JP2011048118A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus that reduces the adverse effect resulting from change of control by setting a lower limit for a toner proportion, while avoiding image failure resulting from adhesion of carrier to a photoreceptor. <P>SOLUTION: Output image density is stabilized in a balanced manner by executing supply control of a triple control system that uses video count, patch detection ATR (Auto Toner Replenishing) control, and toner density sensor. When a target TD ratio calculated in the patch detection ATR control is lower than a lower limit TD ratio, image formation is continued using the lower limit TD ratio. Consequently, a quantity of charged toner Q/M decreases, with the result that white background fogged image and scattering easily occur. However, image formation with a high image ratio has a room for setting a low target TD ratio without causing white spot image failure. Accordingly, the target TD ratio is set to be low using the room. White background fogged image and scatter are suppressed thereby. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an image forming apparatus that performs a replenishment control of a replenishment developer by setting a predetermined lower limit to the ratio of toner in the developer circulating in the developing device, and more specifically, the disadvantages caused by setting a lower limit. It relates to control to reduce.

  An image forming apparatus that develops an electrostatic image written on a photosensitive member into a toner image using a developer (two-component developer) mainly composed of toner (non-magnetic) and carrier (magnetic) is widely used. Yes. In a developing device using a two-component developer, the ratio of toner to the developer (toner concentration, TD ratio) is set so that an electrostatic image formed under a predetermined charging / exposure condition is developed with a predetermined amount of applied toner. The toner charge amount is kept constant by adjusting.

In the control that keeps the toner charge amount constant, if the toner charge amount decreases, the amount of applied toner increases and the image density increases even in the same electrostatic image. Therefore, by reducing the toner ratio and increasing the chance of friction of the developer The toner charge amount is increased. On the other hand, when the toner charge amount is increased, the amount of applied toner is reduced and the image density is lowered even in the same electrostatic image. Therefore, the toner charge amount is reduced by increasing the toner ratio and reducing the chance of developer friction. (Patent Documents 2 to 4).
Patent Document 1 uses a magnetic permeability sensor (inductance sensor) that outputs a signal corresponding to the toner ratio by utilizing the fact that the magnetic permeability of the developer (two-component developer) increases as the carrier ratio increases. Detection means are shown.

  Patent Document 2 discloses an optical that outputs a signal according to the amount of applied toner in a patch image by irradiating a patch image formed at an image interval during continuous image formation with LED light and detecting the amount of specular reflection. A sensor is shown. Here, the toner for replenishing the developer container with the replenishment developer so that the applied toner amount of the patch image formed under a predetermined charging / exposure condition converges to a predetermined value, and occupies the developer (two-component developer). The ratio of is changed. This is so-called patch detection ATR (AutoTonerReplenishing).

  Japanese Patent Application Laid-Open No. 2005-228561 discloses a video count unit that counts and accumulates the number of development dots of the entire binary-modulated image supplied to the light source of the exposure apparatus. Here, the amount of toner consumed when developing one image is processed by processing image data and exposure signals to be formed, and an amount of replenishment developer corresponding to the amount of toner consumed is calculated. By replenishing, fluctuations in the proportion of toner in the developer are suppressed. For this reason, the ratio of the toner to the developer is stable, and the image density is kept substantially constant even if the image ratio changes greatly during continuous image formation.

  Japanese Patent Application Laid-Open No. H10-228561 discloses control for supplying a developer for replenishment to a developer container by using both video count means and patch image detection. Here, the toner replenishment amount obtained by the video count means is corrected based on the detection result of the patch image, and the toner ratio is converged to a target value set so that the image density becomes a predetermined level. By combining the video count means, fluctuations in the toner ratio accompanying changes in toner consumption are suppressed, so that patch detection ATR control can be stably executed.

Japanese Patent Laid-Open No. 01-182750 Japanese Patent Laid-Open No. 06-149057 JP 05-027527 A JP 05-027591 A

  When the toner-only supply developer is supplied to the developing device, the same carrier continues to circulate in the developing device and repeats friction with the toner, so that the surface of the carrier is gradually contaminated and the charging performance is lowered. Further, when a replenishment developer in which a predetermined ratio of carrier is added to toner is replenished to the developing device, the difference in the residence time of the carrier in the developer in the developer container increases, and the variation in charging performance gradually increases.

  In any case, if the developing device accumulates image formation and the number of carriers with reduced charging performance increases in the developer, the toner charge amount tends to decrease, and the amount of toner that adheres to the same electrostatic image increases. The image density increases. For this reason, in the control shown in Patent Document 4, when the toner charge amount decreases, the induction target value of the ratio of non-magnetic toner is lowered in order to increase the toner charge amount and restore the image density.

  However, if the induction target value is lowered and the ratio of the toner is excessively lowered in the state where the number of carriers with reduced charging performance is increased in the developer, an image defect called a white spot image is likely to occur as described later. Become. The white spot image is a defective transfer image in which a round white spot of about 60 to 200 μm is generated in the image by preventing the transfer of the surrounding toner when the carrier adhering to the photoreceptor from the developer carrying member passes through the transfer portion. is there.

  Further, if the ratio of the toner in the developer is too low, the developing device cannot exhibit the required development performance. Another image defect in which the carrier carried on the developer carrying member leaves a rubbing trace on the transferred image is likely to occur.

  For this reason, as shown in FIG. 4, when a predetermined lower limit value (lower limit TD ratio) is provided in the toner ratio and the lower limit value (lower limit TD ratio) is likely to be interrupted, the toner ratio induction target value (target) (TD ratio) is fixed to the lower limit (lower limit TD ratio) (S18a).

  However, when the induction target value (target TD ratio) is fixed to the lower limit value (lower limit TD ratio), the decrease in the toner charge amount is left as a result, and as a result, a white background fogged image or developer carrying member in which toner adheres to the white background portion of the image From this, toner scattering is likely to occur.

  An object of the present invention is to provide an image forming apparatus capable of suppressing generation of a white spot image while suppressing fogging of a white background and toner scattering even in a state where the number of carriers in which charging performance has decreased due to cumulative image formation is increased. Yes.

