Image forming apparatus and image forming method

Abstract

To provide an image forming apparatus and an image forming method with which it is possible to prevent a reduction in developability during use of a toner with a reduced amount of added strontium titanate and fogging after the apparatus is left standing for a long period.SOLUTION: An image forming apparatus uses a developing roller carrying two-component developer on its surface to develop an electrostatic latent image formed on an image carrier to form a toner image, and transfers the toner image formed on the image carrier to a sheet. The image forming apparatus comprises a developing power supply that supplies, to the developing roller, a developing bias that is a predetermined voltage and has a DC component and an AC component, a transfer power supply that supplies, to a transfer device, a transfer current that is a predetermined current; and a control unit that performs control of the developing bias and the transfer current. The control unit checks the cumulative driving time of the developing roller (time obtained by accumulating driving time of developing roller) until immediately before the start of developing the electrostatic latent image, and as the cumulative driving time increases, changes the AC component of the developing bias and the transfer current to be increased from respective reference values.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus and an image forming method that form an image using a two-component developer. [Background technology]

[0002] Some toners used in image forming devices contain strontium titanate as a charge leak agent (charge control agent) for environmental reasons. Patent Document 1 discloses that 0.1 to 10 wt % of strontium titanate is added to stabilize the chargeability of the toner. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2020-091407 Summary of the Invention [Problem to be solved by the invention]

[0004] Strontium titanate takes on an angular shape when it is made into a fine powder, making it highly abrasive to the photosensitive drum. For this reason, if the amount of strontium titanate added to the toner is large, the photosensitive drum is more likely to be scratched, and these scratches can cause filming (a thin layer of toner remaining on the photosensitive drum surface after the cleaning process), resulting in poor images.

[0005] To prevent filming, it is necessary to keep the amount of strontium titanate added below a certain percentage, but in that case, the charge leakage effect in the toner is reduced, so the toner charge increases too much (charge-up) during heavy printing, causing a problem of reduced developability.Heavy printing refers to printing a large number of sheets in a short period of time.

[0006] One possible method for suppressing charge buildup during heavy printing is to increase the toner content (the weight ratio of toner to the total developer) in the developer (toner and carrier) (for example, by increasing the amount of toner replenished to the developer tank during process control), thereby reducing the number of times the carrier comes into contact with the toner and suppressing toner charging. However, with this method, if the developer is left for a long time with an increased toner content, the toner charge will be insufficient when an image is formed after the long period of time, making fogging (a phenomenon in which toner is deposited in non-image areas) more likely to occur. This is because, although the toner charge decreases due to discharge during the long period of time of leaving the developer, the high toner content (relatively low carrier content) makes it difficult for the toner to charge when the image is subsequently formed.

[0007] The present invention has been made in consideration of the above-mentioned problems, and aims to provide an image forming apparatus and an image forming method that can suppress a decrease in developability and fogging after long-term use when using toner with a reduced amount of strontium titanate added. [Means for solving the problem]

[0008] In order to solve the above problems, the image forming apparatus of a first aspect of the present invention is an image forming apparatus that performs an image formation process in which an electrostatic latent image formed on an image carrier is developed to form a toner image using a developing roller that carries on its surface a two-component developer containing toner and carrier, and the toner image formed on the image carrier is transferred to a sheet, and is equipped with a developing power supply that supplies a developing bias having a predetermined voltage and a DC component and an AC component to the developing roller, a transfer power supply that supplies a predetermined transfer current to a transfer device that transfers the toner image formed on the image carrier to the sheet, and a control unit that measures and accumulates the driving time of the developing roller at any time, stores the accumulated time in a memory unit as the accumulated driving time, and controls the developing bias and the transfer current, and is characterized in that the control unit checks the accumulated driving time of the developing roller up to just before the start of development of the electrostatic latent image, and changes the AC component of the developing bias and the transfer current so that they are increased from reference values ​​as the accumulated driving time increases.

