JP4876603B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an electrophotographic image forming apparatus, and more particularly to an image forming apparatus capable of maintaining a high image quality by suppressing deterioration in image quality due to deterioration of a two-component developer used for development.

  In an image forming apparatus that forms an image by an electrophotographic method, a developing apparatus that develops an electrostatic latent image on an image carrier with a two-component developer having a toner made of resin, an external additive, and the like and a magnetic carrier is the most. Many are used. In a developing device using a two-component developer, it is important to properly control the toner density of the developer in order to maintain a constant image quality.

  In such a developing device, as the number of printed sheets advances, the toner stays in the developing device due to an increase in the residence time of the developer in the developing device, so that the external additive on the toner surface is buried in the resin. , Then, the developers come closer to each other, the fluidity is lowered, and the triboelectric charging property is deteriorated. As a result, image quality defects such as a decrease in image quality, scattering due to a decrease in charge amount, and an increase in fog occur. This type of image quality defect tends to become noticeable when printing is performed at a low printing rate with low toner consumption, when the toner stays in the developing device for a long time and is subjected to stress for that amount of time.

  In recent years, with the advancement of advanced information technology, the need for high-quality electrophotographic image forming apparatuses based on digital technology has increased, and measures to maintain high image quality related to development processing have been proposed to meet these demands. .

As an example, the number of rotations of the developing roller (sleeve), which is a developer carrying member, and the number of image dots are stored. When the number of image dots at a predetermined number of rotations of the developing roller is smaller than a predetermined threshold, the developer is added. A method has been proposed in which a developing (discharge) operation is performed from a developing roller onto a photosensitive drum as an image carrier (see, for example, Patent Document 1).
JP 2004-1225829 A

  However, the proposed method does not take into account the developer use environment and life, and is sufficiently effective in the latter half of the developer life when the charging ability of the carrier is low, and in a high temperature and high humidity environment where the charge amount is low. I can't get it. If an attempt is made to obtain an effect in a high-temperature and high-humidity environment state in the latter half of the developer life, useless toner is discharged at the start.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a stable and high-quality image forming apparatus by judging the deterioration state of toner, forcibly discharging the deteriorated toner, and replenishing new toner accordingly.

  The above object is achieved by the following configuration.

A two-component developer stored in a developing unit having a developer carrier to which a developing bias is applied is used to make an electrostatic latent image formed on the image carrier a visible image, and visible on the image carrier. In an image forming apparatus that transfers and fixes an image to a transfer material, a halftone patch is output based on reference gradation input data, the density is detected, and the relationship between the input data and the output density is obtained , and the obtained relationship And a means for controlling the toner density to a predetermined level after the toner is consumed on the photosensitive member .

  Since the deterioration state of the toner can be determined from the γ gradient (curve) of the halftone patch, the deteriorated toner can be effectively discharged (discharged), and high image quality can be maintained.

  Hereinafter, an image forming apparatus according to an embodiment of the present invention will be described.

  FIG. 1 is a schematic diagram showing the overall configuration of the image forming apparatus.

  In FIG. 1, the image forming apparatus includes a double-sided document automatic feeder RADF and an image forming apparatus main body A.

  The double-sided document automatic feeder RADF is located at the upper part of the image forming apparatus main body A and can be opened and closed. The document on the document feed tray a is transported to a paper feed roller b, a separation roller c, a registration roller d, and a transport drum e, and the document is transported.

  Next, the image forming apparatus main body A includes an image reading apparatus 1, an image processing means (including a control unit) 2, an image writing apparatus 3, an image forming means 4, a cassette paper feeding means 5, a conveying means 6, a fixing device 7, The paper unit 8 and the re-conveying unit 9 are included.

  The optical system of the image reading apparatus 1 includes an exposure unit 14 including a light source and a first mirror, a V mirror unit 15 including a second mirror and a third mirror, a lens 16, and a CCD image sensor 17. Document reading by the double-sided document automatic feeder RADF is performed at a position where the exposure unit 14 is stopped at the initial position below the slit exposure glass 13. Reading a document on the platen glass 11 is performed while moving the exposure unit 14 and the V mirror unit 15.