  The image forming apparatus of the present invention includes a photosensitive member, a developer carrying member that carries a developer containing toner and a carrier to develop an electrostatic image of the photosensitive member, and the developer carrying member that is charged with a developer. A developer container carried on the developer container, a supply device for supplying a replenishment developer containing toner to the developer container, and detecting a developer circulating in the developer container and outputting a signal corresponding to the ratio of the toner to the developer And a replenishment development by the supply device so that a predetermined electrostatic image is developed with a predetermined amount of applied toner within a range in which the ratio of the toner detected using the detection unit does not fall below a lower limit value. And a control means for controlling the replenishment of the agent. The image forming apparatus includes a lower limit value setting unit that sets the lower limit value lower for image formation with a large amount of toner consumption.

  In the image forming apparatus of the present invention, the lower the lower limit value is set for the image formation with a larger amount of toner consumption, so that the toner ratio becomes lower as compared with the case where a certain lower limit value is set regardless of the toner consumption amount. The toner charge amount can be controlled and maintained as usual. For this reason, it is possible to suppress the white background fogged image and the toner scattering, which are generated due to insufficient toner charge amount in an image having a large amount of toner consumption. Even when image formation with a high image ratio continues and the toner charge amount temporarily decreases, the toner charge amount control is continued to avoid unnecessary decrease in the toner charge amount, and the subsequent toner Wait for recovery of charge amount.

  As will be described later, in the case of image formation with a large amount of toner consumption, a white spot image is less likely to occur compared to image formation with a small amount of toner consumption. Therefore, by utilizing the lower limit of the lower limit found in image formation with a large amount of toner consumption, it is possible to suppress the white background fog image and toner scattering without generating a white spot image. By setting the lower limit value low, the ratio at which normal control of the toner charge amount is applied increases, so that the reproducibility of the average image density is also increased.

  In particular, when a patch image is formed at a considerable interval and the target ratio of toner is set, and then the supply device is controlled to maintain the target ratio until the next setting, the lower limit value is set low. It is effective to do. This is because by setting the lower limit value low, the number of times that the toner ratio falls below the lower limit value and the target ratio cannot be set is reduced when the target ratio is set.

  Therefore, even when the number of carriers whose image forming performance has been accumulated and the charging performance has decreased is increased, generation of white spot images can be suppressed while suppressing fogging images and toner scattering. It is possible to reduce the adverse effects caused by setting a lower limit for the toner ratio and changing the control at the lower limit.

1 is an explanatory diagram of a configuration of an image forming apparatus. FIG. 6 is an explanatory diagram of a configuration of a yellow image forming unit. It is explanatory drawing of a structure of a developing device. 3 is a flowchart of control according to the first embodiment. FIG. 10 is an explanatory diagram of a conversion table of a difference in TD ratio and a necessary toner supply amount. It is explanatory drawing of the conversion table of a video count value and a toner consumption. It is explanatory drawing of a structure of a Faraday gauge.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the present invention, as long as the lower limit value of the toner density (TD ratio) is lowered in image formation with a large amount of toner consumption, another embodiment in which part or all of the configuration of the embodiment is replaced with the alternative configuration. But it can be done.

  Therefore, if a developing device using a two-component developer is provided, it can be implemented in either a tandem type or a one-drum type, and there is no distinction between an intermediate transfer method, a recording material conveyance method, and a direct transfer method for transferring to a recording material in a single wafer type Can be implemented. It is possible to use a developing device using 100% toner as a replenishing developer, or a developing device that replenishes a replenishing developer containing a carrier and discharges excess developer.

  In the present embodiment, only main parts related to toner image formation / transfer will be described. However, the present invention includes a printer, various printing machines, a copier, a fax machine, a composite machine, in addition to necessary equipment, equipment, and a housing structure. It can be implemented in various applications such as a machine.

  In addition, about the general matter of the image development apparatus and image forming apparatus shown by patent documents 1-4, illustration is abbreviate | omitted and the overlapping description is abbreviate | omitted.

<Image forming apparatus>
FIG. 1 is an explanatory diagram of the configuration of the image forming apparatus. FIG. 2 is an explanatory diagram of the configuration of the yellow image forming unit.

  As shown in FIG. 1, the image forming apparatus 100 is a tandem intermediate transfer type full color printer in which image forming portions Pa, Pb, Pc, and Pd of yellow, magenta, cyan, and black are arranged along an intermediate transfer belt 24. is there.

  In the image forming portion Pa, a yellow toner image is formed on the photosensitive drum 1 a and is primarily transferred to the intermediate transfer belt 24. In the image forming unit Pb, a magenta toner image is formed on the photosensitive drum 1 b and is primarily transferred to the yellow toner image on the intermediate transfer belt 24. In the image forming portions Pc and Pd, a cyan toner image and a black toner image are formed on the photosensitive drums 1c and 1d, respectively, and are sequentially transferred onto the intermediate transfer belt 24 in order to be primarily transferred.

  The four-color toner images primarily transferred to the intermediate transfer belt 24 are transported to the secondary transfer portion T2 and collectively transferred to the recording material P. The recording material P drawn from the recording material cassette 20 is separated one by one by the separation roller 21 and sent to the registration roller 22. The registration roller 22 receives and waits for the recording material P in a stopped state, and sends the recording material P to the secondary transfer portion T2 in time with the toner image on the intermediate transfer belt 24.

  The recording material P on which the four-color toner images are secondarily transferred is heated and pressed by the fixing device 26 to fix the toner images on the surface, and then discharged to the outside of the machine body.

  The image forming portions Pa, Pb, Pc, and Pd are configured substantially the same except that the color of toner used in the developing devices 4a, 4b, 4c, and 4d is different from yellow, magenta, cyan, and black. Hereinafter, the image forming unit Pa will be described, and the image forming units Pb, Pc, and Pd will be described by replacing “a” at the end of the reference numerals attached to the components of the image forming unit Pa with “b”, “c”, and “d”. To do.

  The intermediate transfer belt 24 is supported around a tension roller 27, a driving roller 28, and a counter roller 25, and is driven by the driving roller 28 to rotate in the direction of arrow R2 at a process speed of 300 mm / sec. The secondary transfer roller 23 is in contact with the intermediate transfer belt 24 whose inner surface is supported by the counter roller 25 to form a secondary transfer portion T2. By applying a DC voltage from the power source D2 to the secondary transfer roller 23, the toner image carried on the intermediate transfer belt 24 is secondarily transferred to the recording material P. The belt cleaning device 29 rubs the intermediate transfer belt 24 with a cleaning blade, escapes the transfer to the recording material P, passes through the secondary transfer portion T2, and collects the transfer residual toner remaining on the intermediate transfer belt 24. .

