JP2013092631A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of improving efficiency of charging-member cleaning processing.SOLUTION: An image forming apparatus includes charging-member cleaning processing where, in a location downstream a secondary transfer position but upstream a primary transfer position in the movement direction of an intermediate transfer body, accumulated toner on a charging member which charges toner on the intermediate transfer body by having a voltage applied to the toner is moved to the intermediate transfer body, and then in order that the accumulated toner can be moved from the intermediate transfer body to an image carrier so as to be collected from a cleaning member, a positive voltage and a negative voltage are alternately applied at a predetermined time rate on the charging member. In the charging member cleaning processing, the time rate is set so that the time for applying a voltage of a polarity of higher concentration of charging polarities the accumulated toner has is set longer than the time for applying a voltage of a polarity lower concentration.

Description

  The present invention relates to an image forming apparatus such as a copying machine or a printer.

2. Description of the Related Art Conventionally, as an image forming apparatus such as a copying machine or a printer using an electrophotographic system, a system using an intermediate transfer member is known. In an intermediate transfer type image forming apparatus, a color image can be formed on a transfer material through a primary transfer process and a secondary transfer process.
In the primary transfer step, the toner image formed on the surface of the electrophotographic photosensitive member is transferred onto the intermediate transfer member. By repeatedly executing this primary transfer process for a plurality of color toner images, a toner image having a plurality of colors is formed on the surface of the intermediate transfer member. In the secondary transfer process, the toner images of a plurality of colors formed on the intermediate transfer member are collectively transferred onto the surface of a transfer material such as paper. The toner image transferred to the transfer material is then fixed to the transfer material by a fixing unit. Thereby, a full-color image is formed.
At this time, it is necessary to remove the toner (secondary transfer residual toner) that is not transferred onto the transfer material in the secondary transfer step and remains on the intermediate transfer member from the intermediate transfer member. As a method for removing the remaining toner, Patent Document 1 proposes a method using an electrostatic cleaning member. That is, a charging brush (charging member) is provided as an electrostatic cleaning member, and a voltage is applied to the member to charge the secondary transfer residual toner on the intermediate transfer member to a desired polarity. The residual toner is recovered from the intermediate transfer member by reversely transferring it to the photosensitive member. A part of the transfer residual toner on the intermediate transfer member is also temporarily collected and accumulated in the charging brush. Such toner is discharged from the charging brush between pages (between paper) or after image formation is completed (post-rotation), and is reversely transferred to the photoreceptor and collected (charging member cleaning step).
With the configuration as described above, the intermediate transfer member can be cleaned, and can be prepared for the subsequent primary transfer step of image formation.

JP 2009-205012 A

  The charging polarity of the toner accumulated on the charging member varies. Therefore, when cleaning the toner accumulated on the charging member from the charging member, it is necessary to apply a voltage according to the charging polarity of the toner. However, conventionally, since the tendency of the charging polarity of the toner accumulated in the charging member is not taken into consideration, it has not always been performed efficiently.

  An object of the present invention is to provide an image forming apparatus capable of improving the efficiency of the charging member cleaning process.

In order to achieve the above object, an image forming apparatus according to the present invention includes:
An image carrier for carrying a toner image;
A rotatable intermediate transfer body onto which a toner image is transferred from the image carrier;
A primary transfer member that primarily transfers a toner image from the image carrier to the intermediate transfer body by applying a voltage;
A secondary transfer member that secondarily transfers a toner image from the intermediate transfer member to a transfer material by applying a voltage;
A charging member that charges the toner on the intermediate transfer member by applying a voltage downstream from the secondary transfer position in the rotation direction of the intermediate transfer member and upstream from the primary transfer position;
A cleaning member capable of removing toner on the image carrier;
With
A positive voltage is applied to the charging member to move the accumulated toner accumulated on the charging member to the intermediate transfer member, and then to move the accumulated toner from the intermediate transfer member to the image carrier and collect the accumulated toner from the cleaning member. In an image forming apparatus capable of executing a charging member cleaning process in which a negative voltage and a negative voltage are alternately applied at a predetermined time ratio,
In the charging member cleaning step, the time during which the voltage having the same polarity as the polarity having the high content ratio among the charging polarities of the accumulated toner is applied is longer than the time during which the voltage having the same polarity as the polarity having the low content ratio is applied. The time ratio is set so as to be longer.

  As described above, according to the present invention, the charging member can be efficiently cleaned.

