JP3535582B2 - Image forming device - Google Patents

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,
In particular, the present invention relates to an image forming apparatus which exhibits a uniform image density even when the photoconductor and the developing roller have a peripheral speed difference.

[0002]

2. Description of the Related Art Along with the recent trend toward color electrophotographic apparatuses and reduction in size and weight, a non-magnetic one-component developing system has been attracting attention.
The non-magnetic one-component developing method includes a contact method and a specific contact method, but in most cases, the contact method is used. According to this method, a developing roller to which a bias is applied is brought into contact with a photosensitive member to perform development. The thin toner layer formed on the roller is moved onto the photoconductor to visualize the electrostatic latent image. As a means for forming a thin toner layer on the developing roller, a metal or resin chip or a structure in which the roller is pressed against the developing roller with a constant load is generally used. Is being formed.

However, in such a method, the toner layer is formed very thin on the developing roller, and if the speed ratio of the surface of the photosensitive member and the surface of the developing roller is made approximately the same, the image density is lowered. Therefore, the desired image density is usually obtained by increasing the rotation speed of the developing roller so that the speed of the surface of the developing roller is faster than the speed of the surface of the photoconductor.

However, if the developing roller is rotated so fast that the surface of the developing roller is faster than the surface of the photosensitive member, various problems will occur. An example of this is that the life and performance of the toner charging member is adversely affected, making it difficult to increase the speed and durability of the entire device, and rubbing the surface of the photoconductor with a developing roller reduces image quality. It can be said that it will end. The typical image quality deterioration is
This is the density increase that occurs at the trailing edge of a solid image when a solid image is printed. This is not noticeable with black toner, but it is noticeable when a highly transparent toner such as color toner is used. .

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and occurs at the trailing edge of a solid image during solid printing that occurs in a developing system in which the developing roller and the image carrier have a peripheral speed difference. It is an object of the present invention to provide an image forming apparatus capable of obtaining a uniform solid image without increasing the density.

[0006]

The inventor of the present invention has studied the cause of the above phenomenon and conducted research to take measures against it. As a result, the electrophotographic apparatus such as a digital copying machine or a printer has a surface of a photosensitive member. It was found that it is possible to print a uniform solid image by changing the exposure level at the time of exposure to form an electrostatic latent image, and the present invention has been completed.

That is, the present invention (Claim 1) comprises an electrostatic latent image forming means for forming an electrostatic latent image on the image carrier, and a developing roller which rotates at a peripheral speed difference with respect to the image carrier. Developing means for visualizing the electrostatic latent image, and each pixel of an image divided into a plurality of pixels in a direction perpendicular to the moving direction of the surface of the image carrier before the electrostatic latent image is formed on the image carrier. And a means for previously reducing the output of the electrostatic latent image forming means with respect to all the pixels whose detected length is not less than a certain length. An image forming apparatus is provided.

The present invention (claim 2) comprises an electrostatic latent image forming means for forming an electrostatic latent image on the image carrier and a developing roller which rotates with a peripheral speed difference with respect to the image carrier. Developing means for visualizing the electrostatic latent image, and each pixel of an image divided into a plurality of pixels in a direction perpendicular to the moving direction of the surface of the image carrier before the electrostatic latent image is formed on the image carrier. Means for previously detecting the length of the surface of the image carrier in the moving direction, and electrostatic latent image formation on a portion of a pixel having a detected length equal to or greater than a predetermined distance from a rear end of the pixel. An image forming apparatus comprising: a means for reducing the output of the means in advance.

Further, the present invention (claim 3) comprises an electrostatic latent image forming means for forming an electrostatic latent image on the image carrier, and a developing roller which rotates with a peripheral speed difference with respect to the image carrier, Developing means for visualizing the electrostatic latent image, and an image divided into a plurality of pixels in a direction perpendicular to the moving direction of the surface of the image carrier during the operation of writing the electrostatic latent image on the image carrier. Means for detecting the length of the pixel in the moving direction of the image carrier surface;
An image forming apparatus comprising: a unit that partially reduces the output of the electrostatic latent image forming unit according to the detection signal.