  As shown in FIG. 2, the document to be copied is read by the document reading device 101. The document reading apparatus 101 includes a photoelectric conversion element that converts a document image such as a CCD into an electrical signal, and corresponds to yellow image information, magenta image information, cyan image information, and black image information of a document to be copied. Output image signal.

  In the image forming portion Pa, a charging roller 2a, an exposure device 3a, a developing device 4a, a primary transfer roller 5a, and a cleaning device 6a are arranged around a photosensitive drum 1a that is an example of a photosensitive member. The photosensitive drum 1a has a negatively charged photosensitive layer formed on the outer peripheral surface of an aluminum cylinder, and rotates in the direction of arrow R1 at a process speed of 300 mm / sec.

  The charging roller 2a is driven to rotate in contact with the photosensitive drum 1a, and an oscillating voltage obtained by superimposing an AC voltage on a DC voltage is applied from the power source D3, whereby the surface of the photosensitive drum 1a has a uniform negative polarity dark potential. Charge to VD. The exposure device 3a scans the scanning line image data obtained by developing the yellow separated color image with a rotating mirror, and writes an electrostatic image of the image on the surface of the charged photosensitive drum 1a. As will be described later, the developing device 4a uses a two-component developer to attach toner to the electrostatic image (exposed portion) of the photosensitive drum 1a, and reversely develops the toner image.

  The primary transfer roller 5 a presses the inner surface of the intermediate transfer belt 24 to form a primary transfer portion Ta between the photosensitive drum 1 a and the intermediate transfer belt 24. By applying a positive DC voltage from the power source D1 to the primary transfer roller 5a, the toner image carried on the photosensitive drum 1a is primarily transferred to the intermediate transfer belt 24 passing through the primary transfer portion Ta. The cleaning device 6 a slides the cleaning blade on the photosensitive drum 1 a and collects the transfer residual toner remaining on the photosensitive drum 1 a by escaping from the transfer to the intermediate transfer belt 24.

<Developing device>
FIG. 3 is an explanatory diagram of the configuration of the developing device. As shown in FIG. 3, the developing device 4a carries a developer on a developing sleeve 41, which is an example of a developer carrying member, and develops an electrostatic image on the photoreceptor (1a). In the developing container 40, the developer is stirred and charged by a pair of conveying screw members (44 a and 44 b) and is carried on the developing sleeve 41. A developer cartridge 46, which is an example of a supply device, supplies a replenishment developer containing toner to the developer container 40. The toner concentration sensor 10, which is an example of a detection unit, detects the developer circulating in the developing container 40 and outputs a signal corresponding to the proportion of toner in the developer.

  The developer container 40 contains a developer mainly composed of toner and carrier, and the ratio (toner concentration, TD ratio) expressed by the weight of the toner in the developer in the initial state is 8%. The TD ratio is not limited to 8% because it should be appropriately adjusted depending on the toner charge amount, the carrier particle size, the structure of the developing device 4a, and the like.

  In the developing device 4a, a developing region facing the photosensitive drum 1a is opened, and a developing sleeve 41 made of a non-magnetic material is rotatably disposed so as to be partially exposed from the opening. The magnet 42 which is a magnetic field generating means is constituted by a fixed cylindrical magnet having a plurality of magnetic poles of a predetermined pattern along the circumference of the developing sleeve 41. The carrier having the toner adsorbed on the surface by frictional charging is restrained on the developing sleeve 41 by the magnetic field generated by the magnet 42.

  The developing sleeve 41 rotates in the direction of the arrow A during the developing operation, holds the developer in the developer container 40 in a layered state, carries and transports the developer, and supplies the developer to the developing area facing the photosensitive drum 1a. The layer thickness of the developer carried on the developing sleeve 41 is regulated by a regulating member 43 provided in close proximity to the developing sleeve 41.

  The power source D4 applies an oscillating voltage obtained by superimposing an AC voltage on the negative DC voltage Vdc to the developing sleeve 41. The developing sleeve 41 to which the negative DC voltage Vdc is applied is more negative than the electrostatic image (exposed portion) formed on the photosensitive drum (1a), and is charged to the negative polarity in the developer. The transferred toner is transferred from the developing sleeve 41 to the photosensitive drum (1a). The remaining developer that has developed the electrostatic image on the developing sleeve 41 is collected in the developing container 40 according to the rotation of the developing sleeve 41 and mixed with the developer conveyed by the conveying agitating screw 44a.

  In the developing container 40, conveying and agitating screws 44a and 44b, which are examples of an agitating and conveying member that conveys the developer while stirring, are disposed in parallel with the developing sleeve 41. The developing sleeve 41 and the conveyance stirring screws 44a and 44b are connected to each other by a gear mechanism (not shown) outside the developing container 40, and are integrally rotated by a common driving motor.

  The space in the developing container 40 is divided into two spaces by a partition wall 40F, and a conveying and stirring screw 44a is disposed in the space on the developing sleeve 41 side, and a conveying and stirring screw 44b is disposed in the space on the developer cartridge 46 side. Openings (not shown) are formed at both ends in the longitudinal direction of the partition wall 40F in order to deliver the developer between the two spaces and circulate it in the developing container 40.

  The conveyance agitating screw 44a supplies the developer to the developing sleeve 41 while conveying the developer from the back side to the front side of the sheet. In contrast, the conveyance agitation screw 44b mixes the replenishment developer supplied from the developer cartridge 46 with the circulating developer while conveying the developer from the near side to the far side of the sheet. In this way, the conveying and agitating screws 44a and 44b circulate the developer in the developing container 40, and agitate and charge the toner and the carrier by friction.

<Two-component developer>
The toner of the two-component developer includes colored resin particles containing a binder resin, a colorant, and other additives as necessary, and colored particles to which external additives such as colloidal silica fine powder are externally added. And have. The toner is a negatively chargeable polyester resin produced by a pulverization method, and the volume average particle diameter is preferably 4 μm or more and 8 μm or less. In this example, it was 5.5 μm.