1 is a schematic configuration diagram of an image forming apparatus. FIG. 5 is a diagram illustrating a cleaning sequence for secondary transfer residual toner. FIG. 4 is a diagram illustrating an example of an image density adjustment patch image formed on an intermediate transfer member. The figure which shows the cleaning sequence of a patch image. The chart which shows an example of the execution conditions of a charging brush cleaning process. The figure which shows the longitudinal position dependence of the toner amount of accumulation | storage toner of a charging brush at the time of patch image formation. The figure which shows the cleaning sequence of the conventional charging brush. FIG. 4 is a schematic diagram illustrating how toner is collected from an intermediate transfer member. FIG. 6 is a diagram for explaining toner discharge from a charging brush by bias application. The figure which shows the amount of toner discharged in the conventional charging brush cleaning process. FIG. 6 is a diagram illustrating an example of a polarity distribution of toner accumulated in a charging brush. FIG. 4 is a diagram illustrating a cleaning sequence of the charging brush according to the first embodiment. FIG. 3 is a diagram illustrating an example of an application time of a discharge charging bias in the charging brush cleaning process of the first embodiment. FIG. 3 is a diagram illustrating an amount of discharged toner in a charging brush cleaning process of Example 1. FIG. 6 is a diagram illustrating an amount of discharged toner in a charging brush cleaning process of Example 2. FIG. 4 is a schematic diagram illustrating a transfer behavior of all toner images in a longitudinal direction in a secondary transfer unit. FIG. 7 is a table showing an example of execution conditions for a charging brush cleaning process of Embodiment 2. FIG. FIG. 6 is a diagram illustrating the longitudinal position dependency of the amount of toner accumulated in a charging brush in a wide area printing process. FIG. 6 is a diagram illustrating a cleaning sequence for the charging brush according to the second embodiment. FIG. 6 is a diagram illustrating an example of a discharge charging bias application time in the charging brush cleaning process of the second embodiment.

DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the present invention will be exemplarily described in detail with reference to the drawings. However, the dimensions, materials, shapes, and relative arrangements of the components described in this embodiment should be appropriately changed according to the configuration of the apparatus to which the invention is applied and various conditions. That is, it is not intended to limit the scope of the present invention to the following embodiments.

<Overall configuration of image forming apparatus>
FIG. 1 is a schematic configuration diagram of an image forming apparatus according to each embodiment and comparative example of the present invention. This image forming apparatus is a tandem type color image forming apparatus using an intermediate transfer belt system. In other words, the image forming apparatus uses a toner image of each color formed according to image information separated into a plurality of color components (yellow (Y), magenta (M), cyan (C), and black (K)) as an intermediate transfer member. Primary transfer is performed by sequentially superimposing the images on the image. Thereafter, the superimposed toner images of the respective colors are collectively transferred onto a transfer material to obtain a recorded image. 1Y, 1M, 1C, and 1K are photosensitive drums as electrostatic latent image carriers (photoconductors), and a photoconductor made of an organic photosensitive layer (OPC) is applied to the outer peripheral surface of a cored bar made of aluminum or the like. Configured. The photosensitive drums 1Y to 1K are sequentially arranged along an intermediate transfer belt (hereinafter referred to as ITB) 10 as a second image carrier (intermediate transfer body), and an outer peripheral speed V (hereinafter referred to as process speed). To rotate in the direction of arrow C1.

The ITB 10 is stretched on three rollers, that is, a driving roller 11a, a secondary transfer opposing roller 11b, and a stretching roller 11c, and the driving roller 11a is rotated by a motor (not shown) so that the process speed is increased in the direction of arrow C3. It is driven to rotate. The ITB 10 is made of a single-layer resin and has a volume resistivity of 2 × 10 ^{10 to} 6 × 10 ¹⁰ Ω · cm. The secondary transfer counter roller 11b uses a conductive urethane resin coated on the surface layer. Reference numerals 6Y to 6K denote primary transfer rollers as primary transfer members provided with conductive sponge layers on the shafts, which are in contact with the photosensitive drums 1Y to 1K through the ITB 10, respectively.

  In this image forming apparatus, the charged potential of the photosensitive drum and the charged polarity of the toner in the developing unit are both negative, and after exposing the image information to the negatively charged photosensitive drum, the exposed portion is charged with negative toner. The reversal development method is used.

<Full color printing process>
The photosensitive drums 1Y to 1K are driven in the direction of the arrow C1 by driving means (not shown), and are uniformly charged to a predetermined negative potential by the charging rollers 2Y to 2K. An exposure apparatus that synchronizes with the timing at which the tip of the ITB upper image portion enters the transfer portion of each station so that the image is transferred to an area where the image is to be transferred on the ITB 10 (hereinafter referred to as the ITB upper image portion). Exposure is started by 3Y to 3K. When exposure is started, light based on signals according to the image information of each color is scanned onto the photosensitive drums 1Y to 1K that are uniformly charged at a negative potential on the surface, and a latent image is formed. It should be noted that the detection timing seal (not shown) provided on the ITB 10 is synchronized with the timing of the latent image formation and the timing at which the leading edge of the image portion on the ITB enters the primary transfer portion of each station. This is done by monitoring the timing detected by the optical sensor.

  When the photosensitive drums 1Y to 1K are further rotated in the direction of the arrow C1 after the latent image formation is started, development is performed by the developing devices 4Y to 4K, respectively. The developing devices 4Y to 4K have a developing roller that rotates at a predetermined rotational speed at a portion facing the photosensitive drums 1Y to 1K, and the latent image is visualized by negative polarity toner carried on the surface of the developing roller.

