JP2011242571A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus that improves image deletion due to a nitrate compound without leading to an increase in size and cost of the apparatus.SOLUTION: The relative humidity values RH surrounding a photoreceptor are obtained (S601). If the relative humidity value is less than 30% (YES for S602), an intensity modulation system is applied to an edge part in an image, and a light exposure system 1 using an area modulation system is adopted for the area other than the edge part (S603). Further, if the relative humidity value is 30% or more (NO for S602), a light exposure system 2 using an intensity modulation system is adopted for all areas (S604). Thereafter, when an image forming job is received (YES for S605), the image forming job is performed using the adopted light exposure system (S606).

Description

  The present invention relates to an image forming apparatus, and more particularly to a technique for preventing image deterioration caused by adhesion of nitrogen oxide or the like generated during charging to the surface of a photoreceptor.

  In an electrophotographic image forming apparatus, the surface of a photoreceptor is uniformly charged, then exposed and developed to form a toner image. In this charging step, for example, when corona discharge is used, discharge products such as nitrogen oxides are generated. Nitrogen oxides react with moisture in the air to produce nitric acid, so that various nitric acid compounds are produced. Since the nitric acid compound has high hygroscopicity, the electrical resistance is high in a low humidity environment, while moisture in the air is absorbed in a high humidity environment, and the electrical resistance is low.

When a nitric acid compound adheres to the surface of the photoconductor to form a thin film, in high humidity, the charge on the photoconductor surface flows out of the exposure area via the nitric acid compound, causing abnormalities in the image (hereinafter referred to as “image flow”). ").
In order to solve such a problem, for example, the periphery of a charging device that uniformly charges the surface of the photosensitive member is covered with a case, and the generation of nitrogen oxides is suppressed by introducing a low oxygen gas into the case. Techniques for reducing the amount of compound produced have been proposed (see Patent Document 1).

JP 2004-301958 A

However, in order to introduce the low oxygen gas into the case surrounding the charging device, a cylinder for storing the low oxygen gas, a device for generating the low oxygen gas, a duct for transporting the low oxygen gas, etc. are required. Therefore, problems such as an increase in the size of the apparatus, cost of parts, and an increase in manufacturing cost cannot be avoided.
The present invention has been made in view of the above problems, and provides an image forming apparatus that improves the image flow caused by a nitric acid compound without increasing the size of the apparatus or increasing the cost. Objective.

  In order to achieve the above object, an image forming apparatus according to the present invention exposes a uniformly charged photoreceptor on the basis of image data, and adheres to the surface of the photoreceptor, exposing means for forming an electrostatic latent image. An acquisition unit that acquires an index that indicates the electrical resistance of the discharge product, a determination unit that compares the index with a predetermined threshold to determine the level of the electrical resistance, and an area other than the edge area in the image data A method determining unit that exposes the exposure unit in one of an area modulation method that expresses a gradation according to the size of a halftone dot area and an intensity modulation method that expresses a gradation according to the density of a pixel. The method determining means performs exposure by an intensity modulation method when it is determined that the electric resistance is low, and by an area modulation method when it is determined that the electric resistance is high.

In this way, if the electrical resistance of the discharge product adhering to the surface of the photoconductor is low and charge can move through the surface of the photoconductor via the discharge product, the intensity modulation method is applied to a region other than the edge region. Therefore, it is possible to suppress the fluctuation of the potential on the surface of the photoreceptor. Therefore, it becomes difficult for the electric charge to move through the discharge product, so that the image can be prevented from being deteriorated.
In this case, the index may be a relative humidity in the vicinity of the photoconductor, and the determination unit may determine that the electrical resistance is low when the relative humidity is higher than a predetermined humidity value. The electrical resistance of the discharge product varies depending on the relative humidity level of the atmosphere. By measuring the relative humidity of the atmosphere, the level of electrical resistance of the discharge product can be easily estimated.

  The index is a relative humidity in the vicinity of the photoconductor and a standby time between subsequent image forming processes, and the determination unit determines that the relative humidity is higher than a predetermined humidity value and the standby time. When the predetermined humidity corresponding to the predetermined humidity value is longer than the predetermined time, it may be determined that the electrical resistance is low. The discharge product adheres to the surface of the photoreceptor after the image forming process, and is cleaned and removed from the surface of the photoreceptor after the next image formation. For this reason, the longer the standby time between subsequent image forming processes, the greater the amount of discharge products adhering to the surface of the photoreceptor, and the greater the effect on image quality. Therefore, if attention is paid to the fact that the electrical resistance is lowered if the standby time is long even if the relative humidity value is the same, the deterioration of the image quality can be prevented more accurately.