  A Coulter Counter TA-II type (manufactured by Coulter, Inc.) was used for measuring the volume average particle diameter of the toner. Then, an interface (manufactured by Nikka) and a CX-I personal computer (manufactured by Canon) for outputting the number average distribution and the volume average distribution from the measurement results were used.

  As the electrolytic aqueous solution for the measurement sample, a 1% NaCl aqueous solution prepared using primary sodium chloride was used. A surfactant, preferably 0.1 ml of alkylbenzene sulfonate, was added as a dispersant to 100 to 150 ml of the electrolytic aqueous solution, and 0.5 to 50 mg of a measurement sample was added. The electrolytic aqueous solution in which the measurement sample was suspended was subjected to dispersion treatment with an ultrasonic disperser for about 1 to 3 minutes, and then set in a Coulter counter TA-II type. In the Coulter Counter TA-II type, a particle size distribution of 2 to 40 μm was measured using a 100 μm aperture as an aperture to obtain a volume average distribution. From the volume average distribution thus obtained, a volume average particle size was obtained.

As the carrier, magnetic particles such as surface oxidized or unoxidized iron, nickel, cobalt, manganese, chromium, rare earth, and alloys thereof, or oxide ferrite can be used. There is no particular limitation. The carrier has a volume average particle size of 20 to 50 μm, preferably 30 to 40 μm, and a resistivity of 1 × 10 ⁷ Ωcm or more, preferably 1 × 10 ⁸ Ωcm or more. In this example, the volume average particle diameter of the carrier is φ40 μm and the resistivity is 5 × 10 ⁸ Ωcm.

  The volume average particle size of the carrier was measured using a laser diffraction particle size distribution measuring device HEROS (manufactured by JEOL Ltd.) in a 32 logarithmic division of a particle size range of 0.5 to 350 μm on a volume basis. Then, from the result of counting the number of particles in each channel, the median diameter of 50% volume was defined as the volume average particle diameter.

  For the carrier resistivity, a sandwich type cell having a measurement electrode area of 4 cm and an electrode interval of 0.4 cm was used. Under pressure of 1 kg, an applied voltage E (V / cm) was applied between the electrodes of the cell, and the resistivity of the carrier was measured from the current flowing in the circuit.

Furthermore, as a carrier having a low specific gravity, a resin carrier produced by mixing a phenolic binder resin with a magnetic metal oxide and a nonmagnetic metal oxide at a predetermined ratio and using a polymerization method can be used. Such a resin carrier has a volume average particle size of 35 μm, a true density of 3.6 to 3.7 (g / cm ³ ), and a magnetization of 53 (A · m ² / kg). The amount of magnetization (A · m ² / kg) of the magnetic carrier was measured using an oscillating magnetic field type magnetic property automatic recording apparatus manufactured by Riken Denshi Co., Ltd. The carrier packed in a cylindrical shape was determined by measuring the strength of magnetization of the carrier in an external magnetic field of 79.6 kA / m (1000 oersted).

  The two-component development method has advantages such as stability of image quality and durability of the apparatus as compared with other development methods. On the other hand, as the toner is consumed, the mixing ratio (toner concentration: TD ratio) of the non-magnetic toner and the carrier in the developing container changes, and as a result, the toner charge amount (tribo) changes, thereby developing characteristics. Changes and the output image density changes.

  For this reason, in order to keep the image density of the image formed constant, a toner replenishment control technique for accurately detecting the TD ratio of the developer and the image density and replenishing toner without excess or deficiency has been put into practical use. .

  In Patent Document 1, toner replenishment that adjusts the supply amount of the replenishment developer by determining the TD ratio by detecting an apparent permeability change of the developer that decreases as the TD ratio increases (inductance detection unit). Control is shown.

  In Patent Document 2, a patch image is formed at the time of non-image formation of image formation, and the image density is measured by an image density detection sensor. Then, the density of the initial patch image obtained in advance and the density of the formed patch image are compared, and when a decrease in density is detected, the replenishment developer is replenished.

  In Patent Document 3, the video count detection unit counts the video count number of the density signal of the image information signal for each pixel, and the replenishment developer corresponding to the toner consumption amount predicted based on the video count number is supplied. . However, in the replenishment control using the video count, when fluctuations occur in the replenishment of consumed toner or when the supply capacity of the supply device for the replenishment developer varies, the TD ratio is increased by the variation or the variation. There was an inconvenience that it shifted.

  In this regard, Patent Document 4 employs video count + patch detection ATR control to stabilize the toner charge amount more than Patent Documents 2 and 3. While the toner consumption is predicted and toner is replenished by the feed forward, a change in the replenishment amount and a development change due to a change in the developer are detected by the patch detection ATR, and feedback is applied. By performing the patch detection ATR control, the supply control of the replenishment developer is adjusted so as to keep the density of the patch image constant, so that the image density can be kept constant.

  However, when the toner charge amount is reduced due to the deterioration or deterioration of the developer, if the TD ratio is continuously lowered without any limit, several disadvantages occur as described later. If patch images are formed at every image interval, toner consumption for patch image formation cannot be ignored, and the running cost of the image forming apparatus increases. Problems such as an increase in the load on the belt cleaning device 29 and toner adhesion to the secondary transfer roller (23) also increase. There is also a great burden on the user such as productivity reduction due to downtime for patch image creation.

<Example 1>
FIG. 4 is a flowchart of control according to the first embodiment. FIG. 5 is an explanatory diagram of a conversion table for the difference in TD ratio and the necessary toner replenishment amount. FIG. 6 is an explanatory diagram of a conversion table of video count values and toner consumption.

  In order to solve the above problems in the configurations of Patent Documents 1 to 4, the image forming apparatus 100 shown in FIG. 1 employs triple control type replenishment control by video count + patch detection ATR control + toner density sensor. . Thereby, the execution interval of the patch detection ATR is extended to reduce the patch image formation frequency.

  That is, the TD ratio target value in the developing device 4a is changed according to the patch image density detected by the patch detection ATR control. Then, the replenishment amount of the replenishment developer is calculated so that the TD ratio in the developing device 4a measured using the toner density sensor 10 becomes the changed TD ratio target value. Then, the actual supply amount is calculated by adding the toner consumption amount predicted from the video count value to the calculated supply amount.