  Image information of the document image read by the image reading apparatus 1 is subjected to image processing by the image processing means 2, converted into a signal as image data, and temporarily stored in a memory. Output light from a semiconductor laser (not shown) included in the image writing means 3 is applied to the photosensitive drum 21 which is an image carrier to form an electrostatic latent image.

  In the image forming unit 4, a charge (negative polarity having the same polarity as that of toner in the present embodiment) is applied to the surface of the photosensitive drum 21 by the charger 22, and electrostatic is generated by laser light irradiation from the image writing unit 3. A latent image is formed, and the electrostatic latent image is developed into a toner image by the developing device 23 which is a developing unit. The toner image is transferred onto the paper P, which is a transfer material conveyed from the cassette paper feeding means 5, by the transfer device 29A, the paper P is peeled off by the separator 29B, and the residual toner after transfer is removed by the cleaning device 26. The The sheet P on which the toner image has been transferred is conveyed by a conveying means 6 composed of a belt, fixed by a fixing means 7, and discharged by a paper discharging means 8 to a paper discharge tray 81 outside the apparatus.

  In the case of double-sided copying, the paper P on which the image is formed on the first side is sent to the re-transport unit 9 by the transport path switching plate 82, is reversed, and the image forming unit 4 forms an image on the second side again. Thereafter, the paper is discharged by a paper discharge means 8 to a paper discharge tray 81 outside the apparatus. In the case of reverse paper discharge, the paper P branched from the normal paper discharge path by the transport path switching plate 82 is switched back and upside down in the reverse paper discharge unit 83 and then discharged from the apparatus by the paper discharge means 8. The paper is discharged to the paper tray 81.

  Next, the periphery of the photosensitive drum 21 for forming an image and the developing device 23 will be described.

  FIG. 2 is an enlarged cross-sectional view of the periphery of the photosensitive drum in FIG.

  In FIG. 2, reference numeral 231 denotes a developing roller for converting the latent image into a toner image on the photosensitive drum 21. After the development processing, residual toner (recycled toner) on the photosensitive drum 21 is scraped off by a cleaning blade 26B provided on the downstream side in the rotation direction of the photosensitive drum 21.

  It is guided to the recycled toner supply tank 271 by the guide roller 26A, collected and conveyed by the screw 272, temporarily stored in the recycled toner supply tank 271, and sequentially returned to the developing device 23 via the screw 273 and reused. The screws 272 and 273 are rotated by a driving mechanism (not shown).

  On the other hand, the toner supplied to the developing device 23 is stored in a toner supply container 241 and stored in the toner supply device 24. While the toner supply container 241 rotates by obtaining power from a power source (not shown), the agitating means 242 agitates the toner in the container, and conveys and replenishes it to the developing device 23 via the toner intermediate supply tank 251. .

  Further, the conveyance release stopping unit 243 is a unit for releasing or stopping the conveyance of the toner from the toner supply container 241 toward the developing device 23. In this embodiment, the conveyance cancellation stopping unit 243 is controlled by a control unit included in the image processing unit 2. This is performed by opening and closing the shutter 244 with the power from the toner supply driving unit 10A, but is not limited to this.

  Further, in modes other than the mode in which toner is transported from the toner replenishing device 24 toward the developing device 23, the transport of the toner is stopped by the transport release stopping means 243, and the toner in the toner replenishing container 241 is agitated. ing.

  The toner intermediate supply unit 25 includes a toner intermediate supply tank 251 for temporarily storing toner supplied from the toner supply container 241 to the developing device 23, and supplies toner to the developing device 23.

  The agitating means 252 is a means for agitating the toner in the toner intermediate supply tank 251 and has two internal screws 252a, and agitates and conveys the toner by its rotation.

