JP2008116729A - Image forming apparatus and image forming method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus and an image forming method for reducing waste of developer and time.
Whether to execute or not to perform image density control is determined according to a relationship between a developing bias before a process speed change and a developing bias variable range.
[Selection] Figure 13

Description

  The present invention relates to an image forming apparatus and an image forming method capable of changing a process speed and forming an image of a latent image formed on an image carrier on an intermediate transfer member.

  Conventionally, in a color image forming apparatus, if the density of each color or the tone characteristics of a halftone fluctuate due to a change in usage environment or a change due to long-term use, the color tone of the output image changes. An image density control means is often provided. In the image density control, the image density control sequence is executed after power-on, after the sleep (residual heat) state is released, after outputting a certain number of sheets, and the like, and is designed to always obtain a stable output image (Patent Document 1, 2).

  However, in general, color image forming apparatuses have functions such as a low-speed fixing mode and a high-resolution mode for good fixing on thick paper, and image density control is performed in a normal process speed mode (normal mode). Even if the optimum image forming conditions according to the device and the surrounding environment are determined in advance by the sequence, the optimum image forming conditions are not achieved in the low-speed mode or the like, so there is a problem that the image quality changes. It was.

  In addition, in order to stabilize the formed image even in the low speed mode, it is possible to add a density control sequence similar to the normal mode for the low speed mode. This takes time and leads to a significant increase in the user's printing waiting time.

Therefore, in order to obtain an image of appropriate image quality without taking time even in the low-speed mode, a measurement image is formed at the first process speed when fixing at the normal speed, the density is measured, and the measurement result is The image forming condition is determined and stored based on the image forming condition, and the image forming condition at the second process speed when fixing at low speed is calculated and determined by calculating the stored image forming condition at the first process speed. There is a device (Patent Document 3).
JP 2003-270874 A JP 2003-295532 A JP 2000-231228 A

  However, in the conventional technique, image density control is executed without considering any relationship between the developing bias before the process speed change and the developing bias variable range.

  SUMMARY An advantage of some aspects of the invention is that it provides an image forming apparatus and an image forming method that solve the above-described problems and reduce waste of developer and time.

  The present invention solves the above-mentioned problem, and determines whether image density control is performed or not according to the relationship between the development bias before the process speed change and the development bias variable range. Control is performed only when reliability can be ensured, and wasteful developer and time are reduced.

  In addition, if the development bias before the process speed change is the lower limit of the development bias variable range, printing is executed without changing the development bias setting to the value corresponding to the normal speed before the speed change. As a result, the character quality is prevented from being impaired.

  If the development bias before the process speed change is the lower limit of the development bias variable range and the exposure energy is the lower limit of the exposure energy variable range, the development bias setting is changed to a value corresponding to the normal speed before the speed change. Since the printing is executed in a state where the character is not used, it is possible to prevent the character quality from being impaired due to an erroneous adjustment result.

  If the development bias before the speed change is not the lower limit of the development bias variable range, printing is performed with the development bias setting changed to the initial value corresponding to the speed change. Damage is suppressed.

  Also, if it is determined that the development bias before the speed change is not the lower limit of the development bias variable range, and there is a development bias setting that was used most recently, printing is performed with the development bias setting changed to the most recently used result. Therefore, it is suppressed that character quality is impaired by an incorrect adjustment result.

  In addition, when the development bias before the speed change is determined not to be the lower limit of the development bias variable range and there is a development bias setting used most recently, printing is performed with the development bias setting changed to a correction value. The density can be adjusted accurately without executing the image density control.

  The correction value is a development bias at the time of solid image density control obtained by replacing the target image density at the time of speed change with the target density at the normal speed before the speed change, so the correction value is calculated by the individual to be used. However, the accuracy is higher than the method in which the correction value is previously entered.

  Further, the correction value estimates the relationship between the fine line image density and the exposure energy when the development bias set at the time of the solid image density control is used, and sets the target density of the fine line image at the speed change at the normal speed. Since the exposure energy at the time of fine line image density control obtained by replacing the target density is obtained, the accuracy is higher than the method in which the correction value is calculated for the individual to be used and the correction value is previously entered.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a longitudinal side view of an image forming apparatus of the present invention. In FIG. 1, an image forming apparatus 1 according to the present exemplary embodiment is mounted on a housing body 2, a first opening / closing member 3 that can be opened and closed on the front surface of the housing body 2, and on an upper surface of the housing body 2. And a second opening / closing member 4 (also serving as a paper discharge tray). Further, the first opening / closing member 3 includes an opening / closing lid 3a attached to the front surface of the housing body 2 so as to be freely opened and closed. The opening / closing lid 3a can be opened / closed in conjunction with or independently of the first opening / closing member 3. Has been.

  In the housing body 2, an electrical component box 5 containing a power circuit board and a control circuit board, an image forming unit 6, a blower fan 7, a transfer belt unit 9, and a paper feed unit 10 are disposed, and a first opening / closing member In FIG. 3, a secondary transfer unit 11, a fixing unit 12, and a recording medium conveying means 13 are arranged. The consumables in the image forming unit 6 and the paper feeding unit 10 are configured to be detachable from the main body. In this case, the configuration including the transfer belt unit 9 can be removed and repaired or replaced. It has become.

  A first opening / closing member 3 is mounted on the housing body 2 so as to be openable and closable on both sides of the lower front surface of the housing body 2 via a rotating shaft 3b. In this embodiment, each unit can be attached and detached by accessing only from the front surface of the apparatus, so that the apparatus can be installed in the room in a compact manner. The transfer belt unit 9 is disposed below the housing body 2 and is driven to rotate by a drive source (not shown), a driven roller 15 disposed obliquely above the drive roller 14, and the two rollers. An intermediate transfer belt 16 that is stretched between 14 and 15 and driven to circulate in the direction of the arrow shown in the figure, and a cleaning means 17 that comes into contact with and separates from the surface of the intermediate transfer belt 16.

  The driven roller 15 and the intermediate transfer belt 16 are disposed in a direction inclined to the left in the drawing with respect to the driving roller 14. As a result, the belt surface 16a with the belt conveyance direction downward when the intermediate transfer belt 16 is driven is positioned below. In the present embodiment, the belt surface 16a is a belt tension surface (surface pulled by the drive roller 14) when the belt is driven. The driving roller 14 and the driven roller 15 are rotatably supported by the support frame 9a, and a rotating portion 9b is formed at the lower end of the supporting frame 9a. The rotating portion 9b is a rotation provided on the housing body 2. The support frame 9a is fitted to the housing body 2 so as to be rotatable.