  In this case, the photosensitive member is a rotating member, and includes a counting unit that counts the cumulative number of rotations of the photosensitive member, and the determination unit decreases the predetermined humidity value as the cumulative number of rotations increases. It is more preferable to shorten the predetermined time. As the image forming process is repeated, the amount of discharge products adhering to the surface of the photoreceptor increases. Further, when the photosensitive member is a rotating member and rotates each time image forming processing is performed, the amount of image forming processing can be estimated from the cumulative number of rotations of the photosensitive member. Therefore, since the electrical resistance tends to decrease as the cumulative number of rotations increases, the image quality can be further improved by reducing the threshold values such as the predetermined humidity value and the predetermined time.

Further, the intensity modulation method may always be used for the edge region in the image data. Since the intensity modulation method can express the edge more clearly than the area modulation method, an image with a clearer edge portion can be realized in this way.
The discharge product may be a nitric acid compound. The nitric acid compound adhering to the surface of the photoreceptor has a remarkable increase and decrease in electrical resistance due to the humidity level in the atmosphere, so that high image quality can be realized particularly when attention is paid to the nitric acid compound.

1 is a diagram illustrating a main configuration of a full-color electrophotographic image forming apparatus according to a first embodiment of the present invention. It is a figure which shows the main structures of the image creation part 101K. 2 is an external perspective view illustrating the configuration of a charging device 202. FIG. 3 is a block diagram illustrating a main configuration of a control unit 102. FIG. FIG. 6 is a graph illustrating the photoreceptor potential when various modulation methods are used, in which (a) and (c) are for the area modulation method, and (b) and (d) are for the intensity modulation method. In particular, (c) and (d) show the photoreceptor potential when there is image flow. 3 is a flowchart showing an operation of a control unit 102. It is a flowchart which shows operation | movement of the control 102 which concerns on the modification of the 1st Embodiment of this invention. It is a block diagram which shows the main structures of the control part which concerns on the 2nd Embodiment of this invention. It is a flowchart which shows operation | movement of the control part which concerns on the 2nd Embodiment of this invention. It is a table | surface which illustrates the exposure system table which concerns on the 2nd Embodiment of this invention. It is a flowchart which shows operation | movement of the control part 102 which concerns on the modification of the 2nd Embodiment of this invention. It is a flowchart which shows operation | movement of the control part 102 which concerns on the 3rd Embodiment of this invention. It is a table | surface which illustrates the exposure system table which concerns on the 3rd Embodiment of this invention, Comprising: (a)-(c) shows exposure system table AC, respectively. It is a flowchart which shows operation | movement of the control part 102 which concerns on the modification of the 3rd Embodiment of this invention.

Embodiments of an image forming apparatus according to the present invention will be described below with reference to the drawings.
[1] First Embodiment First, a first embodiment of the present invention will be described. The image forming apparatus according to the present embodiment prevents image quality deterioration due to discharge products by switching the modulation method of the exposure apparatus according to the relative humidity in the apparatus.

(1) Configuration of Image Forming Apparatus First, the configuration of the image forming apparatus according to the present embodiment will be described.
FIG. 1 is a diagram showing the main configuration of a full-color electrophotographic image forming apparatus according to the present embodiment. As shown in FIG. 1, the image forming apparatus 1 includes an image forming unit 101Y to 101K, a control unit 102, a primary transfer roller 103Y to 103K, an intermediate transfer belt 104, a secondary transfer roller pair 105, a fixing device 106, and a paper discharge. A roller 107, a paper discharge tray 108, a cleaner 109, and a paper feed cassette 110 are provided.

  The image forming units 101Y to 101K form toner images of Y (yellow), M (magenta), C (cyan), and K (black), respectively, under the control of the control unit 102. The toner images of each color of YMCK are electrostatically transferred (primary transfer) so as to overlap on the intermediate transfer belt 104 by the primary transfer rollers 103Y to 103K. The intermediate transfer belt 104 is an endless rotating body that rotates in the direction of arrow A and conveys the toner image to the secondary transfer position.

  The paper feed cassette 110 stores recording sheets S, and the recording sheets S are supplied one by one from the top by the pickup roller 111. The supplied recording sheet S is conveyed to the secondary transfer position in parallel with the intermediate transfer belt 104 conveying the toner image. The secondary transfer roller pair 105 electrostatically transfers (secondary transfer) the toner image on the intermediate transfer belt 104 onto the recording sheet S.

The recording sheet S to which the toner image is transferred is conveyed to the fixing device 106. The fixing device 106 is an electromagnetic induction heating type fixing device, and heats the toner image and fuses it to the recording sheet S. The recording sheet S to which the toner image has been fused is discharged onto a discharge tray 108 by a discharge roller 107.
Note that a developer container (so-called toner bottle) containing developer (toner) is detachably attached to the image forming apparatus 1, and the image forming units 101Y to 101K are connected to the developer containers 112Y to 112K, respectively. Get replenished with developer. The toner remaining on the intermediate transfer belt 104 after the secondary transfer is removed by the cleaner 109 and discarded.