  An upper limit TD ratio that is an upper limit value of the TD ratio and a lower limit TD ratio that is a lower limit value of the TD ratio are prepared, and replenishment is stopped when the TD ratio of the developer circulating in the developing container 40 reaches the upper limit TD ratio. When the lower limit TD ratio is reached, forced replenishment is executed.

  As shown in FIG. 3, the developer cartridge 46 has a substantially cylindrical shape for all of yellow, magenta, cyan, and black, and is easily removable from the image forming apparatus 100 via the mounting portion 20. ing.

  A developer supply port 45 is provided on the upper wall 40A of the developing container 40 in the vicinity of the conveyance stirring screw 44b of the developing device 4a. The developer supply port 45 is provided with a developer supply screw 47 for conveying the supply developer. In the developing device 4a, an amount of replenishment developer corresponding to the amount of toner consumed by image formation is supplied from the developer cartridge 46 through the developer replenishment port 45 by the rotational force and gravity of the developer replenishment screw 47 to the developer container. 40 is supplied.

  As shown in FIG. 2, the toner supply control uses a method in which the following three control means are used in combination. Thus, the output image density can be stabilized by combining the first, second, and third control means.

  First control means: toner density control for setting the toner replenishment amount so as to keep the TD ratio detected using the toner density sensor 10 constant. As the toner density sensor 10, an inductance detection sensor that detects an apparent permeability change in the developer that decreases as the TD ratio increases and calculates the TD ratio is employed. The toner density sensor 10 decreases in output when the TD ratio is high and carriers are relatively decreased, and the output is increased when the TD ratio is low and carriers are relatively increased.

  In the first control means, the TD ratio of the developer in the developing container 40 is detected by the toner density sensor 10, the density signal is compared with an initial reference signal stored in advance, and the TD ratio is detected based on the comparison result. Perform replenishment control.

  Second control means: toner charge amount control for setting a toner replenishment amount so as to keep a patch image detected using the image density sensor 7a constant. The toner application amount detection means (7a) detects a patch image formed on the photoreceptor (1a) under predetermined image forming conditions and outputs a signal corresponding to the toner application amount. The image density sensor 7a is a specular reflection type optical sensor that is disposed to face the photosensitive drum 1a and detects regular reflection from the surface of the photosensitive drum 1a by irradiating LED light. As the toner particles on the surface of the photosensitive drum 1a increase, the scattered reflected light increases and the regular reflected light decreases, so that an output signal corresponding to the amount of toner on the patch image can be obtained.

  In the second control means, after a patch image of intermediate gradation is formed on the photosensitive drum 1a under a predetermined image forming condition, a density signal corresponding to the applied toner amount of the patch image is detected by the image density sensor 7a. Then, the density signal and the initial reference signal stored in advance are compared, and patch detection replenishment control is performed based on the comparison result. The patch detection replenishment control as the second control means is executed to periodically correct the initial reference signal used in the TD ratio detection replenishment control as the first control means. In the patch detection replenishment control, a patch image formed on the photosensitive drum 1a at a predetermined number of intervals during continuous image formation is detected by the image density sensor 7a to predict the development characteristics, and the target TD of the developer in the developing device 4a. Change the ratio.

  Third control means: Consumption replenishment control for setting the toner replenishment amount to match the toner consumption amount for each image detected using the video count number measurement circuit 11. In the third control means, the video count processing circuit 11 processes the exposure signal (or the density signal of the image information signal) of the image during image formation, and performs video count detection replenishment control for obtaining the toner consumption amount for each image.

  As shown in FIG. 4 with reference to FIG. 2, the control means (15) detects the patch image by the toner application amount detection means (7a) and sets a target ratio of toner occupying the developer. Is executed during predetermined non-image formation. Then, after the execution of the target ratio setting mode, the supply device (46) is controlled based on the output of the detection means (10) so that the ratio of toner in the developer converges to the target ratio.

  When image formation starts (S1), the control unit 15 detects the density signal VsigTD of the developer TD ratio using the toner density sensor 10 provided in the developing device 4a (S2).

  Next, the target TD ratio Vtar already obtained and recorded in the memory is compared with the density signal VsigTD to obtain a difference (ΔTD) (S3).

  If ΔTD = Vtar−VsigTD <0 (YES in S3), it is determined that the actual TD ratio is lower than the target TD ratio, and the conversion table of FIG. Ttd) is calculated (S4a).

  On the other hand, if ΔTD = Vtar−VsigTD ≧ 0 (NO in S2), it is determined that the actual TD ratio is higher than the target TD ratio, and the conversion table of FIG. ) Is calculated (S4b). In the conversion table of FIG. 5, the horizontal axis is a value obtained by multiplying the difference ΔTD of the actual signal value by the adjustment coefficient α such as the TD ratio sensitivity, the positive direction on the vertical axis is the required toner supply amount, and the negative direction is the excess toner amount. is there.

  At the same time, the video count processing circuit 11 calculates a video count value during image formation (S5). The video count value is a count number obtained by counting the H level of the output signal obtained by pulse width modulation of the output of the image signal processing circuit 12 by the pulse width modulation circuit 13 for each pixel. By integrating this count number over the entire original paper size, a video count number N corresponding to the number of development dots per original is calculated.

  Thereafter, referring to the conversion table of FIG. 6 with the calculated video count number N, a toner consumption amount, that is, a necessary toner supply amount (Tv) is calculated (S6). The conversion table of FIG. 6 is a video count-replenishment amount conversion table, in which the horizontal axis represents the video count number N per document and the vertical axis represents the required toner replenishment amount Tv.

Next, the actual toner supply amount (T) to be actually supplied is determined by the following equation (S7).
T = Ttd + Tv

  When T> 0 (YES in S8), since it is necessary to replenish the developer 4a with the replenishment developer, the actual toner replenishment amount (T) is converted into the replenishment time (Ts) (S9). Then, the developer replenishment screw 47 is rotated for the calculated replenishment time Ts to replenish the developer for replenishment to the developing device 4a (S10a).

  On the other hand, if T ≦ 0 (NO in S7), it is determined that supply of the developing device 4a is stopped, and the supply operation is not performed (S10b).