  A lock lever 9c is rotatably provided at the upper end of the support frame 9a, and the lock lever 9c can be locked to a locking shaft 2c provided in the housing body 2. The drive roller 14 also serves as a backup roller for the secondary transfer roller 19 constituting the secondary transfer unit 11. The driven roller 15 is also used as a backup roller for the cleaning means 17. The cleaning means 17 is provided on the belt surface 16a side facing down in the transport direction.

  Further, on the back surface 16a of the intermediate transfer belt 16 facing downward in the conveying direction, a primary transfer member 21 made of a leaf spring electrode is opposed to the photoreceptor 20 of each image forming station Y, M, C, K described later. The transfer bias is applied to the primary transfer member 21 by contact with the elastic force. A patch sensor 18 is installed on the support frame 9 a of the transfer belt unit 9 in the vicinity of the drive roller 14. The patch sensor 18 is a sensor for positioning each color toner image on the intermediate transfer belt 16, detecting the density of each color toner image, and correcting the color shift and image density of each color image.

  The image forming unit 6 includes a plurality of (four in this embodiment) image forming stations Y (for yellow), M (for magenta), C (for cyan), and K (for black) that form images of different colors. The image forming stations Y, M, C, and K each include a photoconductor 20 as an example of an image carrier, and a charging roller 22 as an example of a charging unit disposed around the photoconductor 20, An exposure unit 23 and developing means 24 are provided. Note that the charging roller 22, the exposure unit 23, and the developing unit 24 are assigned the drawing numbers only to the image forming station Y, and the other image forming stations have the same configuration, and thus the drawing numbers are omitted. Further, the arrangement order of the image forming stations Y, M, C, and K is arbitrary.

  Then, the photoconductor 20 of each image forming station Y, M, C, K is brought into contact with the belt surface 16a facing downward in the transport direction of the intermediate transfer belt 16, and as a result, each image forming station Y, M, C and K are also arranged in a direction inclined to the left in the drawing with respect to the drive roller 14. The photoconductor 20 is rotationally driven in the conveyance direction of the intermediate transfer belt 16 as indicated by an arrow in the drawing. The charging roller 22 is composed of a conductive brush roller connected to a high voltage generation source, and the outer periphery of the brush is in contact with the photosensitive member 20 which is a photosensitive member in the reverse direction and at a peripheral speed of 2 to 3 times. The surface of the photoconductor 20 is uniformly charged by rotating.

Further, when such a conductive brush roller is used in the cleaner-less image forming apparatus 1 as in this embodiment, a bias having the same polarity as the toner charging polarity is applied to the brush roller during non-image formation. Thus, the transfer residual toner attached to the brush roller is discharged to the photoconductor 20, transferred onto the intermediate transfer belt 16 at the primary transfer portion, and collected by the cleaning means 17 of the intermediate transfer belt 16. . By using such a charging roller 22, the surface of the photoconductor 20 can be charged with an extremely small current, so that the inside and outside of the apparatus is not contaminated with a large amount of ozone unlike the corona charging method. In addition, since the contact with the photoconductor 20 is soft, it is difficult for the transfer residual toner generated when the roller charging method is used to adhere to the charging roller 22, and stable image quality and device reliability are ensured. can do.
In this embodiment, the photoconductor 20, the charging unit 22, and the exposure unit 23 of each image forming station Y, M, C, and K are unitized as one photoconductor unit (image carrier unit) 25. These units can be replaced with a support frame 9 a together with the transfer belt unit 9. When the photosensitive unit 25 is replaced, the members including the exposure unit 23 are replaced.

  Next, details of the developing unit 24 will be described on behalf of the image forming station K. In this embodiment, the image stations Y, M, C, and K are arranged in an oblique direction, and the photosensitive member 20 is in contact with the belt surface 16a facing downward in the conveyance direction of the intermediate transfer belt 16, The toner storage container 26 is disposed obliquely downward. Therefore, a special configuration is adopted as the developing unit 24. That is, the developing unit 24 includes a toner storage container 26 as an example of a developer storage container that stores toner (hatched portion in the figure) as an example of the developer, and a developer storage formed in the toner storage container 26. A toner storage part 27 as an example of a part, a toner agitation member 29 as an example of a developer agitation member disposed in the toner storage part 27, and a partition member 30 that is partitioned and formed above the toner storage part 27 Have.

  Further, a toner supply roller 31 as an example of a developer supply member disposed above the partition member 30, a blade 32 provided on the partition member 30 and in contact with the toner supply roller 31, the toner supply roller 31, and A developing roller 33 as an example of a developer carrying member disposed so as to be in contact with the photoconductor 20 and a regulation blade 34 that is in contact with the developing roller 33 are provided. The photoconductor 20 is rotated in the conveyance direction of the intermediate transfer belt 16, and the developing roller 33 and the toner supply roller 31 are driven to rotate in the direction opposite to the rotation direction of the photoconductor 20, as shown by the arrows in the figure. The member 29 is driven to rotate in a direction opposite to the rotation direction of the toner supply roller 31.

  The toner agitated and carried by the toner agitating member 29 in the toner reservoir 27 is supplied to the toner supply roller 31 along the upper surface of the partition member 30, and the supplied toner is rubbed with the blade 32 and slid to the toner supply roller. It is supplied to the surface of the developing roller 33 by the mechanical adhesion force to the surface irregularities 31 and the adhesion force due to the frictional band power. The toner supplied to the developing roller 33 is regulated to a predetermined thickness by the regulating blade 34, and the thinned toner layer is conveyed to the photoreceptor 20, and the developing roller 33 and the photoreceptor 20 come into contact with each other. The latent image portion of the photoconductor 20 is developed at the nip portion and the vicinity thereof.

  In this embodiment, the developing roller 33, the toner supply roller 31, and the contact portion between the developing roller 33 and the regulating blade 34 on the side facing the photoconductor 20 are not buried in the toner in the toner storage unit 27. With this configuration, it is possible to prevent fluctuations in the contact pressure of the regulating blade 34 against the developing roller 33 due to a decrease in the stored toner, and surplus toner scraped off from the developing roller 33 by the regulating blade 34 falls into the toner reservoir 27. Therefore, filming of the developing roller 33 can be prevented.