Further, when the photosensitive drums 1Y to 1K rotate in the direction of the arrow C1 and the developed color toner images come to the primary transfer portion, positive polarity is applied from the high-voltage power supplies 7Y to 7K with the cored bar of the photosensitive drums 1Y to 1K as the counter electrode. A bias is applied to the primary transfer rollers 6Y to 6K. As a result, the developed color toner images are sequentially primary-transferred and superimposed on the ITB upper image portion entering the primary transfer portion in synchronization with the above-described method. The charge amount of the toner on the ITB 10 is about −30 μc / g on average in this image forming apparatus. The primary transfer residual toner on the photosensitive drums 1Y to 1K (on the image carrier) is cleaned by cleaners 14Y to 14K that are scraped by contacting a blade as a cleaning member.

When the four color toner images are primarily transferred onto the ITB 10, the transfer material P is conveyed from the registration roller 12 in synchronization with the rotation of the ITB 10, and the secondary transfer roller 8 as the secondary transfer member is transferred to the transfer material. It contacts ITB 10 via P. The toner images of four colors on the ITB 10 are collectively applied onto the transfer material P by applying a positive polarity bias to the secondary transfer roller 8 from a high voltage power source (not shown) using the secondary transfer counter roller 11b as a counter electrode. Secondary transferred. The secondary transfer roller 8 is made of NBR and has a resistance value of 2 × 10 ^{7 to} 5 × 10 ⁷ Ω. The transfer material P onto which the four color toner images have been transferred is fused and fixed by a conventionally known heating and pressure fixing device 13 to obtain a color image.

  Since the secondary transfer residual toner should not be mixed when the next image is formed on the ITB 10, it can be removed from the ITB 10 by a cleaning process using a charging brush 9b as a charging member. The charging brush 9b is provided downstream of the secondary transfer position in the rotational direction of the ITB 10 and upstream of the primary transfer position. When the secondary transfer residual toner enters the nip portion of the ITB 10 and the charging brush 9b, that is, the charging portion, a voltage is applied to the charging brush 9b to charge the secondary transfer residual toner to a desired polarity and move to the photosensitive drum. By doing so, the secondary transfer residual toner is collected from the ITB 10. The charging brush 9b is made of conductive nylon and has a width of 4 mm and a bristle length of 4 mm. The charging brush 9b contacts the ITB 10 with an intrusion amount of 1 mm with the driving roller 11a as an opposing member.

  This image forming apparatus has an adjustment process (hereinafter abbreviated as a patch detection process) that forms and executes a detection toner image (patch image) directly on the ITB 10 such as image density adjustment and registration adjustment. . The patch inspection process generally uses a dedicated image pattern depending on the purpose, but the same operation as the printing process is performed until the four-color toner images are primarily transferred onto the ITB 10 regardless of the type of pattern. Thereafter, the patch image passes through the secondary transfer portion and is removed from the ITB 10 by a cleaning process using the charging brush 9b.

  In the above secondary transfer residual toner and patch image cleaning process, toner is accumulated in the gap between the charging brushes 9b. The toner accumulated in the charging brush 9b may reduce the charging ability of the secondary transfer residual toner and the patch image by the charging brush 9b, and may cause problems such as image defects. Therefore, the image forming apparatus also includes a cleaning process for the charging brush 9b.

  In the following, the conventional method as a comparative example will be described in detail first for the above-described three types of processes, that is, cleaning of secondary transfer residual toner at the time of image formation, cleaning of a patch image at the time of patch detection, and cleaning of a charging brush. To do. Thereafter, the charging brush cleaning process according to the first embodiment of the present invention will be described.

<Cleaning of secondary transfer residual toner during image formation>
FIG. 2 is a diagram showing a cleaning sequence of the secondary transfer residual toner, and shows the bias application timing in each member. The gray portion indicates that a positive bias is applied to each member, and the bias value is constant with respect to time for each member. A positive bias for realizing good primary transfer such as high transfer efficiency is applied to the primary transfer roller of each color. A positive bias for realizing good secondary transfer such as high transfer efficiency is applied to the secondary transfer member. A positive bias for positively charging the secondary transfer residual toner is applied to the charging brush 9b. In the time zone indicated by each symbol, Tr1 is the primary transfer for each color, Tr2 is the secondary transfer, Ch2 is the charge of the secondary transfer residual toner, and RET is from the intermediate transfer body in the primary transfer portion of the secondary transfer residual toner. Movement to the drum (hereinafter, reverse transfer). Tr1
And RET overlap with each other is a time zone in which reverse transfer is performed simultaneously with primary transfer. In addition, the shift | offset | difference of these application time zones in a figure originates in the distance of each member. For example, the rotation time from the charging portion by the charging brush 9b on the ITB 10 to the first color primary transfer portion is indicated by TC1. ITBCYC represents an ITB cycle.

