JP5039498B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus using a photosensitive drum and a developing device, and more particularly to an image forming device provided with a refresh process for refreshing toner on a developing roller during non-image formation.

  In the conventional image forming apparatus, when the image formation is repeated, particularly when the printing rate on the image (the ratio of the printed area to the area where the image can be formed (paper area); the same applies hereinafter) is low. Since there is little toner used for development by flying from the developer carrier (development roller) to the electrostatic latent image carrier (photoreceptor drum), the toner particles in the developing device are hardly replaced, and the toner is excessively charged. In some cases, image density reduction and fogging may occur. In particular, an image forming apparatus having a plurality of developing devices such as a color machine can handle images with a high printing rate such as photographs and graphic images to images with a low printing rate such as only characters and logo marks. It is necessary to increase the variation in the printing rate for each developing device.

  In such a case, a large amount of toner is ejected from the developing roller to the photosensitive drum side by printing a pattern having a high original printing ratio such as solid, and the toner is transferred to a recording medium to consume the toner. However, if the solid pattern is left unprinted for a long period of time, the toner particles are fixed to the surface of the developing roller due to the effect of charge-up without consumption of the toner, It may not recover.

  Therefore, the toner surface has been improved so that the charge control ability of the toner is stabilized by optimizing the surface shape, material, or external additive of the toner. At present, it has not yet been surely prevented.

In order to solve the above-mentioned problems, various methods for forcibly discharging the toner on the developing roller have been proposed. For example, Patent Document 1 discloses that an AC bias is applied to the developing roller when a non-image is formed. A one-component development type image forming apparatus that forcibly consumes toner on a developing roller by applying the superimposed toner is disclosed. Patent Document 2 proposes an image forming apparatus in which an image area ratio (average printing ratio) per unit driving time of the developing device is calculated and the forced consumption of toner is changed according to the image area ratio. Has been.
JP 2000-206770 A JP 2003-76079 A

  Patent Documents 1 and 2 describe a method for performing the toner discharge forcibly according to the number of printed sheets, or according to the printing rate. However, in the case of an extremely low printing rate, a sufficient effect cannot be obtained only by periodic forced discharge, and image fogging may occur.

  In addition, if printing of an image with biased image data continues in a part of the printing area, the printing rate of the entire printing area is equal to or higher than a predetermined value even if the printing rate is low or there is a part that is not printed. Forced discharge is not executed. For this reason, there is a possibility that the toner stays in a portion where there is no image data (printing is not performed) on the supply roller and the developing roller, and an image defect due to overcharging or toner sticking may occur. In addition, if there are extremely few printed parts and frequently printed parts, the surface of the photoconductor is polished, and the transfer body such as an intermediate transfer belt is also affected by the external toner additive. Since the surface condition would be different, countermeasures were required.

  In view of the above problems, an object of the present invention is to provide an image forming apparatus that allows a refresh process to function effectively even when toner consumption on a developing roller is biased in the image width direction.

  In order to achieve the above object, the present invention has a toner carrier that is disposed opposite to an image carrier, carries toner and supplies the image carrier, and a toner image corresponding to an electrostatic latent image on the surface of the image carrier. And a control unit that controls the amount of toner discharged from the toner carrier based on the printing rate of a printed image, and is configured to supply toner from the toner carrier side to the image carrier side during non-image formation. In the image forming apparatus capable of executing the refreshing process for discharging, the refreshing process includes the necessity of toner discharge based on the printing rate calculated for each of the plurality of areas divided in the image width direction immediately after each printing operation, and A first step of determining the discharge amount for each region, and a second step of determining the necessity and amount of toner discharge based on the average print rate over the entire image width calculated for each predetermined number of printed sheets. combination It is characterized by being configured.

  Further, according to the present invention, in the image forming apparatus having the above-described configuration, the control unit calculates an average printing rate over the entire image width obtained by adding the toner discharge amount in the first step for each predetermined number of printed sheets. The necessity of the second step and the toner discharge amount are determined based on the rate.

  According to the present invention, the image forming apparatus having the above-described configuration includes a plurality of the developing devices, and the calculation timing of the average printing rate in at least two or more of the developing devices is the same period.

  According to the present invention, in the image forming apparatus having the above-described configuration, the image bearing member includes an intermediate transfer member to which a toner image formed on the image bearing member is transferred by the developing device. The primary transfer bias applied between the intermediate transfer member and the intermediate transfer member is turned off.

  According to the first configuration of the present invention, the second step of continuing the refreshing for a predetermined time or more while suppressing the deterioration of the toner on the toner carrier due to the retention of the toner for each printed sheet by the first step. The toner on the toner carrier and further on the toner supply member that supplies toner to the toner carrier is discharged toward the image carrier and is actively consumed. In the first step, the necessity and amount of toner discharge are determined for each area based on the printing rate calculated for each of the plurality of areas in the image width direction. The toner consumption on the carrier can be made uniform in the width direction.

  According to the second configuration of the present invention, in the image forming apparatus having the first configuration, the average print rate over the entire image width obtained by adding the toner discharge amount in the first step is calculated for each predetermined number of printed sheets. Then, by determining the necessity of the second step and the toner discharge amount based on the average print rate, the toner discharge amount at the time of executing the second step becomes the minimum necessary amount corresponding to the print rate. .