In addition, since image formation by electrophotography changes in image quality depending on environmental conditions such as temperature and humidity and usage conditions of the user, the control unit 102 appropriately executes image adjustment control to keep the image quality constant.
(2) Configuration of Image Creating Unit 101K Next, the configuration of the image creating unit 101K will be described by focusing on the portions that are also common to the image creating units 101Y to 101M. Therefore, the following description is applicable to any of the image forming units 101Y to 101K.

FIG. 2 is a diagram illustrating a main configuration of the image forming unit 101K. As shown in FIG. 2, the image forming unit 101 </ b> K includes a photosensitive drum 201, a charging device 202, an exposure device 203, a developing device 204, and a cleaning device 205. The device 203, the developing device 204, and the cleaning device 205 are arranged in this order.
The photosensitive drum 201 is rotated by driving means (not shown) and rotates in the direction of arrow A. The charging device 202 uniformly charges the outer peripheral surface of the photosensitive drum 201 by corona discharge. The exposure apparatus 203 has a semiconductor laser element, and adjusts the amount of laser light irradiated onto the outer peripheral surface of the charged photosensitive drum 201 as described later under the control of the control unit 102, thereby making the photosensitive drum 201. An electrostatic latent image is formed on the outer peripheral surface.

The developing device 204 supplies toner onto the outer peripheral surface of the photosensitive drum 201 to develop (visualize) the electrostatic latent image. The developing device 204 includes a developing roller 206, and a developing bias is applied to the developing roller 206 from a power source (not shown). An electrostatic attraction is applied by the developing bias, and toner is supplied from the developing device 204 to the photosensitive drum 201.
The cleaning device 205 brings the blade 207 into contact with the outer peripheral surface of the photosensitive drum 201 to mechanically scrape off the toner remaining on the outer peripheral surface of the photosensitive drum 201 after the primary transfer. Thereafter, the photosensitive drum 201 is exposed to light by an unillustrated eraser lamp, so that the outer peripheral surface of the photosensitive drum 201 is discharged. By repeating this, image formation is executed one after another.

FIG. 3 is an external perspective view illustrating the configuration of the charging device 202. The charging device 202 is a long device disposed along the rotation axis direction of the photosensitive drum 201. As shown in FIG. 3, the charging device 202 includes a saw-tooth electrode 301 having substantially the same length as the photosensitive drum 202. I have. The saw-tooth electrode 301 is a saw-tooth shaped discharge electrode and is used for corona discharge.
The sawtooth electrode 301 is stored and held in a shield case 302 having a rectangular cross section. The shield case 302 has an opening at a position facing the photosensitive drum 201, and the opening is covered with a grid mesh 303. In FIG. 3, a part of the grid mesh 303 is cut away to show the sawtooth electrode 301 stored in the shield case 302.

The saw-tooth electrode 301 faces the outer peripheral surface of the photosensitive drum 201 with the grid mesh 303 interposed therebetween. When the photosensitive drum 201 is charged, a high voltage (for example, −5 kV to −7 kV when the photosensitive drum 201 having negative charging characteristics is used) is applied to the sawtooth electrode 301. As a result, an air discharge is generated at the tip of the sawtooth electrode 301.
The shield case 302 stabilizes the air discharge by the sawtooth electrode 301 regardless of environmental conditions. As a result, the charge generated by the sawtooth electrode 301 efficiently acts on the photosensitive drum 201. Further, a thin stainless steel plate having a thickness of about 0.1 mm to 0.2 mm is used for the grid mesh 303, and a voltage of −400 V to −1200 V is applied by a power source (not shown). As a result, the potential on the outer peripheral surface of the photosensitive drum 201 is controlled.

(3) Configuration of Control Unit 102 Next, the configuration of the control unit 102 will be described.
FIG. 4 is a block diagram illustrating a main configuration of the control unit 102. As shown in FIG. 4, the control unit 102 includes a central processing unit (CPU) 401, a read only memory (ROM) 402, and a random access memory (RAM) 403.

  The CPU 401 reads out a control program recorded in the ROM 402 and executes various controls using the RAM 403 as a working storage area. The CPU 401 is connected to a relative humidity sensor 410, a photosensitive drum driving motor 411, a charging device 202, an exposure device 203, a developing device 204, and the like. For example, a stepping motor may be used as the photosensitive drum driving motor 411 that rotationally drives the photosensitive drum 302.

  Prior to image formation, the CPU 401 refers to the relative humidity sensor 410 to acquire a relative humidity value. Then, according to the acquired relative humidity value, the image data is processed to generate control data for controlling the output of the semiconductor laser element of the exposure apparatus 203 during image formation. Thereafter, the CPU 401 drives and controls the photosensitive drum driving motor 411, the charging device 202, the exposure device 203, the developing device 204, and the like, and executes image formation.