[0009] According to the above configuration, especially when using toner with a reduced amount of strontium titanate, by changing the development bias and transfer current according to the cumulative driving time of the developing roller, it is possible to suppress deterioration of developability due to toner charge-up during heavy printing. In addition, this method also makes it possible to maintain the toner content in the two-component developer at a standard value and suppress fogging after long-term storage.

[0010] Furthermore, in the image forming apparatus, the control unit may include an accumulated drive time determination unit that measures the drive time of the developing roller within the most recent predetermined time period and determines the accumulated drive time based on the measured drive time of the developing roller, and an unused time determination unit that determines the unused time since the previous image forming process was completed, and the accumulated drive time determination unit may be configured to determine the accumulated drive time by correcting the measured drive time of the developing roller so that the longer the unused time determined by the unused time determination unit is, the shorter the accumulated drive time becomes.

[0011] According to the above configuration, the developing bias and the transfer current can be changed in consideration of the decrease in the charge amount of the toner depending on the time left after image formation.

[0012] The image forming apparatus may be configured such that the storage unit stores a look-up table in which the AC component of the developing bias and the transfer current are set according to the cumulative driving time.

[0013] According to the above configuration, the development bias and transfer current can be changed and controlled through simple control using a lookup table.

[0014] The image forming apparatus further includes an image density sensor that measures the density of a toner image formed on the image carrier, and the control unit includes an image quality adjustment processing unit that performs process control, which is one type of image quality adjustment processing. When performing process control, the image quality adjustment processing unit forms a toner patch on the image carrier using a developing bias whose AC component is changed in accordance with the cumulative driving time, corrects the measured density of the formed toner patch detected by the image density sensor in accordance with a transfer current that corresponds to the cumulative driving time, and determines the DC component of the developing bias based on the corrected toner patch density.

[0015] According to the above configuration, in the process control, it is possible to apply change control of the development bias and transfer current according to the accumulated driving time of the development roller.

[0016] An image forming method according to a second aspect of the present invention is an image forming method in which an electrostatic latent image formed on an image carrier is developed to form a toner image by a developing roller carrying a two-component developer containing toner and carrier on its surface, and the toner image formed on the image carrier is transferred to a sheet to form an image, the image forming method comprising the following steps: a first step of checking the cumulative drive time of the developing roller up to the point just before the start of development of the electrostatic latent image; a second step of changing, in accordance with the cumulative drive time, an AC component of a developing bias, which is a predetermined voltage supplied to the developing roller, and a transfer current, which is a predetermined current supplied to a transfer device that transfers the toner image to the sheet; and a third step of forming an image using the developing bias and transfer current changed in the second step, wherein, in the second step, the AC component of the developing bias and the transfer current are increased from their reference values ​​as the cumulative drive time increases. [Effects of the Invention]

[0017] The image forming apparatus and image forming method of the present invention, particularly when using toner with a reduced amount of strontium titanate added, can suppress a decrease in developability due to toner charging during heavy printing by changing the development bias and transfer current according to the cumulative driving time of the development roller, and can also suppress fogging after long-term storage by maintaining the toner content in the two-component developer at a standard value. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a schematic cross-sectional view showing an example of the configuration of an image forming apparatus to which the present invention can be applied. [Figure 2] FIG. 2 is a schematic diagram illustrating a configuration of a main part of an image forming unit included in a main body. [Figure 3] FIG. 2 is a block diagram showing a control system of the image forming apparatus. [Figure 4] 10 is a flowchart showing control in image forming processing after a job is accepted. [Figure 5] 10 is a flowchart showing control in a density adjustment process during process control. DETAILED DESCRIPTION OF THE INVENTION

[0019] [First embodiment] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. Note that the same components in the embodiments described below will be assigned the same reference numerals, and redundant descriptions of those components will be omitted.

[0020] First, a description will be given of the overall configuration of an image forming apparatus 10 in embodiment 1. Fig. 1 is a schematic cross-sectional view showing an example of the configuration of an image forming apparatus 10 to which the present invention can be applied.