  The developer in the developing device 23 is transported by a transport / stirring member 232, and the toner density is measured by a density sensor LS as a toner density detecting means as shown in FIG. The measured value (detected value) is transmitted to the control unit.

  FIG. 3 is a cross-sectional view showing a density sensor that detects the toner density based on the magnetic permeability of the developer.

  The developer is guided and passed through a coil of a resonance circuit composed of a ferrite core, and the amount of magnetic particle carriers flowing in the magnetic field decreases when the amount of toner is large, and increases when the amount of toner is small, and the inductance changes. . A method is used in which this change is regarded as a frequency change of the resonance circuit, and the control unit calculates a change amount of the toner density corresponding to the change amount.

  Further, the chargeability and life stability of the developer tend to depend on the operating environment of the apparatus, and in this embodiment, as shown in FIG. 4, the optimum condition of the toner density is maintained depending on the humidity and the number of printed sheets. So that it is controlled. That is, a hygrometer (not shown) is installed in the image forming apparatus main body A, and this humidity data is transmitted to the control unit. In FIG. 4, when the humidity RH <30%, a, and when 30% ≦ humidity RH <55%. When b and 55% ≦ humidity RH, the relationship between the number of printed sheets and the toner density is controlled under the condition along c.

  In this embodiment, the toner density is detected by a change in inductance, but a method of controlling the toner density by optically determining the color of the developer (developer) may be used. FIG. 4 is a diagram showing the transition of the number of printed sheets and the toner density according to environmental conditions.

  The developing device 23 includes a developing roller 231 that is rotatably supported facing the photosensitive drum 21. A developing bias is applied to the developing roller 231, and toner carried on the peripheral surface of the roller is transferred onto the photosensitive drum 21. It is attached to the latent image and visualized as a toner image.

  Further, as shown in FIG. 1, a reflection type sensor DS which is an image density detecting means is provided between the transfer device 29A and the cleaning device 26, and a reference formed on the photosensitive drum 21 corresponding to the non-image portion. The density of the multi-stage halftone patch R is measured based on the gradation input data (writing pattern), and the detected value is transmitted to the control unit. However, the measurement is performed once for a predetermined number of printed sheets.

  FIG. 5 shows a multistage halftone patch formed in a non-image area on the photosensitive drum and a reflective sensor for measuring the density thereof.

  FIG. 6 is a diagram showing an output conversion value of the reflection density of the multi-stage halftone patch with respect to the input data of the reference gradation.

  The present invention will be described below.

  The present invention outputs a multi-stage halftone patch R (see FIG. 5) based on specified input data (X) of a reference gradation (10-level density ratio with solid black being 100 and white background being 0), and the density is detected. Then, the degree of fatigue of the developer (toner) is detected from the relationship between the specified input data and the output density, and toner with poor charging efficiency is forcibly discharged out of the developing device (photosensitive drum) according to the degree of fatigue. By replacing the toner, stable image formation is possible.

  That is, in FIGS. 5 and 6, when a multi-stage halftone patch for the specified input data X is formed in the non-image area, the reflection density is read by the reflection type sensor DS, and the output conversion value Y is used (development) The relationship between X and Y in the new developer without fatigue after the agent replacement is a proportional straight line such as Line_1. Thereafter, as the number of printed sheets advances, the toner deteriorates, and the γ gradient (curve) increases as in Lines 2, 3, and 4. This means that toner having poor charging efficiency has increased in the developing device.

  Therefore, it is necessary to forcibly discharge toner with poor charging efficiency from the developing device. In this embodiment, a discharge pattern having a predetermined density is formed on the non-image portion (for example, between sheets) on the photosensitive drum 21 for cleaning. Discard in device 26. That is, a discharge pattern having a toner amount corresponding to the printing rate of the paper being used is formed on the photosensitive member, cleaned and discarded without being transferred to the paper. In this way, the inefficient toner is discharged and the new toner is replenished so that the γ gradient approaches Line_1. Since the control unit stores the toner amount of the discharge pattern corresponding to the γ gradient as a table, a discharge pattern corresponding to the discharged toner amount is formed according to the detection result of the γ gradient.