In each of the above image forming apparatuses, a sensor for detecting the image density on the image carrier is further provided, and an image is printed on the image carrier during the preparatory operation of the apparatus, and the sensor Based on the signal and the detection signal of the means for detecting the length in the moving direction of the image carrier surface of each pixel,
The output of the electrostatic latent image forming means can be determined (claim 4).

[0011]

In the image forming apparatus of the present invention, particularly in a digital copying machine or a printer, it is detected before or during exposure whether or not there is an image (dots) continuous in the output image in a rotating direction of the photoconductor. However, in an image satisfying the conditions, the exposure level of all or a part of the continuous dots is set lower than that of other areas. That is, the exposure amount or the exposure time is set to be low in the portion of the predetermined continuous dots. Therefore, it is possible to prevent an increase in the image density at the trailing edge of the continuous dots, and it is possible to obtain an image having a uniform density over the entire image.

Further, in particular, during the preparatory operation of the apparatus, a solid image in which the exposure level is changed is printed in advance on the photoconductor, the image density is measured by the reflectance sensor, and the exposure level is lowered based on the signal. By doing so, it is possible to always form an image under optimum conditions.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic diagram showing a reversal developing type printer used for this embodiment, which is 8 sheets of A4 paper / min. The printer of FIG. 1 operates as follows. First, the photosensitive drum 1 is uniformly charged to about -500v by the fixed charging brush 2 to which -1kv is applied.
Exposure is performed by the laser optical system 3 of dpi to form an electrostatic latent image on the surface of the photoconductor. Next, a non-magnetic one-component contact developing device 8
To develop and visualize the electrostatic latent image.

The developing roller 4 of the developing unit 8 is made of a conductive urethane elastomer roller having a diameter of 18 mm, and is brought into contact with the photosensitive drum 1 to rotate in the forward direction. The image visualized by the developing device 8 is transferred to a transfer target such as paper by a transfer unit, in this case, the corona charger 5. The transfer residual toner and dust remaining on the photosensitive drum 1 after the transfer is removed by the cleaner 6, and the photosensitive drum 1 is removed by the charge removing means, in this case, the charge removing lamp 7. By repeating such a process, printing is continuously performed.

In the printer described above, FIG. 2 shows the density change of the solid image depending on the speed ratio between the surface of the photosensitive drum 1 and the surface of the developing roller 4. According to FIG. 2, if the surface speed of the developing roller is slower than the surface speed of the photoconductor,
An increase in concentration is observed at the front end of the solid, and when the surface velocities are the same, no increase in concentration is observed. On the other hand, when the speed of the surface of the developing roller is faster than the speed of the surface of the photoconductor, the density is increased at the rear end of the solid.

Further, the length d of the solid image density increasing portion in the moving direction of the photoconductor surface changes only by the speed ratio between the developing roller and the photoconductor, and the larger the speed difference, the longer the density increasing portion. In normal non-magnetic one-component development, the toner layer on the developing roller is thin (theoretically about one toner layer)
Therefore, the developing roller is rotated faster than the photosensitive member (generally 1.5 to 2 times) to obtain the solid density. Therefore, what becomes a problem is an increase in the density of the solid rear end portion when the speed of the developing roller surface is faster than that of the photoconductor.

The cause of this phenomenon will be described by taking reversal development as an example. FIG. 3 is a schematic diagram when the photosensitive drum and the toner layer are treated as one capacitor. Figure 3
As shown in (a), in the case of reversal development, the surface 10 of the photosensitive drum is minus several hundreds V (Vp = -500V in the figure).
When the black portion is exposed after being charged to FIG.
As shown in, the charge on the surface of the photoconductor is almost lost (Vp '= 0 V on the surface 11).

Subsequently, as shown in FIG. 3C, when the developing electrode 13 having a layer of toner particles 12 is connected thereto, the capacitance component Cp of the photosensitive drum and the capacitance component of the toner layer are connected. Due to the capacitance division with Ct, the potential Vp ″ on the surface of the photosensitive drum slightly rises. The respective conditions are as follows.