  Further, the contact portion between the developing roller 33 and the regulating blade 34 is positioned below the contact position between the toner supply roller 31 and the developing roller 33, and the toner is supplied to the developing roller 33 by the toner supply roller 31 and does not shift to the developing roller 33. A path is provided for returning the excess toner and the excess toner regulated and removed from the developing roller 33 by the regulation blade 34 to the toner storage unit 27 below the developing unit. The toner that has returned to the toner reservoir 27 is agitated with the toner in the toner reservoir 27 by the toner agitating member 29 and supplied again to the toner introducing portion near the toner supply roller 31 by the toner agitating member 29.

  Therefore, the excess toner is dropped to the lower portion without being jammed on the sliding portion of the toner supply roller 31 and the developing roller 33 and the contact portion of the developing roller 33 and the regulating blade 34 to stir the toner in the toner storing portion 27. Therefore, it is possible to prevent the toner in the developing unit from gradually deteriorating and abruptly changing the image quality immediately after the developing unit 24 is replaced. Further, the paper feed unit 10 includes a paper feed unit including a paper feed cassette 35 in which the recording media P are stacked and held, and a pickup roller 36 that feeds the recording media P from the paper feed cassette 35 one by one. Yes.

  In the first opening / closing member 3, a registration roller pair 37 that regulates the feeding timing of the recording medium P to the secondary transfer portion, and a secondary transfer unit that is pressed against the drive roller 14 and the intermediate transfer belt 16. A secondary transfer unit 11, a fixing unit 12, a recording medium conveyance unit 13, a paper discharge roller pair 39, and a duplex printing conveyance path 40 are provided. The fixing unit 12 includes a heating roller 45 that includes a heating element such as a halogen heater and is rotatable, a pressure roller 46 that presses and biases the heating roller 45, and is swingable on the pressure roller 46. A belt tension member 47 and a heat-resistant belt 49 stretched between the pressure roller 45 and the belt tension member 47. The color image secondarily transferred to the recording medium is fixed to the recording medium at a predetermined temperature at a nip formed by the heating roller 45 and the heat-resistant belt 49.

  In this embodiment, the fixing unit 12 can be disposed in a space formed obliquely above the intermediate transfer belt 16, in other words, in a space opposite to the image forming unit 6 with respect to the intermediate transfer belt 16. Thus, heat transfer to the electrical component box 5, the image forming unit 6, and the intermediate transfer belt 16 can be reduced, and the frequency of performing the color misregistration correction operation for each color can be reduced.

  FIG. 2 is a block diagram of the image forming apparatus 1 of the present embodiment. In this image forming apparatus 1, when an image signal is given from an external device such as a host computer to the main controller 110 of the control unit 100 as an example of a control device, the engine controller 120 is charged according to a command from the main controller 110. The engine unit EG such as the unit 22, the exposure unit 23, and the developing roller 31 is controlled to form an image corresponding to the image signal on the recording medium P such as copy paper, transfer paper, paper, and OHP transparent sheet.

  The exposure unit 23 is electrically connected to the image signal switching unit 122, and the exposure power control unit 123 controls the exposure unit 23 in accordance with the image signal given through the image signal switching unit 122, and emits light. The photosensitive member 20 is exposed to form an electrostatic latent image corresponding to the image signal on the photosensitive member 20.

  When the image signal switching unit 122 is electrically connected to the CPU 111 of the main controller 110, the light L is exposed on the photoconductor 20 in accordance with an image signal supplied from an external device such as a host computer via the interface 112. Thus, an electrostatic latent image corresponding to the image signal is formed on the photoreceptor 20.

  On the other hand, when the image signal switching unit 122 is electrically connected to the patch creation module 125 based on a command from the CPU 124 of the engine controller 120, the patch image signal output from the patch creation module 125 is sent to the exposure power control unit 123. Given, a patch latent image is formed.

  In FIG. 2, reference numeral 113 denotes an image memory provided in the main controller 11 for storing an image given from an external device such as a host computer via the interface 112, and reference numeral 127 denotes an arithmetic program executed by the CPU 124. A memory (storage unit) for storing calculation results in the CPU 124, control data for controlling the engine unit EG, and the like.

  In the image forming apparatus configured as described above, a predetermined patch image is formed at an appropriate timing such as when the power is turned on, and patch processing is performed to optimize image forming conditions based on the image density. Specifically, the CPU 124 of the engine controller 120 executes a program stored in advance, and performs processing shown in an image density control flowchart described later for each toner color.

  FIG. 3 is a diagram showing the relationship between the exposure and the photoreceptor potential in this embodiment. Usually, the light attenuation characteristics are different between the case of forming a high density patch image and the case of forming a low density patch image. That is, when the photosensitive member 20 charged to a uniform surface potential Vd by the charging unit 22 is partially exposed, the charge of the portion is neutralized and an electrostatic latent image is formed on the surface of the photosensitive member 20. In a high density patch image such as an image, since a relatively wide range of the surface of the photoconductor 20 is exposed, the surface potential profile becomes a well type, and energy E per unit area of exposure (hereinafter simply referred to as “exposure energy”). When the exposure energy E increases, the surface potential of the exposed portion, so-called bright portion potential, gradually decreases. Then, when the residual potential is lowered to about the residual potential determined by the characteristics of the photoconductor 20, even if the exposure energy E is increased, the surface potential of the exposed portion hardly changes. As a result, the high-density patch image has a light attenuation characteristic as shown by a curve Ch in FIG. 3, and this light attenuation characteristic causes the surface potential of the photoconductor 20 to become a predetermined potential as the exposure energy E increases. It has a saturation region asymptotic to Vr.

  On the other hand, in a low density patch image such as a thin line image, the exposed area is narrow, so that the surface potential has a sharp dip profile. For this reason, the actual change in the surface potential of the photoconductor 20 is determined by the change in the surface potential (bright part potential) of the exposed part and the change in the contrast ratio between the non-exposed part and the exposed part. In particular, in this embodiment, the latter change is dominant. As a result, the low-density patch image of the photoconductor 20 has light attenuation characteristics as indicated by the straight line Cl in FIG.