  Further, according to the third configuration of the present invention, when the image forming apparatus having the first or second configuration includes a plurality of developing devices, the calculation timing of the average printing rate in at least two or more developing devices. By using the same period, it is possible to align the execution timing of the second step in each developing device to some extent and suppress a decrease in printing efficiency.

  According to the fourth configuration of the present invention, in the image forming apparatus having the third configuration, the primary transfer bias applied between the image carrier and the intermediate transfer member is turned off when the first step is executed. As a result, the toner can be attached substantially evenly in the width direction of the image carrier and the intermediate transfer member when viewed in total at the time of image formation and execution of the refresh process. The surface state in the width direction can be made uniform.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic configuration diagram of an image forming apparatus of the present invention. In the image forming apparatus (here, a color printer) 100, when performing a copying operation, the photosensitive drum 1 that rotates counterclockwise in the drawing is uniformly charged by the charging unit 2 in the apparatus main body. The exposure unit 3 irradiates the photosensitive drum 1 with a laser beam based on the original image data input to an image input unit (not shown) from a personal computer or the like, and the electrostatic latent image on the photosensitive drum 1. Is formed.

  The photosensitive drum 1 is formed, for example, by laminating a photosensitive layer on an aluminum drum, and the surface is charged by a charging unit 2. Then, an electrostatic latent image in which charging is attenuated is formed on the surface that has received the laser beam from the exposure unit 3. As the photosensitive material for forming the photosensitive layer, an amorphous silicon photoreceptor or an organic photoreceptor (OPC photoreceptor) is used. When positive OPC is used as the photosensitive layer, the generation of ozone or the like is small and charging is stable. In particular, the positive OPC having a single-layer structure has little change in photosensitive characteristics even when the film thickness changes after long-term use. Since the image quality is stable, it is preferably used in a system having a long life.

  The rotary type developing unit 4 that supplies toner onto the photosensitive drum 1 includes cartridge type yellow, magenta, cyan, and black developing devices 4a, 4b, 4c, and 4d in which a developing device and a toner container are integrated. By sequentially rotating the developing devices 4a to 4d to positions facing the photosensitive drum 1, toner is attached to the electrostatic latent image on the photosensitive drum 1 to form toner images of the respective colors. The Toner supply (toner installation) to the developing cartridges 4a to 4d is performed from the toner cartridge 5 through the supply pipe 5a.

  The intermediate transfer belt 6 onto which the toner image is transferred is wound around the intermediate transfer rollers 7a and 7b, the belt driving roller 9 and the driven roller 10, and while being in contact with the photosensitive drum 1, the driving means (not shown) rotates clockwise in the drawing. Rotate to. A sheet made of dielectric resin is used for the intermediate transfer belt 6, and a belt in which both ends thereof are overlapped and joined to form an endless shape, or a belt without a seam (seamless) is used.

  When the user inputs image formation start, a yellow toner image is formed on the photosensitive drum 1 at a predetermined timing. Then, the yellow toner image on the photosensitive drum 1 is transferred onto the intermediate transfer belt 6 by the intermediate transfer rollers 7a and 7b to which a negative transfer bias (when positively charged toner is used) is applied (primary transfer). . Thereafter, the toner remaining on the surface of the photosensitive drum 1 is removed by the cleaning roller 8a and the cleaning blade 8b, and the developing unit 4 rotates by a predetermined amount (here, 90 °). Are formed on the photosensitive drum 1 and transferred onto the intermediate transfer belt 6.

  Thereafter, cyan and black toner images are transferred onto the intermediate transfer belt 6 from the photosensitive drum 1 by the same method as described above. These four color images are formed with a predetermined positional relationship predetermined for forming a predetermined full-color image. A transfer roller 14 is pressed against the intermediate transfer belt 6 at a position facing the belt driving roller 9, and a belt cleaning blade 15 for removing residual toner on the surface of the intermediate transfer belt 6 is disposed downstream of the transfer roller 14. ing.

  The sheet P is conveyed from the sheet feeding mechanism 11 via the sheet feeding roller 12 and the registration roller pair 13 toward the intermediate transfer belt 6 on which the toner image is formed as described above, and sequentially onto the surface of the intermediate transfer belt 6. The formed full-color toner image is transferred onto the paper P at a time (secondary transfer) by the transfer roller 14 to which a negative transfer bias is applied. Then, the sheet on which the toner image is transferred is conveyed to the fixing device 16 and the toner image is fixed. The paper P that has passed through the fixing device 16 is discharged to a discharge tray 19 via a paper conveyance path 17 and a discharge roller pair 18.

  FIG. 2 is a side sectional view of the developing device mounted on the image forming apparatus of the present invention. In the following description, the configuration and operation of the developing device 4a opposite to the photosensitive drum 1 in FIG. 1 will be described. However, the configuration and operation of the developing devices 4b to 4d are basically the same, and the description is omitted. To do.

  As shown in FIG. 2, the developing device 4 a is provided with a toner stirring unit 21 in which toner is stored and a toner supply unit 22 to which toner is supplied from the toner stirring unit 21 in a resin-made developing container 20. The toner stirring unit 21 and the toner supply unit 22 are partitioned by a boundary wall 23. A first opening 28 and a second opening 29 are formed in the boundary wall 23, and the second opening 29 is positioned above the first opening 28 in the drawing.