Ε _p : Relative permittivity of the photoconductor is about 3 ε _t : Relative permittivity of the toner layer is about 1 d _p : Layer thickness of the photoconductor is 20 μm d _t : Toner layer thick experimental values 10 [mu] m V _b: developing bias experimental values _{-200V V p "= [d p} / (ε p d t + d p)] V b in the experimental value V _p is about -80V or more conditions" Since it is 2/5 of V _b , an electric field works between the surface of the developing roller and the surface of the photoconductor to move the negatively charged toner particles to the surface of the photoconductor.
By this, development is performed.

At this time, as shown in FIG. 3D, a new capacitance component Ct 'is generated by the toner layer adhering to the surface of the photoconductor, and as a result, the potential of the surface (toner layer) of the photoconductor is V.
_{From p} ″ to Vp ′ ″, development is performed until this Vb ′ ″ becomes equal to Vb (developing bias), and the toner layer becomes thick. However, in reality, the toner is on the developing roller against the surface of the photoconductor. When the solid image is printed when the speed of the developing roller surface is faster than that of the photoconductor, the amount of toner that the photoconductor receives at the trailing edge of the solid and other parts is not always supplied from above. Makes a difference.

That is, in the nip region where the developing roller and the surface of the photosensitive member are in contact with each other except the rear end of the solid, the electrostatic latent image always appears as one solid after another from the rear to consume the toner. However, since there is no such electrostatic latent image at the rear end of the solid, a larger amount of toner is supplied, especially in the latter half of the nip, as compared with other toner.

As a method for solving this phenomenon in a process manner, there are the following methods. That is, by setting the electric field due to the developing bias and the surface potential of the photoconductor to be smaller than usual, the development on the electric field may be completed even in the portion where the toner supply amount is smaller than the rear end of the solid.

In FIG. 4A, the speed of the developing roller surface is fixed at twice the speed of the photosensitive member surface, and the developing bias (V
It is a characteristic view which shows the rear end concentration increasing phenomenon when b) is changed. According to this, when the developing bias, which is normally used at -200 to -250V, is lowered to -150V or less, the phenomenon of increasing the trailing edge density disappears. However, if the developing bias is set to the + side of -150 V, the density of the entire solid will decrease.

Next, FIG. 4 (b) shows the results of measuring the developing characteristics (reflection density) when the surface potential of the photoconductor was changed at several developing biases. According to this, when the black potential is set so that the reflection density is just saturated (Vb = -150V in the figure), the increase in the density at the rear end is not seen, and it depends on the running test of the device and environmental changes. Considering the margin, allow a margin for the development bias and the black potential (Vb = -200v, -250 in the figure).
V) has occurred. That is, in order to eliminate the concentration increase at the rear end of the solid, set the black potential in the process conditions as follows:
It suffices to limit the area where the development characteristics are saturated.

However, if the black potential is set in this way, the following problems occur. First, in the fine line portion, the exposure state becomes weaker than in the solid portion, so that the potential does not drop as desired and it becomes difficult to obtain a sufficient image density. In particular, in the developing method in which the photoconductor and the developing roller are in contact with each other, the thin line in the longitudinal direction of the photoconductor (hereinafter, referred to as horizontal thin line) is smaller than that in the rotational direction of the photoconductor (hereinafter, referred to as vertical thin line). In addition, the jitter (speed fluctuation) of the photoconductor tends to cause instability. Further, if the developing characteristics are used in the state where the black potential is barely reached, it is conceivable that the solid concentration may not be produced due to the surface potential fluctuation of the photoreceptor due to the environment or running.

Therefore, in the present invention, the entire apparatus has the following structure. In the first aspect of the present invention, the pattern of the image is recognized and stored in advance before the electrostatic latent image is exposed. That is, 2 in succession in the rotation direction of the photoconductor.
Only the part that is exposed by dots is detected and stored,
The exposure level of that portion is set to be lower than that of the other areas by 2 dots. FIG. 5 shows an example of a flowchart in the dot printer.