  Note that the solid image, which is a high-density patch image in the present embodiment, includes not only an image in which dots are formed on the entire surface of the patch image, but also an image in which there is a partial density. When viewed as a whole, an image in which toner adheres to substantially the entire image surface is also included. For example, a solid image substantially includes a surface potential in each part of an electrostatic latent image corresponding to a patch image in a range of 10 V or less. Also included are images in which the area ratio of dots to the entire patch image is about 80% or more. The thin line image, which is a low-density patch image in the present embodiment, includes a thinned image such as 1 on 10 off.

  FIG. 4 shows an alternating voltage applied as a developing bias from the developing bias generator 126 to the developing roller 33. As shown in FIG. 4, the developing bias is an alternating voltage having a waveform in which a rectangular wave voltage having an amplitude Vpp is superimposed on the DC component Vavg. Here, the amplitude is Vpp (= | Vbmax−Vbmin |) as the AC component, and the ratio of the peak potential period Tb on the potential Vmax side with respect to one period Ta of the AC component, that is, the duty percentage (= (Tb / Ta) × 100%. ) Applies a 50% rectangular wave AC component. By applying a developing bias having such a waveform, the flying amount of toner can be controlled by the amplitude Vpp, while the image density can be controlled by the DC component Vavg.

  Note that the waveform of the alternating voltage as the developing bias is not limited to this, and for example, a sine wave or a triangular wave may be superimposed on the DC component. Further, a waveform whose duty ratio is not 50% may be used. In this case, as the DC component Vavg, a weighted average voltage, that is, a value obtained by averaging instantaneous values of a voltage waveform whose amplitude changes with time over a certain time range and converting it into a DC voltage value can be used.

  Next, normal image density control of this embodiment will be described. First, a toner image (patch) having a specific pattern is formed on a photoconductor 20 made of a photoconductor, transferred onto the intermediate transfer belt 16, and the density of the patch is detected by a patch sensor 18. As the test patch, a pattern having a width of about 10 to 20 mm and a length of about 10 to 40 mm is used. The patch sensor 18 mainly includes a light emitting element such as an LED and a light receiving element such as a photodiode. The light emitting element irradiates the pattern with infrared light, and the light receiving element can detect the irregularly reflected light. Since the reflected light detected by the light receiving element has a one-to-one correlation with the toner concentration, the patch sensor 18 can detect the toner concentration as a result.

  The image density is controlled by image forming conditions such as the charging potential, exposure amount, and developing bias of the photosensitive member 20 including a photosensitive drum, and the halftone gradation characteristics are controlled by an image data conversion table. A plurality of images are formed by changing the formation conditions and the image data conversion table in stages. The density of these patches is detected by the patch sensor 18, and the optimum value of the image forming conditions is derived from the result.

  Next, the image density control flow of this embodiment will be described with reference to FIG. In the process shown on the left side of the figure (steps S11 to S18), the development bias necessary for forming a toner image with a high density side target density (OD = 1.3) is obtained as an optimum development condition. In the process shown on the left side (steps S21 to S27), the exposure energy necessary for forming a toner image having a low density side target density (OD = 0.22) is obtained as the optimum exposure condition.

First, in step S11, the exposure energy E is set to a value within the saturation region Vr where the photoreceptor potential V is asymptotic (step S11). The exposure energy E is preferably as low as possible, particularly about 0.3 μJ / cm ^2, which is almost the minimum value in the saturation region Vr. This is because, when the exposure energy E is set to a large value (about 0.5 μJ / cm ² ) in the saturation region Vr, the exposure energy E decreases during the fine line control described later, and the solid image density may decrease. is there. Therefore, if the exposure energy E is set to a substantially minimum value in the saturation region during the control of the solid image in this way, the exposure energy E increases during the fine line control described later, and there is no possibility that the solid image density will decrease. It is.

  Next, the exposure energy E is fixed to the predetermined value set in step S11, and the DC component of the developing bias (hereinafter referred to as “DC developing bias”) Vavg is set to the initial value Vavg_A (step S12). As shown in FIG. 3, the initial value Vavg_A is set to a DC developing bias Vavg corresponding to the high density side target density, that is, a value closer to the residual potential Vr than the optimum developing condition. Therefore, when, for example, a solid image is formed as a high-density patch image with the initial value Vavg_A being set (step S13), a patch image having a density lower than the high-density target density is formed.

  Then, by repeating steps S13 to S15, a patch image is formed under each bias while further changing and setting the DC developing bias Vavg to the bias Vavg_B and Vavg_C. As shown in FIG. 3, Vavg_C is set to a DC developing bias Vavg corresponding to the high density side target density, that is, a value larger than the optimum developing condition. That is, the DC development bias Vavg corresponding to the high density side target density is interpolated between the DC development bias Vavg_A and Vavg_C. In this embodiment, three types of Vavg_A, Vavg_B, and Vavg_C are changed and set in this order as the DC developing bias Vavg, but it goes without saying that the number and order are arbitrary.

  When three types of high-density patch images are thus formed, the image density of each patch image is detected by the patch sensor 18 (step S16). As a result, the image density of the patch image formed under each DC development bias Vavg_A, Vavg_B, Vavg_C is detected, and the correlation between the DC development bias and the image density is obtained based on the combination of the DC development bias and the image density. (Step S17). Next, in step S18, a DC development bias corresponding to the high density side target density is obtained based on the correlation, and this is stored in the memory 127 as the optimum development condition.

  Subsequently, the process on the right side of FIG. 5 is executed. That is, the DC development bias Vavg is set to the optimum development condition obtained previously (step S21), and the exposure energy E is changed and set within the asymptotic range, so that it is used for low density as a halftone image under each energy condition. As the patch image, for example, a line image composed of a plurality of one-dot lines spaced apart from each other is formed (steps S22 to S25). Then, the image density of each patch image formed in this way is detected by the patch sensor 18 (step S26), and the density substantially matches the preset low density target density, in this embodiment, the density OD = 0.22. The exposure energy is calculated and the value is stored in the memory 127 as the optimum exposure condition (step S27).

Here, the Vcont_thin line which is the contrast potential at the time of the thin line in FIG. 3 is Vcont which is the contrast potential when the photoconductor surface potential of the thin line is above the DC development bias Vavg in the graph of FIG. _The direction is the opposite of the solid direction (the direction in which the toner does not fly). However, the photoconductor potential of the fine line is -50V if the average (1on10off) of the exposed fine line part (image part, for example, surface potential -50V) and the non-exposed part (for example, surface potential -500V). Therefore, the measured potential is about -400 V to -300 V when the exposure E is 0.3 to 0.4 μJ / cm ² , and the graph of FIG. 3 is obtained from the DC developing bias Vavg. As described above, the actual development (toner movement) is performed at a contrast potential between the surface potential of the image portion of −50 V and Vavg.