  FIG. 7 is a diagram illustrating the relationship between the γ gradient and the amount of discharged toner corresponding to the printing rate of the paper.

  In FIG. 7, since the developer is not fatigued in Line_1, it is not necessary to discharge the toner. Since the printing rate of the paper used is 0% (the toner amount is 0), the toner amount of the discharge pattern is 0. There is no need for pattern formation. That is, it means a standard γ gradient (Line_1). In the case of Line_3, the toner amount of the discharge pattern formed on the photosensitive drum is a toner amount corresponding to a printing rate of 2% of the paper being used. After exhausting the fatigue toner, new toner is replenished from the toner replenishing device 24 (see FIG. 2) to the developing device 23 (see FIG. 2) to obtain a predetermined toner concentration, and after γ correction is performed again on the specified input data, The normal image forming process is entered.

  Hereinafter, γ correction at the time of replacement of a new developer will be described again using a flowchart.

  FIG. 8 is a flowchart for the γ correction when the new developer is replaced.

  In FIG. 8, in step S1, a new developer is charged into the developing device. In step S2, the solid black density is confirmed and Dmax correction is performed. In step S3, multi-stage halftone patches (10 stages in the present embodiment) are formed on the photosensitive drum with the specified input data (Vin_1). In step S4, the density of the multistage halftone patch is detected and stored as a gradient γ1. In step S5, the input pattern Vin_2 (Vin_2-1, Vin_2-2,..., Vin_2-10 in FIG. 6) is changed and corrected so that the output value becomes the target gradient γ2 (= Line_1). In step S6, the printing operation is started.

  Next, γ correction performed for each predetermined number of printed sheets will be described with reference to a flowchart.

  As described above, the chargeability and life stability of the developer tend to depend on the operating environment of the apparatus. In this embodiment, as shown in FIG. 4, the optimum conditions for the number of prints and the toner density are maintained depending on the humidity. Is controlled to be.

  Therefore, after a predetermined number of printed sheets (every 2000 sheets in the present embodiment) is finished, after Dmax correction is performed, input data of the reference gradation (10 levels with solid black set to 100 and white background 0) (FIG. 6, Vin_2-1, Vin — 2-2,..., Vin — 2-10), multi-stage halftone patches R are output, the density is detected, and the developer (toner) is determined from the relationship between the input data and the output density. ) Is detected, the low charging efficiency toner is forcibly discharged from the developing device, and new toner is replenished for correction.

  FIG. 9 is a flowchart for correction by forced discharge of toner with low charging efficiency.

  In FIG. 9, the optimum condition of the toner density according to the operating environment is maintained in step U1. In step U2, the γ correction mode is set when the main switch is turned on or every predetermined number of prints (in this embodiment, every 2000). In step U3, the solid black density is confirmed and Dmax correction is performed. In step U4, input data Vin_2 (Vin_2-1, Vin_2-2,..., Vin_2-10) having a writing pattern stored in the apparatus after the Dmax correction and γ correction at the time of new developer replacement are multi-staged. A halftone patch is formed, and a gradient γ3 as an output value is detected. In step U5, γ is checked to determine whether or not the gradient γ3 is approximate to the target gradient γ2 (≈Line_1). In the case of γ such as Line_2, 3, 4 in FIG. 6, it is determined that the fatigue toner having poor charging efficiency has increased, and the process proceeds to Step U6. In step 6, the toner is forcibly discharged based on the discharged toner amount (print rate, see FIG. 7) for each pattern stored and stored in advance in the storage unit, and new toner is replenished from the toner replenishing device to the developing device. Control proceeds to step U7 under control of the predetermined concentration. In step U5, if the gradient γ3 is close to the target gradient γ2, the result is Yes and there is no need to forcibly discharge the toner, and the flow directly proceeds to step U7. Thereafter, in step 7 described with reference to FIG. 8, there is a possibility that the charging condition of the developer has changed. Therefore, a multi-stage halftone patch with the prescribed input data (= Vin_1) is formed on the photosensitive drum. To do. In step 8, γ correction is performed, and in step 9, a printing operation is started.