  As the relative humidity sensor 410, for example, by applying an AC voltage between two plate-like electrodes sandwiching the moisture sensitive body, a capacitance between the electrodes caused by a change in dielectric constant accompanying moisture absorption of the moisture sensitive body. A capacitive sensor that measures humidity based on the change in the resistance, or a resistive sensor that utilizes a change in conductivity accompanying moisture absorption by the moisture sensitive body may be used. The relative humidity sensor 410 measures the relative humidity in the vicinity of the photosensitive drum 302.

(4) Modulation method Next, the gradation expression method used in this embodiment will be described. In the present embodiment, two types of gradation expression methods, an area modulation method and an intensity modulation method, are used properly.
When the area modulation method is used, the gray scale is simulated by a dither method.
The dither method uses a plurality of vertically and horizontally adjacent pixels as one matrix, and binarizes with different thresholds corresponding to each pixel in the matrix to turn dots on / off (with or without exposure). Output. Therefore, when the dither method is adopted, the influence of environmental conditions such as temperature and humidity is relatively small, and the matrix is much larger than the pixels, so that gradation expression is excellent.

  FIGS. 5A and 5B are graphs showing examples of the photoreceptor potential when various modulation methods are used. FIGS. 5A and 5C show the case of the area modulation method, and FIGS. 5B and 5D show the intensity modulation method. In particular, (c) and (d) show the photoreceptor potential when there is image flow. In FIG. 5, Vo is a surface potential of the photosensitive drum 302 after charging, Vi is a surface potential of the photosensitive drum 302 after exposure, Vdc is a developing bias applied to the developing roller 206, and GND is a ground potential. At the time of development, toner is electrostatically adsorbed on a region of the outer peripheral surface of the photosensitive drum 302 where the surface potential is in the range of Vi to Vdc.

In the area modulation method, as shown in FIG. 5A, the ratio of the area of the surface potential Vi to the area of the matrix (the area ratio of the toner dots) is determined according to the gradation value for each matrix. Is done. In this way, it is difficult to express a clear edge such as a line image because the gradation is expressed by an aggregate of dots.
The intensity modulation method is a method of expressing the gradation for one dot to be printed by changing the laser light quantity (= light emission time × laser intensity) according to the gradation value at the time of exposure.

  In the intensity modulation method, as shown in FIG. 5B, the entire exposure range is exposed by adjusting the intensity of the laser light amount according to the gradation with respect to the surface potential Vo of the photosensitive drum 302 after charging. A uniform post-exposure potential Vi is formed. Next, dark and light toner dots corresponding to the potential difference Vdc−Vi are formed by the developing roller 206 to which the developing bias Vdc is applied, and gradation is expressed.

As described above, the intensity modulation method can express the edge portion more clearly than the area modulation method, while the area modulation method is excellent in gradation expression and has advantages and disadvantages.
When the nitric acid compound generated by corona discharge of the charging device 202 adheres to the outer peripheral surface of the photosensitive drum 302, the nitric acid compound has conductivity, so that the charges generated by the exposure are generated on the outer peripheral surface of the photosensitive drum 302. Flows beyond the exposure area. The reason for this is that if the surface potential is different on the outer peripheral surface of the photoconductive drum 302, charge transfer due to the nitric acid compound may occur regardless of whether it is the area modulation method or the intensity modulation method.

That is, charge flows from the non-exposed portion having the surface potential Vo to the exposed portion having the surface potential Vi, the surface potential of the non-exposed portion increases, and the surface potential of the exposed portion decreases. As a result, a so-called fog margin, which is a potential difference between the surface potential of the non-exposed portion and the developing roller potential Vdc, is reduced, and background fog occurs.
This background fog occurs at the boundary between the exposed part and the non-exposed part. In the area modulation method, as shown in FIG. 5C, there are many boundaries between the exposed part and the non-exposed part. On the other hand, in the intensity modulation method, since the boundary portion is small as shown in FIG. 5D, the influence of background fogging is limited.

Since charge transfer due to nitrate compounds occurs particularly under high humidity, in the present embodiment, with reference to relative humidity, an intensity modulation method that is less susceptible to background fogging is preferentially applied at high humidity. Thus, the deterioration of the image quality due to the discharge product is prevented.
(5) Operation of Control Unit 102 Next, the operation of the control unit 102 will be described.

  FIG. 6 is a flowchart showing the operation of the control unit 102. As shown in FIG. 6, when the image forming apparatus 1 is turned on, the control unit 102 performs a so-called boot process and then obtains the relative humidity value RH with reference to the relative humidity sensor 410 (S601). ). If the relative humidity value RH is less than 30% (S602: YES), an exposure method 1 described later is adopted (S603). If the relative humidity value RH is 30% or more (S602: NO), the exposure method 2 is adopted (S604).