  Next, an embodiment of print control according to the development bias at the time of changing the process speed will be described. FIG. 6 is a block diagram of the present embodiment. 126 is a developing bias generation unit, 123 is an exposure energy control unit, 201 is a speed change command unit, 202 is a development bias inquiry unit, 203 is an exposure energy inquiry unit, 204 is a determination unit, 205 is an image density control unit, and 206 is normal. Speed correspondence control means, 207 is an initial value use control means, 208 is a latest result use control means, 209 is a correction value control means, 210 is a speed change means, and 211 is a storage means.

  The speed change command means 201 instructs the speed change means 210 to change the process speed in accordance with the paper type in the paper feed cassette 35, and the development bias inquiry means 202 inquires the development bias and makes a determination means. The exposure energy inquiry means 203 inquires the exposure energy and transmits it to the determination means 204. The storage unit 211 is a unit for storing the initial value and the result of the most recent image density control.

  The speed changing unit 210, the developing bias generator 126, and the exposure power controller 123 are for changing the process speed, for changing the DC developing bias Vavg, and for changing the exposure energy E, respectively.

  The determination unit 204 detects image density control (condition 1) by the image density control unit 205 when the developer remaining amount detected by the developer remaining amount detection unit 202 is equal to or greater than a predetermined amount, and detection by the developer remaining amount detection unit 202. When the remaining amount of the developed developer is less than a predetermined value, the development bias setting by the normal speed correspondence control means 206 is not changed to a value corresponding to the normal speed (condition 2), the storage means by the initial value use control means 207 212 referring to the initial value stored in 212 (condition 3), and using the value obtained from the latest low-speed image density control result stored in the storage unit 212 by the latest result use control unit 208 as it is (condition) 4) or one using the correction value by the correction value control means 209 (condition 5) is determined. Note that the initial value at the time of shipment from the factory is used as the initial value of the initial value use control means 207. In the present embodiment, the latest result is stored as a default value as it is, but the default value may be calculated and stored by learning including not only the latest result but also the previous result.

  Here, the correction value by the correction value control means 209 under condition 5 will be described. The correction value by the correction value control means 209 is estimated and obtained from the result of the most recently performed image density control. First, correction values for a solid image will be described. For example, when the solid image density with respect to the DC developing bias Vavg of the most recently performed image density control is as shown in Table 1, the relationship of the solid image density at the low speed with respect to the normal speed is as shown in FIG.

  From the graph of FIG. 7, the relationship between the image density at low speed and the image density at normal speed can be replaced by the following equation.

Normal speed solid image density = Low speed solid image density × 3.75−3.975 (1)
When the target density is 1.35, the normal speed target density for estimating the low-speed solid image density of 1.35 is estimated to be 1.09 from the equation (1). Therefore, referring to the graph of the relationship between the normal speed and the low speed of the solid image density with respect to the DC development bias Vavg when the speed is changed in FIG. The setting of the developing bias Vavg is −200V, but the setting of the DC developing bias Vavg at low speed is −125V.

Next, a correction value for the thin line image is obtained. First, the relationship between the exposure energy E and the fine line image density at the DC development bias Vavg of the solid image is estimated. FIG. 9 is a graph showing the influence of the developing bias Vavg on the relationship between the fine line image density and the exposure energy E. In the control method in which the solid image density control is performed with the DC developing bias Vavg and then fixed with the determined DC developing bias Vavg and the exposure energy E is changed to control the fine line image (1 on 10 off) density, The determined value is affected by how the DC developing bias Vavg is determined. The DC development bias Vavg only needs to match the solid image target density, but the DC development bias Vavg is too small (not enough) to obtain the target density, and is fixed at the DC development bias Vavg. When image control is executed, the exposure energy E is determined to be higher than that in the case of the normal DC developing bias Vavg. On the contrary, if the solid image is determined by an excessive DC developing bias Vavg to obtain the target density, the exposure energy E is induced lower. For example, with respect to the normal speed of the graph showing the relationship between the DC developing bias Vavg and the solid image density in FIG. 8, to obtain the target density of 1.35, the DC developing bias Vavg is originally required to be −200 V. When the DC development bias Vavg is determined to be −150 V due to variations, what the exposure energy E for obtaining the fine line image density will be as seen from the graph of FIG. to obtain 23, if the DC image bias Vavg is -200 V, to 0.24μJ / cm ^2, the direct current developing bias Vavg is the case of -150 V, the 0.4μJ / cm ² required.

  Also, in a state where the developer remaining amount is low in the developing device (for example, there is not enough toner around the supply roller and printing of all solid images is not possible, printing of character images, etc. is possible) At the time of solid image density adjustment, the DC developing bias Vavg is likely to be determined higher than the value that should be originally determined (for example, the value that should be originally determined is -150V, but becomes -200V). The cause is that when the solid image density is controlled, the solid image becomes uneven due to the lack of developer around the supply roller, and the density of the uneven density is read. As a result, the DC developing bias Vavg for obtaining the target density is required to be high. On the other hand, the fine line image density is smaller in the amount of developer used for forming the fine line image than the solid image, and therefore, there is little variation in the density of the fine line image in a short period as in the density adjustment. As described above, when the DC developing bias Vavg is determined to be higher than a normal value, the exposure E is determined to be lower due to the influence. In this state, image formation is continued, especially at a position away from the position of the patch sensor 18 (normally, the patch is set at one position such as the center or the end). For example, when the patch sensor 18 is at the center, exposure is performed at the end. Insufficient concentration occurs because energy E is low.

  As described above, the relationship between the fine line image density and the exposure energy E varies depending on the DC developing bias Vavg. The relationship between the fine line image density and the exposure energy E when the DC developing bias Vavg is -200 V and -150 V is read from the graph of FIG. 9 and the DC developing bias Vavg at the low speed is set to -125 V. Estimate the image density. The slope of the graph in FIG. 9 is considered to be the same depending on the speed, and the intercept is linearly approximated and estimated as shown in Table 2. However, the value for the DC developing bias Vavg-125V is an estimated value.