  The secondary transfer residual toner remaining on the ITB 10 after the primary transfer Tr1 and the secondary transfer Tr2 of each color is applied with a positive polarity bias from the high voltage power source 9a to the charging brush 9b, thereby adding positive, that is, normal toner The charge having the opposite polarity to the charged polarity is uniformly applied. The applied voltage varies depending on the belt resistance of the ITB 10 and environmental conditions, but 1000 V to 1500 V is applied here. However, a small amount of toner mechanically adheres to the charging brush during this charging. Subsequently, the secondary transfer residual toner proceeds to the primary transfer portion of the first color, and is electrostatically transferred to the photosensitive drum 1Y by the recovery bias RET, and is removed from the ITB 10. The applied voltage of RET may be the applied voltage of Tr1. Then, the secondary transfer residual toner transferred to the photosensitive drum 1Y is collected by the photosensitive drum cleaner 14Y, and the cleaning of the secondary transfer residual toner on the ITB 10 is completed. Although a small amount of secondary transfer residual toner may remain on the ITB 10 without being collected on the photosensitive drum 1Y, it is transferred to the photosensitive drums 1M, 1C, or 1K by the primary transfer bias applied in the MCK color. Reverse transcription.

<Cleaning patch images during patch detection>
FIG. 3 shows an example of an image density adjustment patch image formed on the ITB 10. As described above, in the patch inspection process, operations similar to those in the printing process are performed until the primary transfer of the patch image onto the ITB 10. A dedicated image pattern is used according to the purpose, but here, image density adjustment is shown as an example. In FIG. 3, ITB indicates the surface of the ITB 10, and PT indicates a patch image on the ITB.

  The cleaning of the patch image on the ITB will be described with reference to FIG. FIG. 4 shows a patch image cleaning sequence. The gray portion and the shaded portion indicate application of a positive bias and a negative bias in each member, respectively. Tr1 indicates the primary transfer of the patch image onto the ITB 10. Tr2N and Ch2N represent a state in which a negative bias is applied to the secondary transfer roller 8 and the charging brush 9b, respectively, and the patch image passes through the secondary transfer portion and the charging portion. RETN represents a state in which a negative bias is applied to the patch image at the primary transfer unit and the patch image is reversely transferred to the photosensitive drum 1Y. Tr2RN represents a state in which a minus bias is applied to the secondary transfer roller 8 and the remaining toner that has not been collected by the first round, that is, cannot be collected by RETN, is passing. Ch2RP represents a state in which a plus bias is applied to the charging brush 9b to charge the collected residual toner. RETRP represents a state in which a positive bias is applied to the collected residual toner in the primary transfer portion, and the collected residual toner is reversely transferred to the photosensitive drum 1Y.

Since the patch image is not subjected to the secondary transfer, the patch image is passed through the secondary transfer portion while preventing the toner from adhering to the secondary transfer roller 8. Therefore, a negative bias, that is, a bias having the same polarity as the charging polarity of the patch image is applied to the secondary transfer roller 8 in the time zone indicated by Tr2N when the patch image passes through the secondary transfer portion, and the secondary transfer roller 8 repels. Let As the bias value, a value that does not affect the toner charge amount due to discharge was used. According to regular development and primary transfer, the charge polarity of the patch image is negative, but a small amount of positive toner may be developed and primary transferred. However, even if positive toner is present in the patch image before entering the secondary transfer portion, it is attracted to the secondary transfer roller 8 by the above-described negative bias when passing through the secondary transfer portion. There is no plus toner in the patch image that has passed through the portion. Subsequently, also when the patch image enters the charging unit in the time zone indicated by Ch2N, the negative voltage is applied to the charging brush 9b from the high-voltage power source 9a so that it is repelled from the charging brush 9b and passes through the charging unit. However, since the patch image has a large amount of toner unlike the secondary transfer residual toner in the printing process, a larger amount of minus toner mechanically adheres to the charging brush 9b than when the secondary transfer residual toner is charged. Subsequently, the patch image proceeds to the primary transfer portion of the first color, and is electrostatically transferred to the photosensitive drum 1Y by the recovery minus bias in the time zone indicated by RETN, and is removed from the ITB 10. However, since the patch image has a higher density than the secondary transfer residual toner, it cannot normally be collected once, and a minus toner with a relatively small charge, mainly an absolute value of the charge amount of 5 μc / g or less remains. The toner that remains on the ITB 10 without being collected, that is, the collected residual toner, sequentially proceeds to the primary transfer portion of the second to fourth colors, but due to the small amount of charge, the toner remains on the photosensitive drums 1M to 1K. It is hardly transferred electrically. Furthermore, the collected residual toner circulates on the ITB 10 and passes through the secondary transfer portion again in the time zone indicated by Tr2RN. As described above, since the collected residual toner has a negative charge, it is electrostatically repelled from the secondary transfer roller 8 to which a negative bias is applied, and therefore is not attracted. However, since the collected residual toner has a small charge amount and a weak repulsive force, there is a small amount of toner that mechanically adheres to the secondary transfer roller 8. Subsequently, the collected residual toner enters the charging portion, and a positive bias is applied to the charging brush 9b in the time zone indicated by Ch2RP, so that a positive charge is uniformly applied. However, a small amount of toner mechanically adheres to the charging brush 9b during this charging. Subsequently, the collected residual toner proceeds to the primary transfer portion of the first color and is electrostatically transferred to the photosensitive drum 1Y by applying a positive bias in the time zone indicated by RETRP. Similarly, the cleaning is completed.