  In this embodiment, the exposure method 1 divides image data into an edge portion and a region other than the edge portion (hereinafter referred to as “halftone portion”) by edge detection, and an area modulation method is used for the halftone portion. This is an exposure method in which an intensity modulation method is applied to the edge portion. When the relative humidity value RH is less than 30%, the nitric acid compound has a high electric resistance, so that even if the area modulation method is applied to the halftone portion, it is difficult to cause image deterioration, but it is an advantage of the area modulation method. Gradation expression can be obtained.

The exposure method 2 is an exposure method in which the intensity modulation method is applied to both the edge portion and the halftone portion. When the relative humidity value RH is 30% or more, the nitric acid compound has a low electric resistance, so that the intensity modulation method is also applied to the halftone portion to prevent image deterioration.
Note that the edge is clearly expressed because the intensity modulation method is applied to the edge portion regardless of the relative humidity value RH. Further, for edge detection, for example, a Gaussian filter may be applied to the image data, and the effect of the present invention can be obtained even if the edge is detected by other methods.

Thereafter, when accepting the image forming job (S605: YES), the control unit 102 executes the image forming job using the adopted exposure method (S606).
In the present embodiment, the exposure method is changed depending on whether or not the relative humidity value RH is 30% or more. However, it goes without saying that the present invention is not limited to this, and values other than 30% are used as reference values. The exposure method may be changed as follows.

(6) Modification The relative humidity in the image forming apparatus 1 can change at any time depending on the external environment and usage conditions of the image forming apparatus 1. For this reason, the control unit 102 may perform the following operation.
FIG. 7 is a flowchart showing the operation of the control 102 according to this modification. As shown in FIG. 7, when the control unit 102 receives the image forming job after finishing the boot process after turning on the power (S701: YES), the controller 102 refers to the relative humidity sensor 410 and acquires the relative humidity value RH. (S702).

If the acquired relative humidity value RH is less than 30% (S703: YES), the above-described exposure method 1 is adopted (S704). If the relative humidity value RH is 30% or more (S703: NO), the above-described exposure method 2 is adopted (S705). Thereafter, an image forming job is executed using the adopted exposure method (S706).
In this way, even if the relative humidity in the image forming apparatus 1 fluctuates and the electric resistance of the nitric acid compound adhering to the outer peripheral surface of the photosensitive drum 302 fluctuates, the fluctuation is optimized. By adopting an exposure method, excellent image quality can be realized. Note that the relative humidity in the image forming apparatus 1 and the electrical resistance of the nitric acid compound do not fluctuate very quickly, so it is sufficient to select an appropriate exposure method each time an image forming job is received.

[2] Second Embodiment Next, a second embodiment of the present invention will be described. The image forming apparatus according to the present embodiment has substantially the same configuration as that of the image forming apparatus 1 according to the first embodiment, but differs in the manner of determining the exposure method. Hereinafter, the description will be given focusing on the difference.
(1) Configuration of Control Unit First, the configuration of the control unit included in the image forming apparatus according to the present embodiment will be described. FIG. 8 is a block diagram showing a main configuration of the control unit according to the present embodiment. In FIG. 8, since there are members corresponding to those in the first embodiment, the same reference numerals are given, and for those members, refer to the above description.

  In this embodiment, a new feature is that a nonvolatile memory 801 and a timer 802 are connected to the CPU 401. The non-volatile memory 801 is an overwritable memory that can hold data even without power supply. The timer 802 has a so-called backup power source, and continues to count up while the power source of the image forming apparatus main body is turned off.

(2) Operation | movement of a control part Next, operation | movement of the control part 102 which concerns on this Embodiment is demonstrated. FIG. 9 is a flowchart showing the operation of the control unit according to the present embodiment. As shown in FIG. 9, when the image forming apparatus is powered on, the control unit 102 refers to the timer value recorded in the nonvolatile memory 801 (S901). This timer value is obtained by recording the timer value obtained by referring to the timer 802 when the image forming apparatus was previously turned off.

Further, the control unit 102 refers to the timer 802 (S902), obtains the current timer value, compares it with the timer value recorded in the nonvolatile memory, and calculates the waiting time T of the image forming apparatus. (S903). Further, the control unit 102 refers to the relative humidity sensor 410 and detects the relative humidity value RH in the image forming apparatus (S904).
The control unit 102 refers to a table (hereinafter referred to as “exposure method table”) in order to determine an appropriate exposure method from the standby time T and the relative humidity value RH (S905). FIG. 10 is a table illustrating an exposure method table. As shown in FIG. 10, the exposure method table is a table for designating either the exposure method 1 or 2 for each combination of the relative humidity value RH and the standby time T.

  As described above, the exposure method 2 is adopted because the electrical resistance of the nitric acid compound decreases as the relative humidity increases. Conversely, the exposure method 1 is employed because the electrical resistance of the nitric acid compound increases as the relative humidity decreases. . Further, since the nitric acid compound adhering to the photosensitive drum 302 increases as the standby time T is longer, the exposure method 2 is adopted. Conversely, as the standby time T is shorter, the exposure method 1 is adopted.