  Therefore, the fine line image density with respect to the exposure energy E is as shown in Table 3. However, the value for the DC developing bias Vavg-125V is an estimated value.

  From this table, the fine line image density with respect to the exposure energy E is replaced by the following equation.

Normal speed fine line image density = exposure energy x 0.16 + 0.163 (2)
FIG. 10 is a graph showing the relationship between the thin line image density at low speed and the normal line thin image density obtained from Table 3. From this graph, it can be seen that when the DC developing bias Vavg is −125 V, the following equation holds.

Normal speed fine line image density = Low speed fine line image density × 1.6−0.1455 (3)
When the target density is 0.23, the normal speed target value corresponding to the low-speed fine line image density of 0.23 is estimated from the equation (3), and is 0.225. Therefore, when the exposure energy E at which the fine line image density is 0.225 at the normal speed is obtained from the equation (2), it is estimated that the exposure energy is 0.39 μJ / cm ² .

  Under condition 5, the DC development bias Vavg and the exposure energy E thus obtained are used as correction values, and printing is executed without executing image density control when the process speed is changed.

  Next, image density control based on the developing bias Vavg when changing the speed will be described. FIG. 11 shows an example of a normal speed bias variable range lower limit of the solid image density with respect to the developing bias Vavg when the speed is changed, and FIG. 12 shows an example of an upper limit of the normal speed bias variable range of the solid image density with respect to the developing bias Vavg when the speed is changed. Show.

  When the speed is lower than the normal speed, the time from exposure to the development position becomes longer and the potential is attenuated. Therefore, the contrast potential with the development is increased and the development amount is increased for both the solid image and the thin line image. Therefore, there may be a phenomenon that the lower limit sticking of the developing bias Vavg and the upper sticking of the developing bias Vavg occur.

  The lower limit sticking of the developing bias Vavg means a state where the developing bias Vavg is larger than the target developing amount even if the developing bias Vavg is reduced to the full capacity of the power supply in a state where it is easy to develop. For example, as shown in the graph of FIG. 11, when density adjustment by image density control is executed at normal speed, the OD value is 1.40, which is higher than the target value 1.35, and the developing bias Vavg at that time is −100V. In this state, if the process speed is halved for cardboard output and the image density control is executed again in this state, the OD value becomes 1.52, which is a higher density, and the development bias Vavg is -100V. This state is called sticking the lower limit of the developing bias Vavg.

  If the development bias Vavg is stuck at the normal speed and set to a low speed, the development amount further increases. Even if image density control is executed in this state, the result is that the development bias Vavg is again Only the result of the lower limit is returned. Therefore, in the case where the development bias Vavg lower limit is pasted, the image density control is performed although the developer consumption amount is slightly increased by outputting the image at the normal speed development bias Vavg without re-entering the image density control at a low speed. Thus, it is possible to form an image without causing a decrease in productivity due to the implementation. Also, when setting at low speed, a warning that an image with higher density than normal speed will be issued, so that the user can recognize that the image at low speed is shifted from the original target image in a darker direction. Good.

  Next, the upper limit sticking of the developing bias Vavg means a state in which the target developing amount is not reached even when the developing bias Vavg is raised to the full capacity of the power source in a state where it is difficult to develop. For example, as shown in the graph of FIG. 12, when density adjustment by image density control is executed at normal speed, the OD value is 1.30, which is lower than the target value 1.35, and the developing bias Vavg at that time is −250 V. In this state, when the process speed is halved for cardboard output and the image density control is executed again, the OD value becomes larger than the target value of 1.35. The developing bias Vavg is −230V. This state is called sticking the upper limit of the developing bias Vavg.

  If the development bias Vavg is stuck at the normal speed and is set to a low speed, the development amount further increases. Therefore, if image density control is executed in this state, the target density may be reached.

  Further, when the development bias Vavg is not attached to the upper limit or the lower limit at the normal speed, the control is performed according to any of the above-described conditions at the low speed.

  FIG. 13 is a flowchart showing image density control based on the developing bias Vavg when changing the process speed according to the first embodiment. First, in step 31, a process speed change is instructed (ST31). In step 32, the process speed is changed (ST32). Subsequently, in step 33, the development bias Vavg at the normal speed is inquired (ST33). Next, in step 34, it is determined whether the developing bias Vavg is stuck to the lower limit (ST34). If the developing bias Vavg is stuck to the lower limit, in step 35, the image density control is not executed and the DC developing bias Vavg corresponding to the normal speed before the speed change is kept even if the process speed is changed (ST35). If the development bias Vavg is not past the lower limit, the normal image density control shown in FIG. 5 is executed by the image density control means 205 in step 36 (ST36).

  FIG. 14 is a flowchart showing image density control based on the developing bias Vavg when changing the process speed according to the second embodiment. First, in step 41, an instruction to change the process speed is given (ST41). In step 42, the process speed is changed (ST42). Subsequently, in step 43, the development bias Vavg at the normal speed is inquired (ST43). Next, in step 44, it is determined whether the developing bias Vavg is stuck to the lower limit (ST44). If the development bias Vavg is stuck to the lower limit, in step 45, the image density control is not executed, and the DC development bias Vavg corresponding to the normal speed before the speed change is kept even if the process speed is changed (ST45). If the developing bias Vavg is not stuck at the lower limit, it is determined at step 46 whether the developing bias Vavg is stuck at the upper limit (ST46). If the developing bias Vavg is sticking to the upper limit, the normal image density control shown in FIG. 5 is executed by the image density control means 205 in step 47 (ST47). If the developing bias Vavg is not pasted at the upper limit, it is determined in step 48 whether or not the process speed is changed for the first time (ST48). When the process speed is changed for the first time, the latest result cannot be used and the correction value cannot be estimated. Therefore, in step 49, the initial value by the initial value use control means 207 is referred to (ST49). . If the process speed is not changed for the first time, in step 50, the value obtained from the latest low-speed image density control result by the latest result use control unit 208 is used as it is, or the correction value by the correction value control unit 209 is used. Either one (ST48).

  As described above, when the lower limit of the development bias Vavg is attached to the target image density at the normal speed, the development bias Vavg is more easily developed at a lower speed. Therefore, the development bias Vavg is not re-adjusted. By forming an image at the lower limit, toner consumption and time during density adjustment can be reduced.