<Charging Brush Cleaning Process of Comparative Example 1>
When the secondary transfer residual toner is charged or when the patch image is repelled, the charging brush 9b accumulates toner in the brush gap. Since the accumulated toner causes image defects as described above, it is necessary to reliably clean the accumulated toner. In Comparative Example 1, when the accumulated toner reaches a predetermined amount, the charging member cleaning process is executed with the same application time of the positive bias and the negative bias applied regardless of the number of executions of patch detection.

  Hereinafter, setting of conditions for recognizing that the accumulated toner has reached a predetermined amount will be described.

  FIG. 5 is a chart showing an example of execution conditions of the charging brush cleaning process. The charging brush cleaning process always counts the number of printed pages and the number of patch detection executions, and both counts exceed the predetermined number of times in any one of the conditions A to D obtained by combining these predetermined numbers. Executed when. The number of printed pages in FIG. 5 indicates the number of printed pages of the most frequently used image, and it is assumed that the length of the image at that time, that is, the most frequently used image length (for example, A4 length in an A4 printer) is printed. doing. Conditions A to D indicate the number of times of patch detection in which the accumulated toner reaches a predetermined amount in different print page numbers. That is, in the condition A with respect to the condition B, the charge brush cleaning process is executed even when the number of times of patch detection is increased by one and the number of printed pages is reduced by 100 pages. This is because the amount of accumulated toner in one patch detection is about 100 times that in the one-page printing process. This relationship holds for each of the conditions A to D. The reason for this will be described below.

  The above-described decrease in the ability of the charging brush 9b to charge the secondary transfer residual toner due to the accumulated toner occurs only in a part of the ITB 10 in the longitudinal direction (direction perpendicular to the transfer material conveyance direction). It becomes a cause. Therefore, the charging brush cleaning process needs to be executed at a frequency that does not cause image defects in the area where the total amount of accumulated toner by the printing process and patch detection in the longitudinal direction is the largest, that is, the area where the accumulated quantity is the largest. . As described above, the accumulated toner amount becomes 100 times in the patch detection in the most accumulated amount region.

As for the amount of accumulated toner in the actual printing process, since the image pattern is not determined, it may be considered that the accumulated toner amount can be similarly accumulated in the entire region in the longitudinal direction. On the other hand, since the image pattern is determined in patch detection, the amount of accumulated toner is concentrated at a specific location in the longitudinal direction. In consideration of this, in the patch detection, the region having the largest total amount of toner consumption in the circumferential direction in the longitudinal direction is the most accumulated amount region.

FIG. 6 is a diagram illustrating the longitudinal position dependency of the amount of toner accumulated in the charging brush in the patch inspection process. J1 and J2 indicate areas with and without patches in the longitudinal direction. It can be seen that the region where the total amount of accumulated toner amount in the longitudinal direction is the largest, that is, the most accumulated amount region is J1. In the patch detection, the area J1 has an image that is longer than the most frequently used image length in the circumferential direction. As described above, the patch image is charged with a large amount of negative toner as compared with the charge of the secondary transfer residual toner. It adheres mechanically to the brush 9b. For example, it is assumed that the image on the ITB 10 is 297 mm (A4 size) in the printing process, the printing rate is 5% (loading amount 0.025 mg / cm ² ), and the rate of transfer to paper, that is, the transfer efficiency is 90%. In this case, the secondary transfer residual toner per 1 cm in the longitudinal direction is 29.7 × 0.025 × 0.1 = 0.07425 mg, and assuming that 40% adheres to the charging brush 9b, The accumulated toner is 0.0297 mg. On the other hand, five patches (0.1 mg / cm ^{2) with} a loading amount of 0.1 to 0.5 mg / cm ^{2 in} the patch inspection process and an area of 0.1 cm to 0.5 mg / cm ² with an area per patch of 1 cm ² . ^{It is} assumed that 4 colors are used for ( ² intervals). In this case, the patch image per 1 cm in the longitudinal direction is (0.1 +. + 0.5) × 4 = 6.0 mg, and assuming that 50% of the patch image adheres to the charging brush 9b, accumulation per 1 cm in the longitudinal direction. The toner is 3.0 mg.

  For the reasons described above, in the most accumulated amount region, the amount of accumulated toner in one patch detection is much larger than that in the printing process for one page, and the amount is about 100 times. Therefore, as described above, a condition was set such that one patch test corresponds to 100 pages of the printing process. It should be noted that more accurate conditions can be set for values such as the printing rate and transfer efficiency by using values that are more in line with actual usage conditions. Further, it is possible to perform the charging brush cleaning process more optimally by performing the control based on the printing information.

  As described above, all of the conditions A to D are conditions for accumulating the same amount of toner in the maximum accumulation amount region, and it is necessary to execute the charging brush cleaning process.