After determining the exposure method as described above (S906), when the user turns off the main power supply of the image forming apparatus (S907: YES), the control unit 102 refers to the timer 802 (S908), and the read timer value. Is recorded in the nonvolatile memory 801 (S909), and then the main power supply is shut off.
If the main power is not turned off (S907: NO) and an image forming job is received (S910), the image forming job is executed using the exposure method determined as described above (S911).

In the present embodiment, the waiting time T of the image forming apparatus is long. Therefore, when there is a large amount of nitric acid compound adhering to the photosensitive drum 302, image degradation due to the nitric acid compound may occur even if the relative humidity is not so high. According to this embodiment, such image deterioration can also be prevented.
(3) Modification Next, a modification of the present embodiment will be described. The adhesion of the nitric acid compound to the photosensitive drum 302 can occur not only when the power of the image forming apparatus is turned off, but also while the image forming job is not being executed. In this modification, the exposure method is changed by including such a time in the waiting time T.

  FIG. 11 is a flowchart showing the operation of the control unit 102 according to this modification. As shown in FIG. 11, when the control unit 102 receives an image forming job (S1101: YES), the control unit 102 refers to the nonvolatile memory 801 to acquire a recorded value (S1102), and refers to the timer 802 (S1103). The standby time T is calculated from the difference between the current timer value and the recorded value (S1104).

  Next, the control unit 102 detects the relative humidity value RH with reference to the relative humidity sensor 410 (S1105). Then, referring to the exposure method table (S1106), the exposure method is determined from the standby time T and the relative humidity value RH (S1107), and the determined exposure method is used. An image forming job is executed (S1108). After completing the execution of the image forming job, the control unit 102 refers to the timer 802 (S1109), records the timer value in the nonvolatile memory 801, and waits for the next image forming job.

In this way, it is possible to employ an appropriate exposure method even when the waiting time T between image forming jobs is long. Further, similarly to the modification of the first embodiment, the exposure method can be changed following the change in relative humidity.
[3] Third Embodiment Next, a third embodiment of the present invention will be described. The image forming apparatus according to the present embodiment has substantially the same configuration as that of the image forming apparatus according to the second embodiment, but differs in that the cumulative rotation speed of the photosensitive drum is taken into account when determining the exposure method. To do. Hereinafter, the description will be given focusing on the difference. Note that the reference numerals given to the components of the image forming apparatus are the same as those in the second embodiment.

(1) Operation of Control Unit 102 The control unit 102 according to the present embodiment is characterized in that the exposure method is determined from the relative humidity value RH, the standby time T, and the cumulative rotation number of the photosensitive drum.
FIG. 12 is a flowchart showing the operation of the control unit 102 according to the present embodiment. As shown in FIG. 12, when the main power of the image forming apparatus is turned on, the control unit 102 refers to the timer value recorded in the nonvolatile memory 801 (S1201) and refers to the timer 802. (1202) The standby time T is calculated (S1203).

  Further, the control unit 102 refers to the cumulative rotational speed N of the photosensitive drum 302 recorded in the nonvolatile memory 801 (S1204), and refers to the relative humidity sensor 410 to obtain the relative humidity value RH (see FIG. S1205). The control unit 102 records exposure method tables A to C in the non-volatile memory 801, and refers to different exposure method tables depending on the accumulated rotational speed N of the photosensitive drum 302.

  That is, when the cumulative rotation speed N is less than 200,000 times (S1206: 0 to 200k), the control unit 102 refers to the exposure method table A (S1207). If the cumulative rotation speed N is in the range of 200,000 times or more and less than 400,000 times (S1206: 200k to 400k), reference is made to the exposure method table B (S1208). If it is within the range of less than (S1206: 400k to 600k), the exposure method table C is referred to (S1209).

  FIG. 13 is a table illustrating an exposure method table, and (a) to (c) show the exposure method tables A to C, respectively. As shown in FIG. 13, each of the exposure method tables A to C is a table for designating one of the exposure methods 1 and 2 for each combination of the relative humidity value RH and the standby time T. In general, as the cumulative number of revolutions N of the photosensitive drum 302 increases, the amount of nitric acid compound adhering to the outer peripheral surface of the photosensitive drum 302 increases, so that the exposure method table A proceeds to the exposure method table C. Accordingly, the exposure method 2 is often designated.

With reference to any one of the exposure method tables A to C, the control unit 102 determines the exposure method.
Thereafter, when the power of the image forming apparatus main body is turned off (S1211: YES), the timer 802 is referred to (S1215), and the read timer value is recorded in the nonvolatile memory 801 (S1216). This timer value is used for calculating the waiting time T as described above.