  In addition, when the upper limit of the development bias Vavg is stuck to the target image density at the normal speed (the target density is not reached even at the upper limit of the bias), the development is easily performed at a lower speed, so the field density adjustment control is performed. There is a possibility that the target density can be obtained. Since the control can be performed only when image quality adjustment is possible, toner and time can be used effectively.

  Furthermore, the correction value is output in advance for each individual to be used, and the value is used when the speed is changed, so that the density can be adjusted accurately without patch control. Since the correction value is calculated for each individual to be used, the accuracy is higher than the method in which the correction value is previously entered.

  Next, image density control based on the developing bias Vavg and the exposure energy E when changing the process speed will be described. FIG. 15 shows an example of the normal speed exposure energy variable range lower limit of the fine line image density with respect to the exposure energy E when the speed is changed. FIG. 16 shows the upper limit of the normal speed exposure energy variable range of the fine line image density with respect to the exposure energy E when the speed is changed. An example is shown.

The lower limit sticking of the exposure energy E refers to a state where the exposure energy E is larger than the target development amount even if the exposure energy E is reduced to the full capacity of the power source in a state where development is easy. For example, as shown in the graph of FIG. 15, when fine line density adjustment is performed by image density control at normal speed, the OD value is 0.24, which is higher than the target value 0.23, and the exposure energy E at that time is 0. If the lower limit of the variable range of the exposure energy E at ² μJ / cm ² , if the process speed is halved for cardboard output, etc., and the image density control is executed again, the OD value will be higher. The exposure energy E was 0.2 μJ / cm ² . This state is called sticking the lower limit of the exposure energy E.

Next, the upper limit sticking of the exposure energy E refers to a state where the target development amount is not reached even if the exposure energy E is increased to the full capacity of the power source in a state where it is difficult to develop. For example, when fine line density adjustment by image density control is executed at a normal speed as shown in the graph of FIG. 16, the OD value is 0.22, which is lower than the target value 0.23, and the exposure energy E at that time is 0. When the upper limit of the variable range of the power source is 5 μJ / cm ² , when the process speed is halved for cardboard output and the image density control is executed again in this state, the OD value is less than the target value of 0.23. The exposure energy E at that time becomes 0.4 μJ / cm ² . This state is called sticking the upper limit of the exposure energy E.

  Table 4 shows the meaning of image density control for the development bias Vavg and exposure energy E. As can be seen from Table 4, when both the development bias Vavg and the exposure energy E are in the lower limit sticking state, there is no point in performing image density control again at low speed, and in other cases, it is not necessarily avoided. Since there is a possibility of avoidance, control is performed according to any of the above-described conditions at a low speed.

  FIG. 17 is a flowchart showing image density control based on the developing bias Vavg and the exposure energy E when the process speed is changed according to the third embodiment. First, in step 51, an instruction to change the process speed is given (ST51). In step 52, the process speed is changed (ST52). Subsequently, in step 53, the development bias Vavg at the normal speed is inquired (ST53). Next, in step 54, it is determined whether the developing bias Vavg is stuck to the lower limit (ST54). If the development bias Vavg is sticking to the lower limit, it is determined in step 55 whether the exposure energy E is sticking to the lower limit (ST55). If the exposure energy E is a lower limit, the image density control is not executed in step 56, and the DC developing bias Vavg setting corresponding to the normal speed before the speed change is kept even if the process speed is changed (ST56).

  If the developing bias Vavg is not stuck at the lower limit in step 54, the normal image density control shown in FIG. 5 is executed by the image density control means 205 in step 56 (ST56).

  FIG. 18 is a flowchart showing image density control based on the developing bias Vavg and the exposure energy E when the process speed is changed according to the fourth embodiment. First, in step 61, an instruction to change the process speed is given (ST61). In step 62, the process speed is changed (ST62). Subsequently, in step 63, the development bias Vavg at the normal speed is inquired (ST63). Next, in step 64, it is determined whether the developing bias Vavg is stuck to the lower limit (ST64). If the developing bias Vavg is sticking to the lower limit, it is determined in step 65 whether the exposure energy E is sticking to the lower limit (ST65). If the exposure energy E is a lower limit, the image density control is not executed in step 66, and the DC developing bias Vavg setting corresponding to the normal speed before the speed change is kept even if the process speed is changed (ST66).

  If the development bias Vavg is not the lower limit sticking in step 64 or the exposure energy E is not the lower limit sticking in step 65, it is determined in step 67 whether or not the process speed is changed for the first time (ST67). When the process speed is changed for the first time, the latest result cannot be used, and the correction value cannot be estimated. In step 68, the initial value by the initial value use control means 207 is referred to (ST68). . If the change in the process speed is not the first time, the value obtained from the latest low-speed image density control result by the latest result use control unit 208 is used as it is, or the correction value by the correction value control unit 209 is used in step 69. Either one (ST69).

  In this way, when the development bias is stuck to the target image density at normal speed, the development bias tends to be more easily developed when the speed is reduced, so the development bias lower limit is not required again. By forming an image, the toner consumption and time during density adjustment can be reduced.

  In addition, when the upper limit of the development bias is applied to the target image density at the normal speed (the target density is not reached even at the upper limit of the bias), the image is easily developed at a lower speed. There is a possibility that the target concentration can be obtained. Since the control can be performed only when image quality adjustment is possible, toner and time can be used effectively.

  Furthermore, the correction value is output in advance for each individual to be used, and the value is used when the speed is changed, so that the density can be adjusted accurately without patch control. Since the correction value is calculated for each individual to be used, the accuracy is higher than the method in which the correction value is previously entered.