  By the way, in this embodiment, a patch image is formed at an image interval on the photosensitive drum 1a at a predetermined interval during continuous image formation, patch detection and replenishment control is executed, and TD ratio detection is performed according to the measurement result of the patch image. The target TD ratio Vtar for supply control is changed. Specifically, when it corresponds to the timing determined for every 200 sheets of A4 landscape image formation (YES in S11), patch inspection and replenishment control is executed (S12 to S19). However, if it is not the timing, image formation is continued (S19 → S1).

  In the patch detection replenishment control, an electrostatic latent image of a reference toner image (patch image) having a certain area is formed on the photosensitive drum 1a, and this is developed with a predetermined development contrast voltage. Then, the image density sensor 7a detects the patch image to acquire the density signal Vsig (S12), and compares the density signal Vsig with the initial reference signal Vref recorded in the memory in advance (S13).

  Here, when the difference ΔOD = Vsig−Vref ≧ 0 (NO in S13), since it is determined that the density of the patch image is low, that is, the developability is low, the target TD ratio is corrected to be increased (S14b). It is necessary to increase the image density.

The target TD ratio (Vtar) necessary for returning from the difference ΔOD to the initial development characteristics is calculated by the following equation. In the following equation, correction is performed by multiplying the actual signal value (Vsig−Vref) by the adjustment coefficient β such as the TD ratio sensitivity.
Vtar = Vtar + β * ΔOD

  When the calculated new target TD ratio is equal to or less than the upper limit TD ratio (YES in S16b), image formation is continued using the obtained new target TD ratio (Vtar) (S17b) (S19 → S1).

  However, if the calculated new target TD ratio exceeds the upper limit TD ratio (NO in S16b), image formation is continued using the upper limit TD ratio (VHlmt) (S18b) (S19 → S1).

  Here, the upper limit TD ratio (VHlmt) is calculated from the limit of an image defect (white background fogged image in this embodiment) that occurs at a high TD ratio, and specifically, the upper limit TD ratio (VHlmt) with respect to the initial TD ratio of 8%. ) Was set to 12%.

  On the other hand, if the difference ΔOD = Vsig−Vref <0 (YES in S13), since it is determined that the density of the patch image is high, that is, the developability is high, the target TD ratio is corrected to decrease (S14a). It is necessary to lower the image density.

The target TD ratio (Vtar) necessary for returning from the difference ΔOD to the initial development characteristics is calculated by the following equation.
Vtar = Vtar-β * ΔOD

  If the calculated new target TD ratio is equal to or greater than the lower limit TD ratio (YES in S16a), image formation is continued using the determined new target TD ratio (Vtar) (S17a) (S19 → S1).

  However, if the calculated new target TD ratio is below the lower limit TD ratio (NO in S16a), image formation is continued using the lower limit TD ratio (VLlmt) (S18a) (S19 → S1).

  That is, it is determined whether or not the corrected target TD ratio (Vtar) interrupts the “lower limit TD ratio acceptable by the system (VLlmt)” recorded in the memory in advance (S16a). Here, the lower limit TD ratio (VLlmt) is calculated from the limit of image defects (white images due to carrier adhesion in this embodiment) that occur at the time of a low TD ratio. Specifically, the lower limit TD ratio (VLlmt) is lower than the initial TD ratio of 8%. The TD ratio was set to 4-6% (S15).

  In the first embodiment, as shown in Table 1, the control unit 15 that also serves as the lower limit setting unit sets the lower limit TD ratio (VLlmt) according to the image ratio (%) (S15). The image ratio (%) used for setting the lower limit value is obtained from the video count value obtained by the video count processing circuit 11, and the video count value of the yellow overall maximum density image is set to 100 (%). This is a value obtained by calculating a ratio of video count values of images.

  The image forming apparatus 100 stirs the developer to triboelectrically charge the carrier and the toner, thereby applying charge to the toner, and electrostatically applying the charged toner to the electrostatic image on the photosensitive drum 1a. Adhere. For this reason, the toner charge amount of the toner affects the image characteristics. However, as described above, the toner charge amount of the toner is greatly influenced by the charge imparting ability of the carrier in the developer.

  In the first embodiment, it is possible to stabilize the output image density in a well-balanced manner by performing the supply control of the triple control method using video count + patch detection ATR control + toner density sensor. However, when the lower limit TD ratio (VLlmt) is set at a fixed value, the image ratio changes, particularly when the toner charge amount is low, such as scattering or a white background fog image, when images with a high image ratio are continuously formed. In some cases, image defects may occur.

  That is, by continuing to use the developer, the charge imparting ability of the carrier is lowered, and the toner charge amount Q / M (tribo) is lowered. When a large amount of replenishment developer is supplied after image formation with a high image ratio is performed, if the charge imparting ability of the carrier is reduced, the toner charge amount rises slowly, and the desired toner charge amount is reached. It takes a lot of time to get up. If the charge imparting ability of the carrier is reduced, a sudden drop in the toner charge amount temporarily occurs when a large amount of replenishment developer is replenished. Image defects due to a decrease in the amount Q / M are likely to occur.

  Here, in order to suppress the image defect due to the decrease in the toner charge amount Q / M, the toner charge amount Q is set by setting the lower limit TD ratio (VLlmt) low and increasing the friction charging opportunity of the developer. / M needs to be increased.

  However, if the lower limit TD ratio (VLlmt) is simply lowered, carriers adhere to the photosensitive drum 1a and image defects called white spots are likely to occur. The white spot image is a defective transfer image generated when the carrier adhering to the photosensitive drum 1a passes through the primary transfer portion Ta and prevents primary transfer of surrounding toner. When the carrier adhering to the photosensitive drum 1a in the development process passes through the primary transfer portion Ta, a conical gap centered on the carrier is formed and a transfer electric field is not applied to the surrounding toner to cause a transfer failure. It is an image.

  The white spot image becomes obvious in the maximum density image (solid black portion) because a round white background centering on the carrier-attached portion is formed and becomes apparent. The reason why the carrier adheres to the solid black portion is considered to be that the charge polarity of the carrier is reversed by injecting the charge into the leading end portion of the carrier carried on the developing sleeve 41 with the application of the oscillating voltage. It is considered that the carrier whose charging polarity is reversed to the negative polarity by charge injection is developed on the photosensitive drum 1a together with the toner.