  When an image forming job is received (S1212: YES), the image forming job is executed using the determined exposure method (S1213). When executing this image forming job, the CPU 401 calculates the rotational speed of the photosensitive drum 302 from the rotational speed of the photosensitive drum drive motor 411 and the gear ratio between the photosensitive drum drive motor 411 and the photosensitive drum 302. Then, the total value of the calculated rotational speed and the cumulative rotational speed N recorded in the nonvolatile memory 801 is recorded in the nonvolatile memory 801 as a new cumulative rotational speed N (S1214).

In this way, the exposure method is determined in consideration of the cumulative rotation speed N that indicates the amount of nitric acid compound on the outer peripheral surface of the photosensitive drum 302 in addition to the relative humidity value RH and the standby time T. Excellent image quality can be achieved.
(2) Modified Example Next, a modified example of the present embodiment will be described.

FIG. 14 is a flowchart showing the operation of the control unit 102 according to this modification. As shown in FIG. 14, when the control unit 102 accepts an image forming job after the image forming apparatus is activated (S1401: YES), the control value recorded in the non-volatile memory 801 and the photosensitive member are recorded. Reference is made to the cumulative rotational speed N of the drum 302 (S1401, 1405).
Further, the control unit 102 refers to the timer 802 (S1403), acquires the current timer value, calculates the standby time T as described above (S1404), and refers to the relative humidity sensor 410 to determine the relative humidity value. RH is detected (S1406).

  Next, when the cumulative rotation speed N of the photosensitive drum 302 is less than 200,000 times (S1407: 0 to 200k), the exposure method table A shown in FIG. 13 is referred to (S1408). If the cumulative rotation speed N is in the range of 200,000 times or more and less than 400,000 times (S1407: 200k to 400k), reference is made to the exposure method table B (S1409), and 400,000 times or more and 600,000 times. If it is within the range (S1407: 400k to 600k), the exposure method table C is referred to (S1410).

  Then, the exposure method is determined from the relative humidity value RH and the standby time T (S1411), the image forming job is executed (S1412), and the cumulative number of revolutions N recorded in the nonvolatile memory 801 is updated (S1413). ). Thereafter, the timer 802 is referred to (S1414), the current timer value is recorded in the nonvolatile memory 801 (S1415), and the next image forming job is awaited.

In this way, since the exposure method table is selected with reference to the accumulated rotational speed N every time an image forming job is executed, the image forming job can be executed using a more appropriate exposure method.
Although the case where the cumulative rotation number N of the photosensitive drum 302 has reached 600,000 is not particularly mentioned, for example, a display indicating that the useful life of the photosensitive drum 302 has passed has been made on the operation panel, Execution of the image forming job may be prohibited. In addition, an exposure method table may be prepared separately when the cumulative rotation speed N is 600,000 times or more.

Further, when selecting the exposure method table, instead of selecting a different exposure method table every time the cumulative rotation speed N reaches 200,000 times, the exposure method table may be switched based on another number of times. Further, the exposure method table illustrated in FIG. 13 is merely an example, and the setting content of the exposure method table may be different from the setting content illustrated in FIG.
[4] Modifications Although the present invention has been described based on the embodiments, it is needless to say that the present invention is not limited to the above-described embodiments, and the following modifications can be implemented. .

(1) In the above embodiment, the case where the sawtooth electrode is used to charge the outer peripheral surface of the photosensitive drum 302 has been described. However, it goes without saying that the present invention is not limited to this, and a wire is used instead of the sawtooth electrode. Alternatively, a charging roller may be used.
Regardless of the charging device used, according to the present invention, it is possible to prevent image deterioration caused by the deposition of a nitrate compound on the outer peripheral surface of the photosensitive drum.

  (2) Although not particularly mentioned in the above embodiment, since the fixing device is the main heat source in the image forming apparatus and the temperature tends to rise around the fixing device, the relative humidity is It tends to decline. For this reason, when a relative humidity sensor is installed in the vicinity of the fixing device, the obtained relative humidity value tends to be low, and the electrical resistance of the nitrate compound on the outer peripheral surface of the photosensitive drum 302 is not necessarily indicated.

In view of the object of the present invention to prevent the deterioration of image quality caused by the decrease in the electrical resistance of the nitric acid compound adhering to the outer peripheral surface of the photosensitive drum 302 due to moisture in the air, the photosensitive drum 302 is most affected. It is desirable to determine the exposure method with reference to the temperature around the distant photosensitive drum 302.
(3) Although not particularly mentioned in the above embodiment, if an interval for executing an image forming job is longer than a certain time (for example, 2 hours), cleaning at the time of image formation (by the cleaning device 205) Thus, whether or not the discharge product has been removed from the outer peripheral surface of the photosensitive drum 302 may be confirmed by measuring the electric resistance of the photosensitive member surface. If it is confirmed that the nitric acid compound is removed by the cleaning due to the decrease in electrical resistance, excellent gradation expression can be obtained by adopting the area modulation method in the halftone portion. .