[0021] The image forming apparatus 10 is configured to include a main body 11, a document reading unit 12, and a document transport device 13. The main body 11 has an internal image forming unit for forming (printing) an image on paper (sheets). The document reading unit 12 is disposed above the main body 11 and reads documents when copying the documents, etc. In automatic reading mode, the document transport device 13 transports documents placed on a document set tray sequentially toward the document placement table of the document reading unit 12.

[0022] 2 is a schematic diagram showing the configuration of the main parts of the image forming unit included in the main body 11. As shown in FIGS. 1 and 2, the image forming unit includes a photosensitive drum (image carrier) 111, a charger 112, an exposure device 113, a developing device 114, a transfer roller (transfer device) 115, a cleaning device 116, and a fixing device 117.

[0023] The charger 112 uniformly charges the surface of the photosensitive drum 111 to a predetermined potential. The exposure device 113 emits light to scan the photosensitive drum 111, which is the scanned body, exposing the surface of the photosensitive drum 111 to light and forming an electrostatic latent image. The development device 114 develops the electrostatic latent image on the surface of the photosensitive drum 111 to form a toner image on the surface of the photosensitive drum 111. In the development process, a development bias (having an AC component and a DC component) formed by superimposing a DC current and an AC current is supplied to the development roller 114a of the development device 114 by a development power source 114b.

[0024] Transfer roller 115 forms a nip area between itself and photosensitive drum 111, and transports paper transported through the paper transport path while sandwiching it in the nip area. As the paper passes through this nip area, the toner image on the surface of photosensitive drum 111 is transferred to the paper. In the transfer process, a transfer current consisting of a direct current is supplied to transfer roller 115 by transfer power supply 115a. The paper onto which the toner image has been transferred is then transported through the paper transport path to fixing device 117. Fixing device 117 sandwiches the paper onto which the toner image has been transferred between two rollers (a fixing roller and a pressure roller) and applies heat and pressure to fix the toner image to the paper. Cleaning device 116 has cleaning blade 116a that abuts against photosensitive drum 111, and removes residual toner from the surface of photosensitive drum 111 after transfer using cleaning blade 116a, and collects the removed residual toner.

[0025] At least one paper feed cassette 118 for stocking paper is provided in the lower part of the main body 11, and paper is separated one by one from a selected paper feed cassette 118 and transported toward the image forming unit. The image forming unit forms an image on the paper fed from the paper feed cassette 118. The paper on which the image has been formed is discharged to a paper discharge tray 119.

[0026] The main body 11 may have a reverse transport path and be configured to form an image not only on the front side but also on the back side of the paper. Furthermore, the image forming apparatus 10 may be provided with a manual feed tray that can be opened from the side to load paper. Furthermore, the illustrated image forming apparatus 10 is configured to transfer a toner image directly from the photosensitive drum 111 to the paper, but is not limited to this. A belt transfer system may also be used in which the toner image is transferred from the photosensitive drum 111 to an intermediate transfer belt, and then the toner image is transferred from the intermediate transfer belt to the paper.

[0027] The image forming apparatus 10 of the present disclosure uses a two-component developer consisting of toner and carrier. The toner is a resin powder with a particle size of 6 μm, composed of insulating materials such as styrene acrylic and polyester, and therefore becomes charged by friction. A change in the charge amount of this toner changes the developability and image density, so a charge control agent is added to the toner. Strontium titanate is added as a charge control agent for the toner contained in the developer of the present disclosure. Because strontium titanate is conductive, it can suppress so-called charge-up, in which the charge amount of the toner increases excessively due to frictional charging during heavy image formation, in which images are formed continuously on a large amount of paper. The effect of suppressing this charge-up increases with the amount of strontium titanate added. However, because strontium titanate has a particle size of several tens of nanometers and a very angular shape, adding more than 0.4 wt% of strontium titanate can cause strontium titanate that falls from the residual toner on the cleaning blade 116a to scratch the photosensitive drum 111, resulting in the residual toner adhering to the scratched area and causing filming, which leads to poor image quality. Therefore, the strontium titanate content in the toner is set to 0.1 to 0.4 wt%. A strontium titanate content within this range can suppress toner abrasion of the photosensitive drum 111, thereby suppressing poor image quality due to filming. However, under certain conditions, toner charge-up may not be effectively suppressed during heavy image formation. In such cases, it is possible to temporarily control the charge amount of the toner by replenishing toner and increasing the toner ratio (toner content) to the developer, but if the toner charge amount decreases due to natural discharge after a long period of time, the specified charge amount cannot be secured, and there is a risk of image defects (fogging) occurring.