  Therefore, when the occurrence state of the white spot image was examined in detail, it was confirmed that the resistance value of the developer becomes low, that is, it becomes prominent when the TD ratio is low or in a high temperature and high humidity environment. It has been confirmed that this phenomenon becomes conspicuous when the development contrast Vcont (= Vdc−VL) is high or when the AC amplitude of the oscillating voltage employed in the two-component development method is high.

  However, in the case of a carrier whose charge imparting ability has decreased due to cumulative image formation, a white spot image is unlikely to occur if an image formation with a high image ratio is performed immediately and a large amount of replenishment developer is replenished. It has been found.

  For this reason, when a large amount of replenishment developer is supplied based on the video count, a white spot image is unlikely to be generated. Therefore, the lower limit TD ratio (VLlmt) is lowered and normal toner charging is performed up to a TD ratio lower than the conventional one. Control of the quantity Q / M can be continued. For this reason, the toner charge amount Q / M is increased, and image defects due to a decrease in the toner charge amount Q / M such as scattering and a white background fog image are suppressed.

  Therefore, the first feature of the first embodiment is that the replenishment control is performed while changing the target TD ratio value (Vtar) in the developing device according to the image ratio at the time of image formation. When performing video count + patch detection ATR + TD ratio detection replenishment, the lower limit TD ratio (VLlmt) is set according to the image ratio so that a TD ratio lower than the conventional lower limit TD ratio (VLlmt) can be used. . As a result, even when the carrier is deteriorated, it is possible to suppress the image defect due to the decrease in the toner charge amount Q / M while suppressing the occurrence of the image defect due to the adhesion of the carrier.

<Setting of lower limit TD ratio>
FIG. 7 is an explanatory diagram of the configuration of the Faraday gauge. Here, using a Faraday gauge, the change in the toner charge amount due to the difference in image ratio due to carrier deterioration was examined.

  First, the image forming apparatus 100 is used on an A4 chart with an image ratio of 10%, and continuous image formation of 100,000 sheets is performed under the condition of a constant TD ratio, and development in which the charge imparting ability of the carrier is intentionally lowered is developed. An agent (durable developer) was prepared. Then, continuous image formation is performed with different image ratios in a plurality of stages between a durable developer whose carrier charging ability is lowered and a new developer (initial developer) whose carrier charging ability is not lowered. The change in the toner charge amount Q / M at that time was examined in detail.

  Specifically, the developer filling amount was 300 g, and continuous image formation was performed in a normal temperature and humidity environment with a TD ratio of 8% and 23 ° C. and 50%. Immediately after the start, the toner charge amount Q / M of the initial developer was −30 C / Kg, whereas the toner charge amount Q / M of the durable developer was −24 C / Kg. Then, the developing device 4a adjusts the toner charge amount Q / M difference (−30−24 = −6 C / Kg) between the initial developer and the durable developer by the TD ratio, so that the durable developer and the initial developer are separated from each other. The image density can be stabilized by controlling the same development characteristics.

Here, the toner charge amount Q / M was measured using a Faraday-Cage as shown in FIG. The Faraday gauge includes a double cylinder in which metal cylinders having different shaft diameters are arranged coaxially, and a toner collecting filter (filter) 93 for further taking toner into the inner cylinder of the double cylinder. Yes. If the inner cylinder 92 and the outer cylinder 91 of a double cylinder are insulated by an insulating member 94, and charged particles having a charge amount q are put into the inner cylinder 92, a metal cylinder having an amount of electricity q is as if due to electrostatic induction. It will be the same as it exists. The charge amount induced in the double cylinder is expressed as KEITHLEY 616.
Measured by DIGITAL ELECTROMETER, and the amount of charge measured divided by the weight of toner in the inner cylinder was defined as the toner charge amount Q / M.

  Next, using the initial developer and the durable developer, the toner charge amount Q / M before and after the endurance of 100 sheets continuously by changing the image ratio to 10/30/50/100% and controlling the TD ratio to be constant. And confirmed the image defect characteristics. The image defect characteristics were confirmed as a white background fogged image and a blank image due to carrier adhesion (hereinafter referred to as white spots). The measurement of the toner charge amount Q / M and the fogging image on the white background were confirmed after 100 sheets of continuous durability.

  Next, after that, the toner charge amount Q / M in which the durable developer is reduced is controlled only by the patch detection ATR control without setting the lower limit TD ratio, and when the stable state is confirmed, the white background fog image and the white spot are confirmed again. did. The experimental results are shown in Table 2.

  In the evaluation in Table 2, strict paper type, for example, coated paper, white background fog image is 2.5% or less, and the number of white spots is evaluated as “O” in the A3 plane. In addition, the case of 2.6 to 3.0% for the fogged image on the white background, and the case of 1 to 3 in the A3 plane for the white spots were indicated by Δ. Any more than that was evaluated as x.

  As shown in Table 2, it has been found that the toner charge amount Q / M decreases according to the image ratio. Further, it has been found that the toner charge amount Q / M in the durable developer is greatly reduced as compared with the initial developer. This is because, in an image having a high image ratio, a large amount of replenishment developer is replenished to the developing device 4a, so that the toner charge amount Q / M is lowered unless sufficient stirring time is provided. In the case of a durable developer having a reduced charge imparting capability, it is considered that the amount of decrease in the toner charge amount Q / M is increased because the charge characteristics are further deteriorated.

  Then, as a result of checking the image defect characteristics, as expected, as the image ratio increases, it is confirmed that the white background fog image increases based on the decrease in the toner charge amount Q / M of the durable developer, and the TD ratio is adjusted. It was confirmed that the fogging image on the white background improved. However, contrary to expectation, it has been newly found that the white spot image has a tendency to improve as the image ratio becomes higher among the image defect characteristics.

<Reason why the white spot image improves as the image ratio increases>
The reason why the white spot image is improved as the image ratio is increased is considered to be that the improvement in developability due to the decrease in the toner charge amount Q / M is superior to the adhesion of the charged carrier to the photosensitive drum 1a. In other words, the development process with toner is superior to the speed of carrier adhesion due to the improvement in toner developability. For this reason, it is considered that the electrification injection to the carrier is reduced by developing the toner as an insulator on the surface of the photosensitive drum 1a in advance and reducing the development contrast Vcont.