  In the toner agitation unit 21, an agitation paddle 24 in which an agitation blade such as a PET film is attached to a rotation shaft is rotatably supported in a counterclockwise direction in FIG. The toner supply unit 22 faces a photosensitive drum (see FIG. 1) carrying a latent image and develops the latent image, and a supply roller 26 for supplying toner to the development roller 25. A metal regulating member 27 for regulating the toner layer thickness on the developing roller 25 and charging the toner is provided.

  The toner layer on the developing roller 25 is regulated in layer thickness by a regulating member 27 (for example, a SUS foil having a thickness of 0.08 mm and regulated pressure = 25 N / m) and is triboelectrically charged. 1 is used for development of an electrostatic latent image on 1. A seal member 30 (for example, a conductive high molecular weight PE film is used as a gap between the developing roller 25 and the developing container 20 on the side opposite to the regulating member 27 and is backed up with a urethane sponge so as to uniformly contact the developing roller 25. The seal member 30 prevents toner leakage.

  Further, since the developing roller 25 and the supply roller 26 rotate clockwise in FIG. 2, the second opening 29 is formed downstream of the first opening 28 in the rotation direction of the supply roller 26, and the second opening 29 is positioned above the upper end of the supply roller 26. The first opening 28 is positioned below the rotation axis of the stirring paddle 24.

  The developing process by the developing device will be described. The toner in the toner stirring section 21 is sent to the toner supply section 22 through the first opening 28 by the rotation of the stirring paddle 24. The toner sent to the toner supply unit 22 side is conveyed to the development roller 25 by the supply roller 26, is regulated to a thin layer by the regulating member 27 and is conveyed to the development nip portion, and the electrostatic latent image on the photosensitive drum 1 is converted. develop. The toner that is not used for development and remains on the developing roller 25 passes through the seal member 30, and then is peeled off by the supply roller 26 and returned to the toner supply unit 22.

  Of the toner conveyed by the supply roller 26, excess toner regulated by the regulating member 27 stays in the toner supply unit 22 together with new toner supplied (filled) from the first opening 28. When the amount of toner in the supply unit 22 increases, excess toner passes through the second opening 29 and returns to the toner stirring unit 21 from the toner supply unit 22 side, and the internal pressure of the toner supply unit 22 is reduced.

  Next, the control path of the image forming apparatus of the present invention will be described. FIG. 3 is a block diagram showing an example of a control path used in the image forming apparatus of the present invention. It should be noted that since various control of each part of the apparatus is performed when the image forming apparatus 100 is used, the control path of the entire image forming apparatus 100 becomes complicated. Therefore, here, a portion of the control path that is necessary for the implementation of the present invention will be mainly described.

  The control unit 90 includes a central processing unit (CPU) 91 as a central processing unit, a read only memory (ROM) 92 that is a read-only storage unit, a random access memory (RAM) 93 that is a read / write storage unit, A plurality of (two in this case) that transmit a control signal to each device in the temporary storage unit 94, the counter 95, and the image forming apparatus 100 for storing image data and the like, and receive an input signal from the operation unit 50. The I / F (interface) 96 is provided. Further, the control unit 90 can be arranged at an arbitrary location inside the apparatus main body.

  The ROM 92 stores a control program for the image forming apparatus 100, data necessary for control, and the like that are not changed during use of the image forming apparatus 100. The RAM 93 stores necessary data generated during the control of the image forming apparatus 100, data temporarily required for controlling the image forming apparatus 100, and the like. In addition, the ROM 92 (or RAM 93) calculates the print length of the toner discharge pattern in addition to the number of printed sheets for determining whether or not the refresh process is necessary and the reference print rate used when determining whether or not the refresh process is necessary. The outer peripheral lengths of the developing roller 25 and the supply roller 26 and the linear speed ratio data for the photosensitive drum 1 are also stored. The counter 95 adds up the number of printed sheets and counts it. Note that the number of times may be stored in the RAM 93, for example, without providing the counter 95 separately.

  In addition, the control unit 90 transmits a control signal from the CPU 91 to the respective units and apparatuses in the image forming apparatus 100 through the I / F 96. In addition, a signal indicating the state and an input signal are transmitted from each part or device to the CPU 91 through the I / F 96. The parts and devices controlled by the control unit 90 include, for example, the photosensitive drum 1, the exposure unit 3, the developing unit 4 (developing devices 4a to 4d), the paper feeding mechanism 11, the fixing device 16, the image input unit 40, and the operation. Part 50 and the like.

  When the image forming apparatus 100 is a printer as shown in FIG. 1, the image input unit 40 is a receiving unit that receives image data transmitted from a personal computer or the like. When the image forming apparatus 100 is a copying machine, Scanning optical system equipped with a scanner lamp that illuminates the document during copying and a mirror that changes the optical path of the reflected light from the document, a condensing lens that focuses the reflected light from the document and forms an image It is an image reading unit composed of a CCD or the like that converts image light into an electrical signal. The image signal input from the image input unit 40 is converted into a digital signal and then sent to the temporary storage unit 94.

  The operation unit 50 is provided with a liquid crystal display unit 51, LEDs 52 indicating various states, and a numeric keypad 53, and the user operates the operation unit 50 to input instructions, thereby making various settings of the image forming apparatus 100. And execute various functions such as image formation. The liquid crystal display unit 51 displays the state of the image forming apparatus 100, displays the image forming status and the number of copies, and can be used as a touch panel to perform various settings such as functions such as double-sided printing and black-and-white reversal, magnification setting, and density setting. It has become. The numeric keypad 53 is used for setting the number of copies to be printed and for inputting the other party's FAX number when the image forming apparatus 100 also has a FAX function.