[0028] The image forming apparatus 10 of the present disclosure maintains the toner content in the developer at a standard value (approximately 4 to 7%), while suppressing the deterioration of developability due to the buildup of toner charge and fogging after being left unused for a long period of time using a controlled method. The method will be described below.

[0029] FIG. 3 is a block diagram showing a control system for forming an image on paper in the image forming apparatus 10. As shown in FIG. 3, the image forming apparatus 10 includes a control unit 20, which controls the image forming unit, the drive source 21, and the image density sensor 120 (see FIG. 2). The image forming apparatus 10 may also include a communication unit 22, in which case the control unit 20 also controls the communication unit 22. The control unit 20 controls the charging power supply 112a, the exposure device 113, the development power supply 114b, the transfer power supply 115a, and other components of the image forming unit. The control unit 20 controls the development bias and transfer current by controlling the development power supply 114b and the transfer power supply 115a. The control unit 20 controls the drive source 21, which controls the drive sources of the photosensitive drum 111, the development roller 114a, and other components. The control unit 20 includes a processor for performing arithmetic operations, such as a CPU (Central Processing Unit) or an ASIC (Application Specific Integrated Circuit), and a memory for storing data and computer programs required for each process.

[0030] The control unit 20 includes an accumulated drive time determination unit 201 , a non-operating time determination unit 202 , an image quality adjustment processing unit 203 , and a storage unit 204 .

[0031] The control unit 20 measures and accumulates the driving time of the developing roller 114a at any time, and stores the accumulated time in the storage unit 204 as the accumulated driving time.

[0032] At the start of development of an electrostatic latent image, the cumulative drive time determination unit 201 checks and determines the cumulative drive time of the developing roller 114a up to that point. In this embodiment, the cumulative drive time of the developing roller 114a checked by the cumulative drive time determination unit 201 is reset when no image formation has been performed for 12 hours or more after the image formation process. In other words, the cumulative drive time determination unit 201 measures the drive time of the developing roller 114a within the most recent 12 hours, and can determine the cumulative drive time based on the measured drive time of the developing roller 114a.

[0033] The idle time determination unit 202 checks and determines the idle time since the previous image forming process ended. The image quality adjustment processing unit 203 performs process control, which is one type of image quality adjustment, every time images are formed on a predetermined number of sheets of paper. In process control, multiple toner patches are formed on the surface of the photosensitive drum 111 using developing biases with different DC components, the densities of these toner patches are detected by the image density sensor 120, and the DC component of the developing bias is adjusted to obtain a predetermined density. The memory unit 204 stores various data necessary for controlling the image forming apparatus 10.

[0034] Next, a description will be given of control when the image forming apparatus 10 accepts the execution (job) of image forming processing. Fig. 4 is a flowchart showing control in image forming processing after the job is accepted.

[0035] When image forming apparatus 10 accepts a job, cumulative drive time determination unit 201 checks the cumulative drive time, and idle time determination unit 202 checks the idle time (S1: first step). Furthermore, the cumulative drive time is corrected based on the idle time (S2). This correction is performed by multiplying the cumulative drive time by a correction factor corresponding to the idle time.

[0036] Table 1 below is an example of a lookup table showing the correction rate according to the unused time. The lookup table of Table 1 is stored in the storage unit 204. For example, if the cumulative drive time confirmed in S1 is 46 minutes and the unused time is 7 hours, the correction rate for the unused time is 5% according to Table 1, so the corrected cumulative drive time (corrected cumulative drive time) is 46 x 0.05 = 2.3 (minutes). Also, if the cumulative drive time confirmed in S1 is 46 minutes and the unused time is 2 hours, the corrected cumulative drive time is 46 x 0.6 = 10.2 (minutes).

