 6 is different from the image forming apparatus shown in FIG. 1, and an additional exposing device 13a and the like are required.

FIG. 9 shows a sequence example of an image forming process in the image forming apparatus in FIG. In this case, the color order of the image to be formed may be arbitrary.

Effects of the Invention As described above, in the present invention, in forming a plurality of color images on the image carrier in a superimposed manner, the wavelength range used for the latent image forming exposure or the sensitivity wavelength range of the image carrier photosensitive layer is used. On the other hand, since the images of a plurality of colors are exposed and developed in the order of decreasing the light reflectance of the developer, the phenomenon of color misregistration and blurring of the images of the plurality of colors can be reduced or eliminated, and the color reproducibility of the images can be reduced. And resolving power can be greatly improved.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing an embodiment of the image forming apparatus of the present invention. FIGS. 2A and 2B are explanatory diagrams showing the order of image formation in the image forming apparatus of FIG. 3 (a) and 3 (b) are explanatory diagrams showing examples of improper order of image formation. FIG. 4 is a sequence diagram showing a sequence of an image forming process in the image forming apparatus of FIG. FIG. 5 is a sequence diagram showing another example of the sequence. FIG. 6 is a configuration diagram showing another embodiment of the image forming apparatus of the present invention. FIG. 7 is a configuration diagram showing another example of the photoreceptor belt in the image forming apparatus of the present invention. FIG. 8 is a configuration diagram showing still another embodiment of the image forming apparatus of the present invention. FIG. 9 is a sequence diagram showing a sequence of an image forming process in the image forming apparatus of FIG. 1: Photoreceptor belt 5: Temporary charger 6: Exposure means 7-10: Developing unit 12: Static eliminator 14: Transfer charger

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G03G 13/01 G03G 15/01-15/01 117 G03G 15/04-15/056

Claims (1)

(57) [Claims] An image exposure is performed on a photosensitive layer of an image carrier having a translucent rear surface by an exposure means from the rear side of the image carrier, and the obtained latent image is developed with a developer of a corresponding color. Developing and visualizing is repeatedly performed for a plurality of color images, and in an image forming apparatus for forming a plurality of color images on the image carrier in an overlapping manner, a wavelength range of light used for exposing the image is used. An image forming apparatus comprising: forming an image of a plurality of colors from an image of a developer of a color having the lowest reflectance.