  In addition, the operation unit 50 includes a start button for instructing the user to start image formation, a stop / clear button used when the image formation is stopped, and various settings of the image forming apparatus 100 in a default state. A reset button or the like is provided for use.

  When the image forming apparatus of the present invention is not transferred to a recording medium, for example, when the image forming apparatus is started up from a power-off state or a sleep (power saving) mode to a copy start state, or a predetermined number of prints are performed. Sometimes, a refreshing process for discharging the toner on the developing roller 25 in the developing devices 4a to 4d to the photosensitive drum 1 side can be executed.

  In the refreshing process, the printing rate is calculated every predetermined time, and the duration of the refreshing process, that is, the printing length of the toner discharge pattern is determined by comparing with the reference printing rate. Was regularly executed according to. However, when images with an extremely low printing rate are continuous, the toner consumption on the developing roller 25 becomes insufficient only by executing the refresh process periodically and forcibly consuming the toner, and the deterioration of the toner proceeds. Problems such as density reduction and image fogging are likely to occur. Further, when the printing of the image with the biased image data continues in a part of the printing area, the toner consumption on the developing roller 25 may be partially insufficient. Therefore, it is necessary to devise a method for increasing the printing rate (toner consumption) for each printed sheet for each region in the image width direction.

  The present invention combines a step of discharging toner immediately after each printing operation (hereinafter referred to as a first step) and a step of discharging toner for each predetermined number of printed sheets (hereinafter referred to as a second step). By executing the refreshing process, a higher refreshing effect than that of the prior art is brought out. According to this configuration, the second step of continuing the refreshing for a predetermined time or more on the developing roller 25 and further on the supply roller 26 while suppressing the deterioration of the toner due to the retention of toner for each printing sheet by the first step. The toner is discharged to the photosensitive drum 1 side and actively consumed.

  In the first step, the development area width is divided into a plurality of areas in the image width direction, and the necessity and amount of toner discharge are determined for each area based on the printing rate calculated for each area. That is, since the toner discharge amount can be increased in the region where the printing rate is low, the toner consumption amount on the developing roller 25 can be made uniform in the width direction even when the printing rate of the formed image has a deviation in the width direction. . When printing images of various patterns at random, there is little bias in the printing rate, but especially when printing with margins at both ends in the width direction or printing of the same pattern with images only at a part in the width direction continues. Is effective.

  4 and 5 are diagrams illustrating an example of the toner discharge pattern formed in the first step and the second step. 4 and 5, a method for calculating the toner discharge amount (print length of the toner discharge pattern) in the first step and the second step from the print rate, and a method for determining the necessity of each step. This will be described in detail with reference to FIGS. 4 and 5 show only one color among the four color images, the other three colors will be described in the same manner.

  First, the first step will be described. The example of FIG. 4 shows a print pattern of a left-justified horizontal document, and the development area width H (see FIG. 5, 216 mm for an A4 size apparatus) is divided into a plurality of areas (here, R1 to R8 are 8 etc.) R1 in the first division (left end) and R8 in the eighth division (right end) are margins, so the printing rate is 0%, and R2 to R5 in the second to fifth divisions are printing rates of 5.75%, The print ratios R6 and R7 in the seventh division are 0.7% and 0.3%. The total printing rate of this image is 3%. Assuming that the threshold of the printing rate when determining whether the first step is necessary (hereinafter referred to as the first reference printing rate) is C1 (%) (here 1.5%), the total printing rate and the reference printing rate Since the comparison with C1 exceeds the reference printing rate C1, the first step is not executed even though the portions R1 and R6 to R8 having extremely low printing rates exist.

  Therefore, in the present invention, the control unit 90 calculates the printing ratios b1n to b8n (%) for the regions R1 to R8 based on the digital signal in the temporary storage unit 94, and prints the regions R1 to R8 for each printed sheet. It is determined whether or not the ratios b1n to b8n are smaller than C1. If there is an area where b1n to b8n <C1, only the difference between the reference printing ratio C1 and the printing ratios b1n to b8n for each printing area. A first step of discharging toner is executed. The printing length Lmn of the toner discharge pattern in the region Rm is calculated by Lmn = L0 × (C1−bmn) / 100, where L0 is the circumferential length of the image size that is the reference (100%) for calculating the printing rate. The

  For example, when the image shown in FIG. 4 is printed on the first sheet, assuming that the reference print rate C1 is 1.5%, it is only necessary to discharge the toner corresponding to 1.5% difference from C1 in R1 and R8. Therefore, when an A4 size image (circumferential length 297 mm) is used as a reference for calculating the printing rate, L11 = L81 = 297 (= L0) × (1.5-0) /100≈4.46 mm, and R1 and R8 The print lengths L11 and L81 of the toner discharge pattern formed in step 4 are 4.46 mm, and the respective print widths are 216 × 1/8 = 27 mm.