1 is a diagram illustrating an embodiment of an image forming apparatus of the present invention. 1 is a block diagram of an image forming apparatus. It is a figure which shows the light attenuation characteristic of a photoreceptor. It is a figure which shows the voltage applied to a developing roller. It is a figure which shows the flowchart of normal image density control. It is a block diagram regarding control at the time of process speed change. It is a figure which shows the relationship between normal speed and low-speed image density. It is a figure which shows the solid image density with respect to the developing bias at the time of speed change. It is a figure which shows the fine line density with respect to exposure energy. It is a figure which shows the relationship between normal speed and low-speed image density. It is a figure which shows the lower limit sticking of a developing bias. It is a figure which shows the upper limit sticking of developing bias. It is a figure which shows the flowchart of 1st Embodiment. It is a figure which shows the flowchart of 2nd Embodiment. It is a figure which shows the lower limit sticking of exposure energy. It is a figure which shows the lower limit sticking of exposure energy. It is a figure which shows the flowchart of 3rd Embodiment. It is a figure which shows the flowchart of 4th Embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus, 2 ... Housing main body, 2b ... Rotating shaft (rotating fulcrum), 2c ... Locking shaft, 3 ... First opening / closing member, 3a ... Opening / closing lid, 3b ... Rotating shaft, 4 ... First 2 opening / closing members, 5 ... electric component box, 6 ... image forming unit, 7 ... blower fan, 9 ... transfer belt unit, 9a ... support frame, 9b ... rotating part, 10 ... feeding unit, 11 ... secondary transfer Unit: 12 fixing unit 13 recording medium conveying unit 14 driving roller 15 driven roller 16 intermediate transfer belt 16a belt surface 17 cleaning unit 18 patch sensor 19 secondary transfer Roller, 20 ... photosensitive member (image carrier), 21 ... primary transfer member, 22 ... charging roller (charger), 23 ... exposure unit, 24 ... developing means, 25 ... photosensitive unit (image carrier cartridge), 26 ... toner storage capacity (Developer storage container), 27 ... toner storage part (developer storage part), 29 ... toner stirring member (developer stirring member), 30 ... partition member, 31 ... toner supply roller (developer supply member), 32 ... Blade 33, developing roller (developer carrier) 34, regulating blade 35, paper feed cassette 36, pickup roller 37, registration roller pair 39, paper discharge roller pair 40, double-sided printing conveyance path, 45 ... heating roller, 46 ... pressure roller, 47 ... belt tension member, 49 ... heat resistant belt, 100 ... control unit (control device), 110 ... main controller, 111 ... CPU, 112 ... interface, 113 ... image memory, DESCRIPTION OF SYMBOLS 120 ... Engine controller 121 ... Charging bias generating part 122 ... Image signal switching part 123 ... Exposure power control part 124 ... CPU DESCRIPTION OF SYMBOLS 125 ... Patch preparation module, 126 ... Development bias generation part, 127 ... Memory, 201 ... Speed change command means, 202 ... Development bias inquiry means, 203 ... Exposure energy inquiry means, 204 ... Judgment means, 205 ... Image density control means, 206 ... Normal speed correspondence control means, 207 ... Initial value use control means, 208 ... Latest result use control means, 209 ... Correction value control means, 210 ... Speed change means, 211 ... Storage means

Claims (16)

  Speed change means for changing the process speed, development bias inquiry means for inquiring a development bias before the speed change by the speed change means, development bias before the speed change inquired by the development bias inquiry means, and a development bias variable range An image forming apparatus comprising: a determination unit that determines whether image density control is performed or not according to the relationship.   When the determination means determines that the development bias before the speed change inquired by the development bias inquiry means is the lower limit of the development bias variable range, the development bias setting remains in a state corresponding to the normal speed and remains unchanged. The image forming apparatus according to claim 1, further comprising a speed correspondence control unit.   Exposure energy inquiry means for inquiring exposure energy before speed change by the speed change means, and the determination means before the speed change inquired by the development bias inquiry means is a lower limit of a development bias variable range; When it is determined that the exposure energy before the speed change inquired by the exposure energy inquiry means is the lower limit of the exposure energy variable range, the development speed setting is set to a value corresponding to the normal speed and remains unchanged. The image forming apparatus according to claim 1, further comprising:   When the determination unit determines that the development bias before the speed change inquired by the development bias inquiry unit is not the lower limit of the development bias variable range, an initial value in which the development bias setting is changed to an initial value corresponding to the speed change 4. The image forming apparatus according to claim 1, further comprising a usage control unit.   The determination means determines that the development bias before the speed change inquired by the development bias inquiry means is not the lower limit of the development bias variable range, and when the development bias setting used most recently exists, the development bias setting was used most recently. The image forming apparatus according to claim 1, further comprising: a latest result use control unit configured to change to a result.   The determination means determines that the development bias before the speed change inquired by the development bias inquiry means is not the lower limit of the development bias variable range, and if there is a development bias setting used most recently, the development bias setting is changed to a correction value. The image forming apparatus according to claim 1, further comprising a correction value control unit configured to perform the correction.   The correction value control means replaces the target image density at the time of speed change with a target density at a normal speed before the speed change, and sets a development bias at the time of solid image density control. Image forming apparatus.   The correction value control means estimates the relationship between the fine line image density at the developing bias set at the time of the solid image density control and the exposure energy, and sets the target density of the fine line image at the speed change to the target density at the normal speed. The image forming apparatus according to claim 7, wherein the exposure energy for fine line image density control is set instead.   An image forming method, wherein whether or not image density control is performed is determined according to a relationship between a developing bias before a change in process speed and a developing bias variable range.   The printing is executed in a state where the development bias setting remains unchanged at a value corresponding to the normal speed before the speed change when the development bias before the process speed change is the lower limit of the development bias variable range. 10. The image forming method according to 9.   When the development bias before the process speed change is the lower limit of the development bias variable range and the exposure energy is the lower limit of the exposure energy variable range, the development bias setting remains unchanged at a value corresponding to the normal speed before the speed change. The image forming method according to claim 9, wherein the printing is executed by.   12. The printing according to claim 9, wherein when the development bias before the speed change is not the lower limit of the development bias variable range, the printing is executed in a state where the development bias setting is changed to an initial value corresponding to the speed change. The image forming method according to any one of the above.   If it is determined that the development bias before the speed change is not the lower limit of the development bias variable range, and there is a development bias setting used most recently, printing is executed with the development bias setting changed to the result of the most recent use. The image forming method according to claim 9, wherein:   It is determined that the developing bias before the speed change is not the lower limit of the developing bias variable range, and when the most recently used developing bias setting exists, printing is executed with the developing bias setting changed to a correction value. The image forming method according to claim 9.   The correction value is a development bias at the time of solid image density control obtained by replacing the target image density at the time of speed change with the target density at the normal speed before the speed change. Image forming method.   The correction value estimates the relationship between the fine line image density and the exposure energy when the developing bias set at the time of the solid image density control is used, and sets the target density of the fine line image at the speed change to the target density at the normal speed. The image forming method according to claim 15, wherein the exposure energy at the time of fine line image density control obtained by substituting

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