[0037] [Table 1] Once the corrected cumulative drive time is calculated, a cumulative drive time area is determined based on this (S3). The following Table 2 is an example of a lookup table for determining the cumulative drive time area. The lookup table of Table 2 is stored in the storage unit 204. From Table 2, for example, if the corrected cumulative drive time is 2.3 (minutes), the cumulative drive time area is "1", and if the corrected cumulative drive time is 10.2 (minutes), the cumulative drive time area is "4".

[0038] [Table 2] Once the cumulative drive time area is determined, the AC component of the developing bias and the transfer current are changed based on this cumulative drive time area (S4: second step). In this embodiment, the larger the cumulative drive time area, the longer the corrected cumulative drive time, making it more likely that toner charge-up will occur. Therefore, in the process of S4, the larger the cumulative drive time area, the more the developing bias and transfer current are changed to compensate for the deterioration of developability due to toner charge-up. Table 2 below is an example of a lookup table showing the AC component of the developing bias and the change value of the transfer current according to the cumulative drive time area. The lookup table of Table 3 is stored in memory unit 204.

[0039] [Table 3] As shown in Table 3, the AC component of the developing bias is changed so that it becomes larger than the reference value of 500 V, which is the value of cumulative driving time area 1, as the value in the cumulative driving time area increases. This makes it possible to compensate for the decrease in developability due to toner charge-up and improve developability.

[0040] Furthermore, the larger the value in the cumulative drive time area, the larger the transfer current is changed to be above the reference value of 15 μA, which is the value in cumulative drive time area 1. This is because toner charging reduces not only the developability but also the transferability, so increasing the transfer current improves the transfer rate from the photosensitive drum 111 to the paper. In other words, by compensating for the decrease in transferability by increasing the transfer current, the desired image density can be obtained in the image formed on the paper.

[0041] In addition, the above control may also involve changing the toner content in the developer. That is, as the value of the cumulative driving time area increases, the toner ratio (toner content) in the developer may be increased to suppress toner charge-up. However, as mentioned above, if the toner content is increased too much, the toner charge amount decreases after a long period of use, causing the problem of fogging. Therefore, the toner content is changed within a standard value range that does not cause fogging after a long period of use.

[0042] When the development bias and transfer current are changed in S4, the image forming process (that is, printing of the job) is performed under these conditions (S5: third step).

[0043] Table 4 below shows the results when the control of the present disclosure is performed. Note that Table 4 only shows examples when the cumulative drive time area is "1" and "6" to "8." The AC component of the development bias and the transfer current in this control follow the lookup table in Table 3. Note that "roughness" in Table 4 refers to image defects caused by toner charge-up (particularly when a halftone image is formed, the surface is not uniform and appears rough).

[0044] [Table 4] As shown in Table 4, the toner charge amount increases as the cumulative drive time area value increases, but the application of the control of the present disclosure has the effect of suppressing roughness. For comparison, in a control in which only the AC component of the development bias was changed according to the cumulative drive time, and the transfer current was not changed, the effect of suppressing roughness was not sufficient when the cumulative drive time area was "6" to "8".

[0045] Furthermore, when the control of the present disclosure is applied, the toner content in the developer can be maintained within the standard value range. Therefore, even when an image formation process (job printing process) is performed after three days of use from a state where the cumulative drive time area is "8," the toner charge amount can be quickly increased by normal stirring, and it was confirmed that the occurrence of fogging can be suppressed.

[0046] Second Embodiment In the first embodiment, the control of the present disclosure is applied at the start of image formation processing. However, this control is not limited to the above application, and can also be applied during process control (image quality adjustment processing).