  Similarly, since the printing rate of R6 is 0.7%, L61 = 297 × (1.5−0.7) /100≈2.38 mm, and the printing length L61 of the toner discharge pattern formed by R6. Is 2.38 mm, and the printing width is 27 mm. Since the printing rate of R7 is 0.3%, L71 = 297 × (1.5−0.3) /100≈3.56 mm, and the printing length L71 of the toner discharge pattern formed by R7 is The print width is 3.5 mm and the print width is 27 mm. On the other hand, since the printing rate of R2 to R5 is 5.75% (≧ C1), L21 to L51 = 0, and the first process is not executed in R2 to R5. In this way, the toner discharge amounts (print lengths L11 to L81) in the respective regions R1 to R8 are determined, and the toner as shown in FIG. 4 is formed on the photosensitive drum 1 after the first image is formed. A discharge pattern T1 is formed.

  Hereinafter, the printing rates b1n to b8n of R1 to R8 are compared with C1 until the counter 95 counts a predetermined number A (10 in this case) for determining whether the second step is necessary, and b1n to b8n <C1. In this case, the printing lengths L1n to L8n are calculated for each of R1 to R8 based on the difference between C1 and b1n to b8n to form the toner discharge pattern T1. As a result, the actual printing rate (image + toner ejection pattern T1) for each of R1 to R8 is raised to 1.5% or more, and a portion where the printing rate is extremely low (<1.5%) in the printing width direction. Even in such a case, since a constant amount of toner is always consumed from the developing roller 25, the deterioration of the toner can be effectively suppressed until a second step described later is executed.

  Here, the development area width H is divided into eight equal areas R1 to R8. However, the development area width H may be divided into smaller areas such as two equal parts, four equal parts, or more than eight equal parts. It may be divided into regions. However, although the toner discharge amount can be strictly controlled as the number of divided areas increases, the load on the control unit 90 increases. Therefore, it is preferable to determine the number of divided areas in consideration of these.

  The toner discharge pattern T1 discharged from the developing roller 25 onto the photosensitive drum 1 may be collected by the cleaning roller 8a and the cleaning blade 8b (see FIG. 1), or the intermediate transfer belt 6 by the intermediate transfer rollers 7a and 7b. After the primary transfer, the belt cleaning blade 15 (see FIG. 1) may be used for collection. Here, in order to make the surface states of the photoconductive drum 1 and the intermediate transfer belt 6 uniform in the width direction, the photoconductive drum 1 and the intermediate transfer belt 6 are seen when viewed in total during the image formation and the refresh process. It is necessary to deposit the toner substantially uniformly in the width direction.

  Therefore, the application of bias to the intermediate transfer rollers 7a and 7b is turned off, that is, the primary transfer bias applied between the photosensitive drum 1 and the intermediate transfer belt 6 is turned off, and the discharged toner is subjected to physical force. Thus, it is preferable that the toner is attached to the photosensitive drum 1 side and the intermediate transfer belt 6 side by approximately ½ each. As a result, the polishing state of the photosensitive drum 1 and the surface state of the intermediate transfer belt 6 due to the influence of the toner external additive become uniform in the width direction, and variations in the surface state of the photosensitive drum 1 and the intermediate transfer belt 6 can be suppressed. Become.

  Next, the second step will be described. When the number of printed sheets reaches A, the control unit 90 calculates an integrated printing rate Σbn obtained by integrating the total printing rate bn (3% in FIG. 4) for each printing for A sheets. Then, the average printing rate B (%) per printing sheet A is calculated by dividing the integrated printing rate Σbn by the printing sheet A. Assuming that the threshold of the printing rate when determining whether the second step is necessary (hereinafter referred to as the second reference printing rate) is C2 (%), it is determined whether or not B <C2, and B <C2 In the case of (2), the second step is executed. The second reference print rate C2 is set to a value larger than the first reference print rate C1. Then, a toner discharge pattern T2 is formed on the photosensitive drum 1 after the A-th image (here, the tenth image) is formed.

  An actual toner discharge pattern T2 is shown in FIG. The toner discharge pattern T2 is a toner discharge length (printing length in the drum circumferential direction) L obtained by multiplying the developing area width H of the developing roller 25 (see FIG. 2) and the toner discharge amount per image by the number of printed sheets. Is a rectangular shape with two sides.

  In the second step of collectively discharging a predetermined number of under-consumed toners, toner discharge to the photosensitive drum 1 side continues for a certain period of time, so the toner on the developing roller 25 and further on the supply roller 26 is transferred to the photosensitive member. It is discharged to the drum 1 side and actively consumed. Therefore, by performing the refresh process that combines the first process and the second process, the refresh effect can be further enhanced, and the occurrence of image fog, density reduction, and the like can be more effectively prevented.

  In the second step, it is conceivable that the toner is discharged by dividing the region in the width direction as in the first step, but the effect is lower than that in the case where the toner is discharged over the entire width direction. . This is because the developer moves to some extent in the longitudinal direction (image width direction) of the developing roller 25 in the developing devices 4a to 4d, and a sufficient effect is obtained by toner discharge for each printing sheet in the first step. It is thought that the cause is obtained. In addition, when the second step is executed by dividing the region in the width direction without performing the first step, almost no effect is seen.

  The toner discharge amount calculation method in the second step is because the difference (C2-B) between the second reference print rate C2 and the average print rate B is the required discharge amount per image (shortage of consumption). Then, (C2-B) × A (%) obtained by multiplying this by the number of printed sheets A is the toner discharge amount in the second step. For example, assuming that the number of printed sheets A is 10, the average printing rate B is 3%, and the second reference printing rate C2 is 5%, the toner of (5-3)% × 10 sheets may be ejected.