  Therefore, in the first embodiment, in view of the result of the above examination, the higher the image ratio, the lower the lower limit TD ratio (VLlmt) of the target TD ratio in the developing device 4a, and the range in which normal control is applied is expanded. ing.

  Specifically, the setting of the lower limit TD ratio is controlled by using the table shown in Table 1 above according to the image ratio in Example 1 as a result of confirming the white fogging image level and the white spot level. .

  As a result, when continuous durability is performed at an image ratio of 10% or more as the image formation is accumulated, the white background fogged image in which the density of the white background fogged image is 3.0% or higher of the solid image has a lower limit TD ratio of 6. It was confirmed that it was reduced to 3.0% or less by lowering from 0.0% to 4%. Of course, no white spot image has occurred.

  On the other hand, in image formation with an image ratio of 10%, as shown in Table 2, white spots occurred when the TD ratio was set lower than 6.0%. However, the lower limit TD ratio should be set to 6.0%. Thus, it is possible to achieve both a white background fog image and a white spot generation level.

When the image ratio changes from a high image ratio to a low image ratio during image formation, for example, when the image ratio is changed from 100% to 10%, the lower limit TD ratio is changed from 4.0% to 6.% based on the table in Table 1. Change to 0%. However, in that case, the lower limit TD ratio is increased stepwise in the order of 4.0 → 4.5 → 5.0 → 5.5 → 6.0 at intervals of every 100 sheets of image formation. As a result, sudden toner replenishment and the like can be avoided, so that problems such as color fluctuations do not occur. Furthermore, it is preferable that the change number interval is changed according to the image ratio change amount. This is because, as described above, when the image ratio is low, the white spot occurrence rate increases, and thus it is necessary to quickly return the TD ratio. Specifically, in this example, a very good result was obtained by changing the number of intervals according to the following equation.
Changed sheet interval = Image ratio change / 0.001

<Example 2>
As described in the first embodiment, the amount of change in the toner charge amount Q / M depending on the image ratio differs depending on the degree of deterioration of the initial developer and the durable developer, that is, the carrier. Therefore, in Example 2, as shown in Table 3, the change width of the lower limit TD ratio to be changed according to the image ratio is changed according to the degree of carrier deterioration.

  Specifically, as shown in Table 3, the ROM 18 is provided with a conversion table for changing the lower limit TD ratio for each increment of the number of image formations, which is an example of the cumulative amount of image formation.

  Using the conversion table in Table 3, the lower limit TD ratio corresponding to the cumulative amount of image formation and the image ratio is changed and controlled. Utilizing the fact that white spots are less likely to occur as the cumulative amount of image formation increases, the lower the lower limit TD ratio is set as the cumulative amount of image formation increases, and the toner charge reaches a lower TD ratio range. Continue control of quantity Q / M. As a result, it was possible to suppress the occurrence of a white background fog image and white spots through the accumulation of image formation.

  Table 3 is a conversion table used as a lower limit setting unit for controlling the developing device 4a of the image forming unit Pa. Actually, it is optimal for each image forming unit Pa, Pb, Pc, Pd, that is, for each development color. Thus, four conversion tables are prepared.

  Further, at times other than the control change timing (that is, for example between the initial period and 20K), the lower limit TD ratio is set by linearly interpolating the lower limit TD ratio table value. For example, at an image ratio of 30%: 10K, 5.9% is set by interpolation calculation. This is preferable because there is no sudden change in the TD ratio.

  Further, although the deterioration degree of the carrier is expressed by the cumulative number of image formations in the second embodiment, it may be controlled by the total rotation time of the developing sleeve 41.

  As described above, according to the control of the second embodiment, in the image forming apparatus 100 that performs triple supply control using video count + patch + TD ratio detection, the lower limit TD is set according to the image ratio and the cumulative number of image formations. Change the ratio. Thereby, even when a carrier deteriorates, generation | occurrence | production of defective images, such as a white background fog image and carrier adhesion, can be suppressed.

1a, 1b, 1c, 1d Photosensitive drum (photoconductor)
2a, 2b, 2c, 2d Charging rollers 3a, 3b, 3c, 3d Exposure devices 4a, 4b, 4c, 4d Developing device 7a Image density sensor 10 Toner density sensor 41 Developing sleeve 42 Magnets 44a, 44b Conveying stirring screw 46 Toner bottle ( Supply device)

Claims (5)

A photosensitive member, a developer carrying member that carries a developer containing toner and a carrier to develop an electrostatic image of the photosensitive member, a developer container that is charged with the developer and carried on the developer carrying member, and toner A supply device for supplying the developer for replenishment to the developer container; a detector for detecting the developer circulating in the developer container and outputting a signal corresponding to a ratio of toner in the developer; and the detector Control means for controlling replenishment of the replenishment developer by the supply device so that a predetermined electrostatic image is developed with a predetermined amount of applied toner within a range in which the ratio of the toner detected using the toner does not fall below a lower limit value In an image forming apparatus comprising:
An image forming apparatus, comprising: a lower limit value setting unit configured to set the lower limit value lower for image formation with a large amount of toner consumption. A toner application amount detection unit that detects a patch image formed on the photoconductor under predetermined image forming conditions and outputs a signal corresponding to the toner application amount;
The control means executes a target ratio setting mode for detecting the patch image by the toner applied amount detecting means and setting a target ratio of toner occupying the developer at the time of predetermined non-image formation. 2. The image forming apparatus according to claim 1, wherein after the execution, the supply device is controlled based on an output of the detection unit so that a ratio of toner in the developer converges to the target ratio. Video counting means for processing the exposure signal of the image to be formed to form data corresponding to the toner consumption;
3. The image forming apparatus according to claim 1, wherein the lower limit value is set based on an output of the video count unit.   The image forming apparatus according to claim 1, wherein the setting unit sets the predetermined lower limit value lower as the cumulative amount of image formation increases. A plurality of different developing containers for each developing color;
The image forming apparatus according to claim 1, wherein the predetermined lower limit is set for each of the developing containers.

Images