[0047] Figure 5 is a flowchart showing the control in the density adjustment process during process control. In the control of Figure 5, the processes of S1 to S3 are the same as the control of Figure 4. Once the cumulative drive time area is determined in the process of S3, the AC component of the development bias is changed based on this cumulative drive time area (S14). Furthermore, while applying the development bias with the changed AC component, toner patches (multiple toner patches created by development biases with different DC components) are formed on the photosensitive drum 111, and the density of each of the formed toner patches is measured (S15).

[0048] Next, the DC component of the developing bias that will provide the required image density is determined from the density information of each toner patch obtained in S15 (S16). That is, the DC component of the developing bias that will provide the desired toner patch density is set.

[0049] The embodiments disclosed herein are illustrative in all respects and are not intended to be limiting. Therefore, the technical scope of the present invention should not be interpreted solely by the above-described embodiments, but should be determined based on the claims. [Explanation of symbols]

[0050] 10 Image forming device 11 Main body 111 Photosensitive drum (image carrier) 114 Developing device 114a Developing roller 114b Development power supply 115 Transfer roller (transfer device) 115a Transfer power supply 20 Control Unit 201 Accumulated drive time determination unit 202 Standing time determination unit 203 Image quality adjustment processing unit 204 Storage section

Claims

1. An image forming apparatus that performs an image forming process in which an electrostatic latent image formed on an image carrier is developed by a developing roller that carries a two-component developer containing toner and carrier on its surface to form a toner image, and the toner image formed on the image carrier is transferred to a sheet, a development power supply that supplies a development bias having a predetermined voltage and a DC component to the development roller; a transfer power supply that supplies a transfer current, which is a predetermined current, to a transfer device that transfers the toner image formed on the image carrier onto a sheet; a control unit that measures and accumulates the driving time of the developing roller as needed, stores the accumulated time in a storage unit as an accumulated driving time, and controls the developing bias and the transfer current; The control unit checks the cumulative driving time of the developing roller up to the point just before the start of development of the electrostatic latent image, and the longer the cumulative driving time, the greater the AC component of the developing bias and the transfer current are increased from the reference values.

2. 2. The image forming apparatus according to claim 1, The control unit an accumulated driving time determination unit that measures a driving time of the developing roller within a most recent predetermined time period and determines an accumulated driving time based on the measured driving time of the developing roller; a leaving time determination unit that determines the leaving time since the previous image forming process was completed, The image forming apparatus is characterized in that the cumulative drive time determination unit determines the cumulative drive time by correcting the measured drive time of the developing roller so that the longer the unused time determined by the unused time determination unit, the shorter the cumulative drive time becomes.

3. 2. The image forming apparatus according to claim 1, The image forming apparatus is characterized in that the storage unit stores a look-up table in which the AC component of the developing bias and the transfer current corresponding to the cumulative driving time are set.

4. 4. The image forming apparatus according to claim 1, further comprising an image density sensor for measuring the density of a toner image formed on the image carrier; the control unit includes an image quality adjustment processing unit that performs process control, which is one type of image quality adjustment processing; The image quality adjustment processing unit, when performing process control, forming a toner patch on the image carrier using a developing bias whose AC component is changed according to the accumulated driving time; a correction is made to the measured density of the formed toner patch detected by the image density sensor in accordance with a transfer current corresponding to the accumulated driving time; An image forming apparatus, characterized in that a DC component of a developing bias is determined based on a corrected toner patch density.

5. 1. An image forming method comprising: forming a toner image by developing an electrostatic latent image formed on an image carrier using a developing roller having a surface carrying a two-component developer containing toner and a carrier; and transferring the toner image formed on the image carrier to a sheet, the method comprising: a first step of checking an accumulated driving time of the developing roller up to the time immediately before the start of development of the electrostatic latent image; a second step of changing an AC component of a development bias, which is a predetermined voltage supplied to the development roller, and a transfer current, which is a predetermined current supplied to a transfer device that transfers the toner image onto the sheet, according to the accumulated driving time; a third step of forming an image using the development bias and transfer current changed in the second step; In the second step, the AC component of the developing bias and the transfer current are increased from the reference values ​​as the cumulative driving time increases.

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