  The printing length L of the toner discharge pattern T2 is calculated by L = L0 × (C2−B) × A / 100, where L0 is the circumferential length of the image size that is the reference (100%) for calculating the printing rate. The Therefore, when an A4 size image (circumferential length 297 mm) is used as a reference for calculating the printing rate, L = 297 (= L0) × (5-3) × 10/100 = 59.4 mm, and the toner discharge pattern T2 The printing length L is 59.4 mm. Assuming that the rotational speed of the photosensitive drum 1 is 150 mm / second, it is sufficient to discharge the toner for 59.4 / 150≈0.4 seconds.

  In addition, for example, it is conceivable that the printing ratio of R1 to R7 out of R1 to R8 is 10% (≧ C1) and the printing ratio of R8 is 0.5% (<C1). In this case, only R8 is the first. The process is executed. When 10 images are continuously printed, the average printing rate B exceeds the reference printing rate C2, and thus the second step is not executed.

  Further, when calculating the average printing rate B, it is preferable to calculate by adding the toner discharge amount in the first step to the printing rate bn for each printing. For example, in the image shown in FIG. 4, in the first step (toner discharge amount at R1 and R8) + (toner discharge amount at R6) + (toner discharge amount at R7) = {(1.5− 0) × 2/8} + {(1.5−0.7) × 1/8} + {(1.5−0.3) × 1/8} = 0.625 (%) of toner. Is consumed.

  That is, the toner of 3.625% is consumed for each printing together with the image printing rate of 3%, and when the image shown in FIG. The difference from the reference print rate C2 is discharged as 3.625%. Therefore, the printing length of the toner discharge pattern T2 is L = 297 × (5−3.625) × 10 / 100≈40.8 mm.

  As described above, by calculating the average printing rate B including the toner discharge amount in the first step, an appropriate toner discharge amount corresponding to the shortage of toner consumption is calculated, so that a high refresh effect is maintained. Useless toner consumption can be suppressed. The first and second reference print rates C1 and C2 are appropriately set according to the characteristics of the toner of each color. Although the number of printed sheets A for calculating the average printing rate B is 10 here, it can be arbitrarily set according to the usage status of the user.

  FIG. 6 is a flowchart showing an example of the refresh process executed in the image forming apparatus of the present invention. The execution procedure of the refresh process will be described according to the steps in FIG. 6 with reference to FIGS. Here, the procedure for executing the refresh process for one of the developing devices 4a to 4d will be described, but the same procedure is performed for the other three developing devices.

  First, when the image forming process is started by the user operating the operation panel 50 or a personal computer, the control unit 90 counts the number of printed sheets n by the counter 95 (step S1), and outputs the digital signal in the temporary storage unit 94. Based on this, the printing rate bmn for each region Rm (R1 to R8 in FIG. 4) is calculated for each printed sheet (step S2). Next, the printing rate bmn and the first reference printing rate C1 are compared (step S3), and the toner discharge amount for each region is obtained using the difference between C1 and bmn for the region where bmn <C1. The printing length Lmn for each is calculated (step S4). Then, the first process is executed to discharge the toner discharge pattern T1 onto the photosensitive drum 1 (step S5).

  At this time, the bias application to the intermediate transfer rollers 7a and 7b is turned off, and the toner is adhered to the photosensitive drum 1 and the intermediate transfer belt 6 by about ½, and the toner on the photosensitive drum 1 is cleaned by the cleaning rollers 8a, The toner on the intermediate transfer belt 6 is collected by the belt cleaning blade 15 by the cleaning blade 8b. On the other hand, for the region where bmn ≧ C1 in step S3, the process proceeds to the next step without executing the first step.

  Thereafter, it is determined whether or not the number n of printed sheets has reached the predetermined number A (step S6). If n <A, steps S1 to S5 are repeated until n = A. When n = A, the control unit 90 calculates an integrated print rate obtained by adding the toner discharge amount in the first step to the total print rate bn for each printed sheet and integrating the A sheets, thereby calculating The average print rate B is calculated by dividing the integrated print rate by the predetermined number A (step S7).

  Next, the average print rate B and the second reference print rate C2 are compared (step S8). If B <C2, the print rate difference C2- between the second reference print rate C2 and the average print rate B is calculated. Multiply B by the number of printed sheets A to calculate the toner discharge amount (C2−B) × A (%). The calculated toner discharge amount and the circumferential length of the image size serving as a reference (100%) for calculating the printing rate Based on the length L0, the printing length L of the toner discharge pattern T2 is calculated (step S9). Then, the second step is executed, and a toner discharge pattern T2 having a printing length L is discharged onto the photosensitive drum 1 (step S10). On the other hand, if B ≧ C2 in step S8, the process proceeds to the next step without executing the second step. Thereafter, the count number n of the counter 95 is reset to 0 (step S11), and the process returns to step S1 again.

  Since the image forming apparatus 100 of the present embodiment is a color printer having four developing devices 4a to 4d, it is necessary to execute the refreshing process for each of the developing devices 4a to 4d. Normally, monochrome and color printing frequencies are different. Therefore, when the second step of ejecting toner to each developing device 4a to 4d for each predetermined number of sheets is performed separately, development of cyan, magenta and yellow used for color printing is performed. There is a possibility that the timing for determining whether the second step is necessary is different between the devices 4a to 4c and the black developing device 4d used for monochrome printing, and the second step is frequently executed, resulting in a decrease in printing efficiency.