  The charging brush cleaning process will be described with reference to FIGS. FIG. 7 is a diagram showing a sequence of a conventional charging brush cleaning process as Comparative Example 1. ChP and ChN represent a state in which accumulated toner is discharged onto the ITB 10 to the charging brush 9b by applying positive and negative biases for discharging the toner to the charging brush 9b, respectively. PRETP and PRETN represent a state where the discharged toner on the ITB 10 is reversely transferred to the photosensitive drums 1Y and 1M by applying a primary transfer plus and minus bias for collecting discharged toner in the first and second colors, respectively. Yes. FIG. 8 is a schematic diagram showing how toner is collected by reverse transfer.

  The positive and negative toners accumulated in the charging brush 9b are electrostatically repelled from the brush and applied to the ITB 10 by applying + 2000V and -2500V to the charging brush 9b in the time zones indicated by ChP and ChN, respectively. Discharged. ChP and ChN are the same time Tch. By repeating this process a predetermined number of times, 90% or more of the accumulated toner is discharged. Subsequently, the positive toner among the discharged toner is electrostatically reversely transferred to the photosensitive drum 1Y by applying a 1500 V positive bias to the primary transfer roller 6Y of the first color. Similarly, the negative toner is electrostatically reversely transferred to the photosensitive drum 1M by applying a negative bias of −1500 V to the primary transfer roller 6M for the second color. Thereby, the charging brush cleaning process is completed.

Next, the time required for the charging brush cleaning process will be described. In order to shorten the time required for the sequence and improve usability, the application time Tch in one bias application to the charging brush is devised as shown below.

  FIG. 9 is a diagram for explaining how much toner is discharged from the charging brush by a single bias application. FIG. 9A shows the relationship between the application time and the bias value in one bias application to the charging brush. FIG. 9B shows the relationship between the application time and the discharge amount in one bias application to the charging brush. FIG. 9C shows a time average from the bias ON of the discharged toner amount in one bias application to the charging brush. These relationships shown in FIGS. 9A to 9C apply to both the positive toner and the negative toner. Tsw is the time from when the bias is turned on until the bias rises. Tp0 is the time from when the bias rises to when the bias is turned off, and can be expressed as Tch-Tsw. A time when the time average from the bias ON of the discharged toner amount becomes maximum is defined as Tpmax. Hereinafter, paying attention to two time zones of Tsw and Tp0, the discharge amount in one bias application will be described.

  As can be seen from FIG. 9B, the discharge amount during Tsw after the start of bias switching increases gradually at the beginning and then abruptly in the same manner as the bias rising profile, but the total amount is small. . On the other hand, the discharge amount at the time Tp0 applied after the bias switching has a peak immediately after the completion of the bias switching and then decreases, but unlike the rapid rise at Tsw, it gradually decreases. For this reason, as shown in FIG. 9C, the time average from the start of switching of the discharged toner amount becomes maximum at a time of Tpmax from the bias ON, which is several times Tsw from the time when the bias rises.

  Considering the above, the application time Tch in one bias application to the charging brush 9b is the average of the amount of toner discharged in the first minus bias application in all the conditions A to D on average. The value that increases is 400 ms. Here, the reason why the value related to the minus bias application is adopted instead of plus is that, as will be described later, the accumulated toner is mostly minus and priority is given to the ejection of minus toner.

  As described above, in the charging brush cleaning step, the cleaning can be completed by repeating the application of plus and minus biases ChP and ChN at Tch until 90% or more of the accumulated toner is discharged. In all of the conditions A to D, since 6 repetitions were necessary, this setting was adopted.

  The amount of toner discharged in this setting will be described with reference to FIG. FIG. 10 is a diagram showing the amount of toner discharged in the conventional charging brush cleaning process. In addition, the discharge amount per time of plus and minus bias application states ChP and ChN is shown as an average. If the ejection capability is the same, the toner is ejected at a substantially predetermined ratio from the accumulated amount, so that the amount of ejection by bias application per time gradually decreases as shown in FIG. Here, there is a difference between the positive and negative discharged toner amounts in FIG. 10 because the accumulated toner is mostly negative.

  The toner accumulated on the charging brush 9b by cleaning the secondary transfer residual toner at the time of image formation and cleaning the patch image at the time of patch detection was cleaned by the charging brush cleaning process as described above in the comparative technique.

In the conventional charging member cleaning process, only the amount of the accumulated toner is considered, so depending on the polarity distribution, the discharge ability is excessive for one or both of the positive polarity toner and the negative polarity toner, An extra time extension was invited. On the other hand, in each embodiment of the present invention, the application time of the negative voltage having a high content ratio among the charging polarities of the accumulated toner is alternately applied so as to be longer than the application time of the positive voltage having a low content ratio. The positive / negative application time ratio is set. The setting of the application time ratio takes into account that the higher the execution ratio of the step of cleaning the toner image formed on the intermediate transfer body without secondary transfer and the higher the execution ratio of the wide area printing step, the larger the accumulated amount of negative polarity toner. That is, the higher the ratio of the number of executions of the process, the higher the negative toner content in the accumulated toner is higher than the positive toner content. Therefore, the application time ratio of the positive polarity and the negative polarity bias in the charging member cleaning process is biased toward the negative polarity side so that the application time ratio of the negative polarity bias becomes larger according to the difference in the content.