In the system shown in the flowchart of FIG. 5, as shown in FIG. 6A, the pixel row in the longitudinal direction of the photoconductor of the input image is x, the pixel row in the photoconductor rotation direction is y, and the pixel is ( (x, y) coordinates, and pixels (x _n , y _n ), (x _n ,
For y _{n + 1} ) ..., the exposure amount of those pixels is reduced. FIG. 6B schematically shows an example of the input image in this case. As can be seen from FIG. 6B, in two or more consecutive dots in the vertical direction in which the trailing edge density increase phenomenon may occur, the exposure amount decreases in any case.

In the case of the printer used in the experiment, the exposure level to be lowered was set so that the surface potential after exposure of a black character would be about -50V since the developing bias was -200V. With such a configuration, in the case of horizontal thin lines and diagonal thin lines that are likely to be unstable due to photoreceptor jitter,
It is possible to reduce an increase in the trailing edge density that occurs in a solid image or a vertical fine line while maintaining a normal exposure level.

However, if the exposure level is constantly lowered by a fixed amount, the increase in the trailing edge density may not be completely extinguished due to environmental fluctuations and surface potential fluctuations of the photoreceptor. In such a case, as shown in FIG. 7, the reflectance sensor unit 20 is installed between the developing device and the cleaner,
During the preparation operation of the printer (during aging), the solid image is printed on the photosensitive member in advance while the transfer target and the transfer device are OFF, and the reflectance is measured. The exposure level may be determined in consideration of the data. 8 is a perspective view showing the reflectance sensor unit 20 taken out. As shown in FIG. 8, the reflectance sensor unit 20 is provided with a plurality of reflectance sensors 21.

The inventor of the present invention conducted an experiment in which the reflectance of an image was measured by using such a reflectance sensor unit and correspondingly the exposure amount was sequentially increased from a low level. In this experiment, the exposure dose was increased from normal 60% to 90% to 10%.
Gradually increase the reflectance to a solid density of 1.
When the value reaches 4, the exposure amount is adopted. FIG. 10 shows a flowchart of the preparation operation. By adopting such a method, it is possible to print a solid image under optimum conditions at all times, and it is possible to completely eliminate the increase in the trailing edge density.

Rather than installing only one reflectance sensor and rotating the photoconductor many times, several reflectance sensors are arranged side by side in the longitudinal direction of the photoconductor to print solids at several exposure levels at a time, It is more convenient to determine the exposure level of since it can shorten the preparatory operation time of the apparatus. FIG. 9 shows the concentration of the solid obtained by this method. Figure 9
From the results shown in (1), it can be seen that when the surface speed of the developing roller is doubled that of the photoconductor, the increase in the trailing edge density disappears.

Next, the second aspect of the present invention will be described. In this mode, the exposure amount of only the portion where the trailing edge density increase occurs is lower than that of other areas. The length of increase in density at the rear end of the solid hardly changes when the same developing device is used at a fixed speed. Therefore, the length of the trailing edge density increasing portion may be measured in advance and the exposure amount may be adjusted by the length from the trailing edge of the solid or the vertical fine line. In the experiment in this example, the length of the concentration increase at the rear end portion of the solid was 1 mm, so 1 from the rear end portion.
The exposure amount was decreased by the same length as the first mode by a length of about mm. As a result, the concentration increase at the rear end portion was improved in the same manner as in the first aspect.

In order to obtain a more stable effect, the length of the density increasing portion at the rear end of the solid is detected by a reflectance sensor, and the exposure level of only the rear end of the solid can be lowered based on the signal. Good. According to this method, it is possible to reliably change the density setting of only the rear end portion without fear that the density of the unnecessary portion will decrease. Further, it is more effective to optimize the exposure level to be reduced by measuring the length of the trailing edge density increase and at the same time measuring the reflectance of the solid portion. The optimization method is
As described above, it is convenient to judge several kinds of exposure levels at once by using several reflectance sensors.

FIG. 11 shows an example of a flow chart for determining the exposure amount according to the density by measuring the length of the density-increased portion at the rear end portion and the reflectance as described above. However,
According to this aspect, before writing the electrostatic latent image on the photoreceptor,
It is necessary to memorize the image pattern in advance and recognize all the continuous dots in the rotation direction of the photoconductor.