  Therefore, it is preferable that the timings for determining the necessity of the second step (calculation timing for the average printing rate B) be made as uniform as possible for each of the developing devices 4a to 4d. For example, in a usage situation in which monochrome printing frequency is high, when the number of monochrome prints becomes A, the average print rate B for monochrome A sheets is calculated to determine whether or not the second step is necessary for the developing device 4d. At the same time, when the number of color prints becomes A and the average print rate B for color A is calculated to determine whether or not the second step is necessary for the developing devices 4a to 4c (monochrome print number A + a). The average printing rate B for monochrome a sheet is calculated, and the necessity of the second step is determined for the developing device 4d. By controlling in this way, it is possible to synchronize the determination and execution timing of the second step in the developing device 4d with any one or more of the developing devices 4a to 4c, and suppress a decrease in printing efficiency. .

  Alternatively, when the number of printed sheets of either color or monochrome reaches A and the other number of printed sheets n is in the range from Aa to A (Aa ≦ n ≦ A), at that point It is also possible to calculate the average printing rate B up to and determine simultaneously whether or not the refreshing steps of the developing devices 4a to 4c and the developing device 4d are necessary.

  In the above embodiment, toner discharge is calculated by calculation using the number of printed sheets A, the printing ratios bmn and bn for each printed sheet, the average printing ratio B, the first reference printing ratio C1, and the second reference printing ratio C2. Although the amounts (printing lengths Lmn and L of the toner discharge patterns T1 and T2) are obtained, the printing lengths Lmn and L may be obtained by other methods. For example, the print length Lmn using a toner discharge amount setting table that stores a plurality of toner discharge modes for defining the toner discharge amount discharged from the developing roller 25 for each print rate level when the first and second steps are executed. , L can also be determined.

  An example of the toner discharge amount setting table is shown in FIG. In FIG. 7, the printing rate bmn of the region Rmn is assigned to each row of the toner discharge amount setting table from 0% to 5% or more at 1% intervals, and 6 stages of toner discharge modes from 0 to 5 are assigned to each column. It has been. In each toner discharge mode, the toner discharge amount in each region corresponding to the print rate level is defined by the print length (mm). In mode 0, toner discharge is not performed regardless of the average print rate, and the toner discharge amount at each print rate level is set to increase stepwise up to mode 5. In each mode, the toner discharge amount is set to decrease as the average printing rate increases. When the average printing rate is 5% or more, toner discharge is not performed in any mode. The toner discharge mode may be set manually or may be automatically set according to the temperature and humidity inside the apparatus.

  When the printing rate bmn of the region Rmn is calculated by the control unit 90, the calculated printing rate bmn and the toner discharge amount at the position where the row and the column corresponding to the setting mode intersect (printing length Lmn of the region Rmn) are determined. Is done. For example, when the calculated printing rate bmn is 1.5% and the setting mode is MODE4, the printing length 4 per printed sheet at the position where the row of 1 ≦ bmn <2 and the mode 4 column intersect. 0.0 mm is selected. Here, the method for determining the toner discharge amount in the region Rmn in the first step using the toner discharge amount setting table has been described, but the average print rate B and the toner discharge mode are also determined for the toner discharge amount in the second step. Can be determined in the same manner using a toner discharge amount setting table stored in association with each other.

  In addition, this invention is not limited to the said embodiment, A various change is possible in the range which does not deviate from the meaning of this invention. For example, in the above embodiment, the printing rate and the average printing rate are calculated by the control unit 90, but an arithmetic unit that calculates the printing rate and the average printing rate may be provided separately from the control unit 90. In the present invention, only a rotary development type color printer has been described as an example. Of course, the present invention can be applied to various image forming apparatuses equipped with a developing device including a developing roller and a supply roller.

  When printing was performed using the image forming apparatus of the present invention shown in FIG. 1, the relationship between the toner ejection pattern and the image defect during the refresh process was investigated. The test conditions were 14 mm and 13 mm in diameter of the developing roller 25 and the supply roller 26, 1.1 times the linear speed ratio S1 of the developing roller 25 to the photosensitive drum 1, and the linear speed ratio S2 of the supply roller 26 to the developing roller 25. Is 1.2 times, the rotational speed of the photosensitive drum 1 is 150 mm / second, and the first step of discharging toner for each area after each printing operation, and the toner discharge of the entire image width every predetermined number (10 sheets) 4 is continuously displayed when the refresh process combined with the second process is performed (the present invention) and when only the second process is performed (comparative example). The density reduction of the solid image when printed and the occurrence of image fogging were investigated.

  In the evaluation method, the density of the printed image is measured by using a reflection densitometer (RD918) manufactured by Macbeth Co., Ltd., and the density reduction is evaluated by the reduction of the ID in the solid image. Image fogging was evaluated by the increase in ID at the part. The results are shown in FIGS.

  As is clear from FIG. 8, in the present invention (indicated by □ in the figure), the ID value was 1.33 even after printing 10,000 sheets, and was hardly lowered from the initial value (1.4). . On the other hand, in the comparative example (indicated by black circles in the figure), the ID value decreased to 1.1 after printing 10,000 sheets.