<Example 1>
The image forming apparatus according to the first embodiment of the present invention will be described with reference to FIGS. FIG. 11 is a diagram illustrating an example of the polarity distribution of toner accumulated in the charging brush. FIG. 12 is a diagram illustrating a charging brush cleaning sequence according to the present exemplary embodiment, and illustrates the application timing of each bias in the charging brush cleaning process. FIG. 13 is a diagram illustrating an example of the application time of the discharge charging bias in the charging brush cleaning process of the present embodiment. FIG. 14 is a diagram illustrating the amount of toner discharged in the charging brush cleaning process of the first embodiment.

<Charging Brush Cleaning Process of Example 1>
In this embodiment, a method for shortening the time required for the charging brush cleaning process of Comparative Example 1 will be described. Also in this embodiment, as described in Comparative Example 1, one bias application time can be determined so that the amount of toner discharged per unit time in one bias application to the charging brush 9b is maximized. is necessary. Further, in the present embodiment, in consideration of the fact that the polarity of the accumulated toner is different under the conditions where the execution ratio of the patch detection process as shown in the conditions A to D is different, the positive and negative biases are individually determined in each condition. In addition, the time for applying the bias once is determined. This process is repeated until 90% or more of the accumulated toner is discharged. By doing so, the charging brush cleaning process can be made shorter than that of Comparative Example 1.

  Also in this embodiment, as in the comparative example, when both the number of printed pages and the number of patch detection execution times exceed the predetermined number of times in any one of the conditions A to D combining the predetermined number of times, charging is performed. Perform a brush cleaning process. Before the detailed description of this process, how the polarity of the accumulated toner changes depending on the execution frequency of the patch detection process, that is, the execution ratio with respect to the number of printed sheets, in conditions A to D will be described below.

  As described above, in the patch inspection process, when the patch image on the ITB 10 first enters the charging unit, a larger amount of minus toner is mechanically applied to the charging brush 9b than when the secondary transfer residual toner is charged in the printing process. Adhere to. In addition, as described above, the amount of toner that adheres to the charging brush when the collected residual toner again enters the charging portion is extremely small compared to the amount that the negative patch image first enters the charging portion. . That is, the toner accumulated in the charging brush in the two bias application states Ch2N and Ch2RP at the charging unit in the patch detection process is a large amount of negative toner and a very small amount of toner, and the polarity is almost negative.

  On the other hand, in the printing process, a small amount of positive toner may be developed and primary-transferred as in the case of patch images. In secondary transfer, the amount of charge is small and sufficient electrostatic transfer force is applied regardless of polarity. Unused toner is not transferred. Therefore, the secondary transfer residual toner is mainly a plus toner and a minus toner having an absolute charge amount of 5 μc / g or less. Therefore, the toner that mechanically adheres to the charging brush 9b during charging includes plus toner.

That is, the accumulated toner in the patch inspection process (second image forming process)
Compared with the accumulated toner in the image forming step), the polarity distribution is biased to the minus side. For this reason, as shown in FIG. 11, the accumulated toner during the charging brush cleaning process has a larger amount of minus toner and a smaller amount of accumulated plus toner on the condition A side where the patch detection process is frequently performed. .

  As described above, the amount of accumulated minus toner increases and the amount of accumulated plus toner decreases as the condition A side where the execution ratio of the patch detection process is high. The charging brush cleaning process in the present invention in consideration of this will be described below.

  FIG. 12 shows a sequence of the charging brush cleaning process. ChP1 and ChN1 represent a state in which toner accumulated on the charging brush is discharged onto the ITB 10 by alternately applying a positive voltage and a negative voltage for discharging the toner to the charging brush 9b. TchP1 and TchN1 represent application times per time of ChP1 and ChN1, respectively. RETP1 and RETN1 represent a state in which the discharged toner on the ITB 10 is reversely transferred to the photosensitive drums 1Y and 1M by applying a primary transfer plus and minus bias for collecting discharged toner in the first and second colors, respectively. ing.

  The positive and negative toners accumulated in the charging brush 9b are discharged onto the ITB 10 by repeating the discharge bias application states ChP1 and ChN1, respectively, a predetermined number of times, like the ChP and ChN of the first comparative example. . Further, the toner is transferred to the photosensitive drums 1Y and 1M by RETP1 and RETN1, and collected by the photosensitive drum cleaners 14Y and 14M, and the cleaning process of the charging brush 9b is completed.

  Next, the time required for the charging brush cleaning process will be described. Similar to Comparative Example 1, a method of determining the bias application time once is also applied here so that the time average of the amount of toner discharged by one bias application to the charging brush 9b becomes large. However, in consideration of the fact that the amount of minus toner accumulated is larger and the amount of plus toner accumulated is smaller as the condition A is above, minus and plus discharge bias application time TchN1, in each case of conditions A to D, The optimum values of TchP1 are all different. Therefore, when the optimum values of TchN1 and TchP1 in each of the conditions A to D were obtained from experiments, the values shown in FIG. 13 were obtained. It should be noted that the number of repetitions was 6 times.