FIG. 12 shows an example of a recognition flow chart in this case, and FIG. 13 is a schematic diagram of an example of printing in the case where the density increase at the trailing edge portion occurs for only two dots. In this example, although it is necessary to store and determine the data for one sheet of paper to be printed, there is an advantage that there is no concern of lowering the density of the extra portion.

It is the third aspect of the present invention to minimize such storage / discrimination. An example of the flow chart is shown in FIG. 14 and a schematic diagram of a printing example is shown in FIG. In this aspect, the process is performed while writing the latent image on the photoconductor instead of recognizing the image pattern before writing the latent image on the photoconductor. After performing normal printing on the line in the longitudinal direction of the photoconductor, store the pattern printed on that line, and then compare the pattern of the line to be written next with the print pattern stored before writing, and then in the photoconductor rotating direction. Extract the part where dots are continuous. Then, the output of only the continuous portion is reduced as compared with the other portions.

By repeating the above operation,
In the case of a solid image or a vertical line, only the 1-dot line at the front end becomes the normal exposure, and the other areas become low. Then, a horizontal thin line or an oblique thin line is in a normal exposure state. With this method, it is not possible to print the entire solid area under good conditions, but when printing an image, it is only necessary to store one line of the image in the longitudinal direction of the photoconductor, and without significantly consuming printing time or memory. It is possible to make the concentration increase at the rear end part inconspicuous.

Also in this system, a reflectance sensor is provided between the developing device and the cleaner to print a solid image with different exposure conditions on the photoconductor in advance during the printer preparatory operation, and the solid image is exposed. If printing is performed after optimizing the amount, it is possible to obtain a stable solid density even if the environment changes.

The respective aspects of the present invention described above can be applied not only to the dot printer but also to any electrophotographic apparatus for digitally storing image data. In the case of a dot printer, the exposure level is adjusted by the amount of light, but in the case where the pulse width is modulated to form an image, the pulse width is changed as the exposure level. In the case of performing pulse width modulation exposure, the present invention is applied when a pixel having a pulse width of a certain fixed width or more continues twice or more than a fixed number of times in the photoconductor rotation direction, and the pulse width is lowered by a fixed level.

When the pulse width is changed, unlike the case where the light amount is changed, it seems difficult to perform development by using the optimum portion of the development characteristics, but the unit of pixel becomes smaller. And in the case of pulse width modulation,
Inevitably, the latent image in the fine line part is blunted, and it becomes like a dot printer or analog copying machine. That is,
Since the apparent density can be adjusted sufficiently by the pulse width, it is possible to eliminate the increase in the density at the rear end of the solid by taking the same measure.

[0041]

As described above, according to the image forming apparatus of the present invention, whether the output image has continuous images (dots) in the rotation direction of the photoconductor for a certain amount or more is determined before or during the exposure. For an image that is detected and satisfies the conditions, the exposure level of the entire continuous dot or a part of the continuous dot is set to be lower than that of the other areas. It is possible to obtain an image of uniform density over the entire image.

[Brief description of drawings]

FIG. 1 is a diagram showing an outline of a printer used in an embodiment of the present invention.

FIG. 2 is a characteristic diagram showing a change in density of a solid image depending on a speed ratio between a surface of a photoconductor and a surface of a developing roller.

FIG. 3 is a schematic diagram in which a photoconductor and a toner layer are treated as a capacitor.

FIG. 4A is a characteristic diagram showing a phenomenon in which the rear end density increases when the developing roller surface speed is fixed to twice the photosensitive member surface speed and the developing bias (Vb) is changed; 4) is a characteristic diagram showing the results of measuring the developing characteristics (reflection density) when the surface potential of the photoconductor is changed at several developing biases.

FIG. 5 is a flowchart showing the first aspect of the present invention.

FIG. 6 is a diagram showing a pixel row in the longitudinal direction of the photoconductor of an input image,
The pixel row in the photoconductor rotation direction is y, and the pixel is (x, y)
Figure expressed in coordinates

FIG. 7 is a schematic view of a printer in which a reflectance sensor unit is installed between a developing device and a cleaner.