  Note that during the γ correction mode, the normal printing operation is stopped.

  As described above, γ correction is performed to approximate the standard γ gradient, and stable image formation is maintained.

  Next, an experiment for confirming the effect of the present invention will be described.

・ Experimental conditions
AC development bias frequency during running: 5 kHz
AC bias Vp-p voltage during running: 1 kV
θ (= developing sleeve linear velocity Vs / photosensitive drum linear velocity Vp): 2
DS (space between the photosensitive drum and the developing sleeve): 0.3 mm
Fog margin (photosensitive member charging voltage Vo−development DC bias Vdc): 200V
Developer transport amount: 200-240
(G / m ² )
Under the above conditions, only the toner density control according to the operation environment of the apparatus is applied (comparative example), and the one to which the γ correction of the present invention is applied (example), 50 kp (original with a printing rate of 0.01% to 10%) A running test (50,000 prints) was performed, and the degree of halftone, fog, and scattering was compared.

  The results of the confirmation experiment are shown in Table 1.

  In Table 1, Table 1 (a) shows the result of the comparative example, and Table 1 (b) shows the result of the example.

  As is apparent from Table 1, in the case of the comparative example, in the case of a low printing rate of 0.01% to 1.0% in which the toner consumption is low by 50,000 prints, the problems of halftone, fogging and scattering are conspicuous. In the example, it was found that any document with any printing rate is in a pass level without any problem.

  However, in the table, “x” indicates a defect, “Δ” indicates a slight defect, and “◯” indicates a pass level.

1 is a schematic diagram illustrating an overall configuration of an image forming apparatus. FIG. 2 is an enlarged cross-sectional view of the periphery of the photosensitive drum in FIG. 1. FIG. 6 is a cross-sectional view illustrating a density sensor that detects toner density based on magnetic permeability of a developer. FIG. 6 is a diagram illustrating changes in the number of prints and toner density according to environmental conditions. 2 shows a multi-stage halftone patch formed in a non-image area on a photosensitive drum and a reflective sensor for measuring the density thereof. It is a figure which shows the output conversion value of the reflection density of the multistage halftone patch with respect to the input data of a reference gradation. 6 is a diagram illustrating a relationship between a γ gradient and an amount of discharged toner corresponding to a printing rate of a sheet. FIG. FIG. 6 is a flowchart for γ correction when a new developer is replaced. FIG. 6 is a flowchart for γ correction by forced discharge of low charging efficiency toner.

Explanation of symbols

2 Image processing means 21 Photosensitive drum 23 Developing device 231 Developing roller 232 Conveying / stirring member 24 Toner replenishing device 26 Cleaning device 3 Image writing device DS Reflective sensor R Halftone patch

Claims (3)

A two-component developer stored in a developing unit having a developer carrier to which a developing bias is applied is used to make an electrostatic latent image formed on the image carrier a visible image, and visible on the image carrier. In an image forming apparatus for transferring and fixing an image to a transfer material,
A halftone patch is output based on the input data of the reference gradation, the density thereof is detected, the relationship between the input data and the output density is obtained , and a discharge pattern having a discharge toner amount corresponding to the obtained relationship is formed. An image forming apparatus comprising means for controlling the toner density to a predetermined level after the toner is consumed on the photosensitive member.   The image forming apparatus according to claim 2, wherein the halftone patch is divided into predetermined gradations in multiple stages.   The image forming apparatus according to claim 1, wherein the toner density is controlled to a predetermined value in accordance with a change in inductance for measuring a magnetic permeability of the developer.

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