Note that the test pattern is exposed to the photosensitive drum between the papers, that is, between the subsequent two image formations, and the surface resistance of the photosensitive member is measured in a non-contact manner using a surface potential sensor. You can ask for it.
(4) In the above embodiment, several exposure method tables have been exemplified. However, it goes without saying that the present invention is not limited to these, and even if the contents of the exposure method table are different from the above examples, the relative humidity values are not limited. The effect of the present invention is the same if the intensity modulation method is used for the halftone portion as the RH is higher or the standby time H is longer.

(5) Although not particularly mentioned in the above embodiment, the image forming job may be received by, for example, an operation panel, or may be received from another device such as a personal computer via a communication network.
(6) In the above embodiment, an appropriate exposure method is used by estimating the electrical resistance of the discharge product adhering to the outer peripheral surface of the photosensitive drum 302 from the relative humidity value RH, the standby time T, and the cumulative rotational speed N. Although the case of selecting is described, it is needless to say that the present invention is not limited to this. Instead, the electric resistance of the discharge product deposited on the outer peripheral surface of the photosensitive drum 302 by other means, or the electric resistance is changed. An exposure method may be selected by measuring an index to be indexed.

(7) Although not particularly mentioned in the above embodiment, for example, when a plurality of photosensitive drums are provided as in a tandem type color image forming apparatus, an exposure method is provided between the plurality of photosensitive drums. May be matched. In such a case, for example, the relative humidity sensor 410 may be disposed in the vicinity of the photosensitive drum 302 farthest from the fixing device 106.
Regarding the cumulative rotational speed N, for example, when the cumulative rotational speed N of the K photosensitive drum 302 in YMCK is larger than the cumulative rotational speed N of the other photosensitive drums 302, the K photosensitive drum By adopting the cumulative rotation speed N of 302, it is possible to prevent image quality deterioration.

(8) In the above embodiment, the exposure method is determined using the exposure method table. However, the present invention is not limited to this, and the exposure method may be determined using other methods.
For example, in the case of the second embodiment, a threshold value of the standby time T is determined for each relative humidity value RH, and if the standby time T is longer than the threshold value determined from the relative humidity value RH, the strength of the halftone portion is increased. A modulation method may be employed, otherwise an area modulation method may be employed.

  Alternatively, if the product of the relative humidity value RH and the standby time T is larger than a predetermined threshold value, the intensity modulation method may be adopted for the halftone part, and otherwise the area modulation method may be adopted. Further, the predetermined threshold value may be a threshold value that decreases as the cumulative rotational speed N increases.

  The image forming apparatus according to the present invention is useful as an apparatus for preventing image deterioration caused by adhesion of nitrogen oxides or the like generated during charging to the surface of the photoreceptor.

1 ... Image forming apparatuses 101Y to 101K ... Image forming unit 102 ... Control units 103Y to 103K ... Primary transfer roller 104 ... ... Intermediate transfer belt 201 ……………… Photosensitive drum 202 ……………… Charging device 203 ……………… Exposure device 204 ……………… Developing device 205 ……………… Cleaning device 206 ……………… Developing roller 207 ……………… Blade 301 ……………… Sawtooth electrode 302 ……………… Shield case 303 ……………… Grid mesh 401 ………………… CPU
402 …………… ROM
403 …………… RAM
410... ...... Relative humidity sensor 411... ...... Photosensitive drum drive motor 801.

Claims (6)

Exposure means for exposing a uniformly charged photoconductor based on image data to form an electrostatic latent image;
An acquisition means for acquiring an index indicating the electrical resistance of the discharge product attached to the surface of the photoreceptor;
A determination means for comparing the index with a predetermined threshold to determine the level of the electrical resistance;
In the image data other than the edge region, the exposure means may be one of an area modulation method for expressing a gradation by the size of a halftone dot area and an intensity modulation method for expressing a gradation by the density of a pixel. A method determining means for exposing to,
The image forming apparatus according to claim 1, wherein the method determining unit performs exposure using an intensity modulation method when it is determined that the electric resistance is low, and using an area modulation method when the electric resistance is determined to be high. The index is the relative humidity in the vicinity of the photoreceptor,
The image forming apparatus according to claim 1, wherein the determination unit determines that the electrical resistance is low when the relative humidity is higher than a predetermined humidity value. The index is a relative humidity in the vicinity of the photoconductor and a standby time between subsequent image forming processes,
The determination means determines that the electrical resistance is low when the relative humidity is higher than a predetermined humidity value and the standby time is longer than a predetermined time corresponding to the predetermined humidity value. The image forming apparatus according to claim 1. The photoconductor is a rotating body,
Comprising a counting means for counting the cumulative number of rotations of the photoreceptor;
The image forming apparatus according to claim 3, wherein the determination unit decreases the predetermined humidity value and shortens the predetermined time as the cumulative number of rotations increases. The image forming apparatus according to claim 1, wherein an intensity modulation method is always used for an edge region in the image data. The image forming apparatus according to claim 1, wherein the discharge product is a nitric acid compound.

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