  Further, as is clear from FIG. 9, in the present invention (indicated by □ in the figure), the ID value of the white portion is 0.07 even after printing 10,000 sheets, and the image fog is not noticeable with the naked eye. It was a grade that did not become. On the other hand, in the comparative example (indicated by black circles in the figure), the ID value increased to 0.12 after 10,000 sheets were printed, and noticeable image fogging was observed.

  From this result, in the present invention in which the first step and the second step are combined, a higher refreshing effect is obtained compared to the comparative example in which only the second step is executed, and the density reduction and image fog are effectively reduced. It was confirmed that it can be suppressed.

  The present invention comprises a developing device that has a toner carrier that is disposed opposite to an image carrier and carries toner and supplies the toner to the image carrier, and forms a toner image corresponding to the electrostatic latent image on the surface of the image carrier. And a control means for controlling the amount of toner discharged from the toner carrier based on the printing rate of the printed image, and capable of executing a refresh process for discharging toner from the toner carrier to the image carrier during non-image formation. In the apparatus, the refresh step determines the necessity and amount of toner discharge for each region based on the printing rate calculated for each of the plurality of regions divided in the image width direction immediately after each printing operation. The process is combined with a second process for determining the necessity and amount of toner discharge based on the average print ratio of the entire image width calculated for each predetermined number of printed sheets.

  As a result, even when an image with an extremely low printing rate or an image with a biased printing rate in the printing area is continuously printed, the refreshing effect can be further enhanced, and the occurrence of image fogging, density reduction, etc. is effective. It is possible to provide an image forming apparatus that can prevent the problem. Further, since the necessity of the second step and the toner discharge amount are determined based on the average printing rate calculated by adding the toner amount discharged in the first step, wasteful toner consumption and the refresh step are performed. An image forming apparatus that suppresses a decrease in image forming efficiency due to execution as much as possible.

  Further, when a plurality of developing devices are provided like a color image forming apparatus, if the average printing rate in at least two developing devices is calculated at the same time, the execution timing of the second step in each developing device can be determined. A reduction in printing efficiency can be suppressed by aligning to some extent, and a reduction in image formation efficiency due to printing interruption during continuous printing is prevented as much as possible, and a psychological burden on the user due to waiting time is reduced. Furthermore, if the primary transfer bias is turned off during the first step so that the toner adheres substantially evenly in the width direction of the image carrier and the intermediate transfer member, variations in the polishing state of the image carrier and toner external addition can be achieved. Variations in the surface state of the intermediate transfer member due to the influence of the agent can be suppressed.

FIG. 1 is a schematic cross-sectional view showing an overall configuration of an image forming apparatus of the present invention. These are sectional views of a developing device used in the image forming apparatus of the present invention. FIG. 3 is a block diagram showing a control path of the image forming apparatus of the present invention. FIG. 5 is a diagram illustrating an example of a print image having a predetermined print rate distribution and a toner discharge pattern formed in the first step on the print image. FIG. 4 is a diagram illustrating an example of a toner discharge pattern formed in a second step. These are flowcharts showing the execution procedure of the refresh process in the image forming apparatus of the present invention. FIG. 4 is a diagram showing an example of a toner discharge amount setting table used in the present invention. These are graphs showing the relationship between the number of printed sheets and the image density of a solid image when the first and second steps are combined (the present invention) and when only the second step is executed (comparative example). These are graphs showing the relationship between the number of printed sheets and image fogging when the first and second steps are combined (the present invention) and when only the second step is executed (comparative example).

Explanation of symbols

1 Photosensitive drum (image carrier)
4 Developing unit 4a to 4d Developing device 6 Intermediate transfer belt (intermediate transfer member)
7a, 7b Intermediate transfer roller 25 Developing roller (toner carrier)
26 Supply roller 90 Control section (control means)
91 CPU
92 ROM
93 RAM
94 Temporary Storage Unit 95 Counter 100 Image Forming Apparatus T1 Toner Discharge Pattern (First Step)
T2 Toner ejection pattern (second process)
Lmn, L Print length in the circumferential direction (of the toner ejection pattern)

Claims (3)

A developing device that has a toner carrier that is disposed opposite to the image carrier and carries toner and supplies the toner to the image carrier, and forms a toner image corresponding to the electrostatic latent image on the surface of the image carrier;
Control means for controlling the amount of toner discharged from the toner carrier based on the printing rate of the printed image,
In an image forming apparatus capable of executing a refresh process for discharging toner from the toner carrier side to the image carrier side during non-image formation,
The refreshing step includes a first step of determining the necessity and amount of toner discharge for each region based on a printing rate calculated for each of a plurality of regions divided in the image width direction immediately after each printing operation. The second step of determining the necessity and amount of toner discharge based on the average printing rate of the entire image width calculated for each predetermined number of printed sheets ,
The control means calculates an average print rate over the entire image width obtained by adding the toner discharge amount in the first step for each predetermined number of printed sheets, and determines whether the second step is necessary based on the average print rate. An image forming apparatus that determines a toner discharge amount . 2. The image forming apparatus according to claim 1, wherein a plurality of the developing devices are provided, and an average printing rate calculation timing in at least two of the developing devices is set at the same time . A primary transfer member that includes an intermediate transfer member to which a toner image formed on the image carrier is transferred by the developing device, and is applied between the image carrier and the intermediate transfer member during the execution of the first step; The image forming apparatus according to claim 2 , wherein the transfer bias is turned off .