FIG. 8 is a diagram showing a reflectance sensor unit.

FIG. 9 is a characteristic diagram showing a relationship between a peripheral speed ratio of a photoconductor and a developing roller and an image density when a reflectance sensor unit is used.

FIG. 10 is a flowchart of a preparatory operation when a reflectance sensor unit is used.

FIG. 11 is a flowchart of a method for determining the exposure amount according to the density by measuring the length of the density-increased portion of the rear end portion and also measuring the reflectance according to the second aspect of the present invention.

FIG. 12 is a flowchart of a method of memorizing an image pattern in advance and recognizing how many continuous dots exist in the rotation direction of the photoconductor before writing the electrostatic latent image on the photoconductor.

FIG. 13 is a schematic diagram of a print example in which the density increase at the trailing edge occurs for only two dots.

FIG. 14 is a flow chart diagram of a system according to a third aspect of the present invention for minimizing the storage / discrimination of image patterns.

FIG. 15 is a schematic diagram of a printing example in the method of FIG.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Photosensitive member, 2 ... Charging device, 3 ... Exposure device, 4 ... Developing roller, 5 ... Transfer device, 6 ... Cleaner, 10, 11 ... Photosensitive member surface, 12 ... Toner, 13 ... Developing electrode, 15 ... Pixel,
20 ... Reflectance sensor unit, 21 ... Reflectance sensor.

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP 5-183711 (JP, A) JP 5-183712 (JP, A) JP 5-83501 (JP, A) JP 5- 336324 (JP, A) JP-A-5-252366 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) H04N 1/29

Claims (4)

(57) [Claims] 1. A developing device comprising an electrostatic latent image forming means for forming an electrostatic latent image on an image bearing member and a developing roller rotating at a peripheral speed higher than that of the image bearing member, for visualizing the electrostatic latent image. Means for forming an electrostatic latent image on the image carrier, and a plurality of pixels of the image divided in a direction perpendicular to the moving direction of the image carrier surface are continuous in the moving direction of the image carrier surface. Output of the electrostatic latent image forming means to the entire image composed of pixels that are continuous in the moving direction of the image carrier surface when the detected length is equal to or greater than a predetermined length. An image forming apparatus comprising: 2. A developing device which comprises an electrostatic latent image forming means for forming an electrostatic latent image on an image carrier and a developing roller which rotates at a peripheral speed higher than that of the image carrier, and which visualizes the electrostatic latent image. Means for forming an electrostatic latent image on the image carrier, and a plurality of pixels of the image divided in a direction perpendicular to the moving direction of the image carrier surface are continuous in the moving direction of the image carrier surface. Means for detecting the height of the electrostatic latent image, and when the detected length is equal to or more than a certain length, the electrostatic latent image is detected with respect to a portion at a predetermined distance from the rear end of the image composed of pixels continuous in the moving direction of the image carrier surface. An image forming apparatus comprising: a unit that reduces the output of the image forming unit in advance. 3. A developing device comprising an electrostatic latent image forming means for forming an electrostatic latent image on an image bearing member and a developing roller rotating at a peripheral speed higher than that of the image bearing member, for visualizing the electrostatic latent image. Means, and during the operation of writing the electrostatic latent image on the image carrier, a plurality of image pixels divided in a direction perpendicular to the moving direction of the image carrier surface are continuous in the moving direction of the image carrier surface. A unit for detecting the length and a unit for partially reducing the output of the electrostatic latent image forming unit with respect to an image composed of pixels continuous in the moving direction of the image carrier surface in accordance with the detection signal. An image forming apparatus comprising: 4. A sensor for detecting an image density on the image carrier, wherein an image is printed on the image carrier during a preparatory operation of the apparatus, a signal of the sensor and the image carrier of the pixel. 4. The image forming apparatus according to claim 1, wherein the output of the electrostatic latent image forming unit is determined based on a detection signal of a unit that detects a continuous length in the moving direction of the body surface. .