  In order to compare with the time required for the charging brush cleaning process in Comparative Example 1, the amount of toner discharged under condition A is shown in FIG. In the method of Comparative Example 1 shown in FIG. 10, the amount of plus toner discharged is extremely small around the fourth time, and wasteful application time is generated at the fifth and sixth times. On the other hand, in the present embodiment shown in FIG. 14, the generation of useless application time can be suppressed, and sufficient ejection can be performed in a short time. This tendency was the same in the conditions B, C, and D. Note that the execution conditions and the discharge bias application time may be appropriately changed from those shown in FIG. 5 and FIG.

  In the charging brush cleaning process of Comparative Example 1, 400 ms that is TchN1 in the condition A, which is the longest time for one bias application according to the present embodiment, is common to all of the conditions A to D and both plus and minus. It was applied only for the time. According to the present embodiment, it is possible to reduce the time for the charging brush cleaning process compared to Comparative Example 1 under all conditions. Specifically, (400 × 2−120−400) × 6 = 1680 ms can be shortened under condition A. When calculated in the same manner, the conditions B, C, and D can be shortened by 1680 ms, 1680 ms, and 1740 ms, respectively.

Here, in the patch detection process, when the patch image on the ITB 10 first enters the charging portion, a method of applying a negative bias to the charging brush to repel it is shown. On the other hand, even when a positive bias is applied to positively charge the patch image and then collected at the positive primary transfer bias when entering the primary transfer portion, negative toner that cannot be charged adheres. Therefore, as a result, when the patch image first enters the charging portion, the accumulated toner is more polar than the toner accumulated by charging the secondary transfer residual toner, as in the case of applying a negative bias to the charging brush. Since the distribution is biased to the minus side, this configuration is effective.

  For example, even when a jam occurs in the printing process, an image that is not secondarily transferred on the ITB 10 may be cleaned in the same manner as the patch detection. Therefore, a better effect can be obtained by adding a condition such as the number of jams to the execution condition of the charging brush cleaning process.

  As described above, it can be determined that the accumulation amount (content ratio) of minus toner increases as the execution ratio of a process such as patch detection for cleaning the toner image formed on the intermediate transfer member without performing secondary transfer increases. In consideration of this, in this embodiment, the higher the ratio of the number of executions of the process, the more biased the application time ratio of the positive voltage and the negative voltage in the charging member cleaning process. Thus, the charging brush cleaning process can be completed in a short time without taking an extra margin.

<Example 2>
With reference to FIGS. 15 to 20, an image forming apparatus according to a second embodiment of the present invention will be described. FIG. 15 is a diagram showing the amount of toner discharged in the charging brush cleaning process of this embodiment, and shows the application timing of each bias in the secondary transfer residual toner removing process in the wide-area printing process. FIG. 16 is a schematic diagram showing how the entire toner image in the longitudinal direction is transferred at both ends of the transfer material at the nip portion between the ITB 10 and the secondary transfer roller 8, that is, the secondary transfer portion. FIG. 17 is a chart showing an example of execution conditions of the charging brush cleaning process of the present embodiment. FIG. 18 is a diagram illustrating the longitudinal position dependency of the amount of toner accumulated in the charging brush in the wide-area printing process. FIG. 19 is a diagram illustrating a charging brush cleaning sequence according to the present exemplary embodiment, and illustrates application timing of each bias in the charging brush cleaning process. FIG. 20 is a diagram illustrating an example of the application time of the discharge charging bias in the charging brush cleaning process of the present embodiment.

  In this embodiment, an invention for shortening the charging brush cleaning process in an image forming apparatus capable of printing a borderless image will be described.

  First, cleaning of secondary transfer residual toner at the time of borderless image formation will be described. Also in this example, the image forming apparatus shown in FIG. 1 was used as in Comparative Example 1 of Example 1.

<Cleaning of secondary transfer residual toner during borderless image formation>
The borderless image is printed by forming a toner image on the ITB 10 in a wider area than the transfer material in the same image forming process as that of the conventional example, thereby printing on the transfer material without upper, lower, left and right margins. Hereinafter, this printing process is referred to as a wide area printing process (fourth image forming process), and in contrast to this, a printing process for forming a toner image in the transfer material, that is, a blank for not forming an image on the edge of the transfer surface of the transfer material The image forming process for providing the region is distinguished as a normal printing process (third image forming process). Hereinafter, cleaning of the secondary transfer residual toner in the wide-area printing process will be described with reference to FIG.

In FIG. 15, the gray portion and the shaded portion indicate application of plus and minus biases in each member, respectively. Tr1, Tr2, Ch2, and RET indicate the same as in FIG. In Tr2RN2, a negative bias is applied to the secondary transfer roller 8, and the first round, that is, RET
This represents a state in which the uncollected toner that could not be collected in FIG. Ch2R represents a state in which a plus bias is applied to the charging brush 9b and the collected residual toner is charged. RETR represents a state in which a positive bias is applied to the collected residual toner in the primary transfer portion, and the collected residual toner is reversely transferred to the photosensitive drum 1Y.

  FIG. 16 schematically shows the state of secondary transfer in the longitudinal direction of ITB 10 (direction orthogonal to the transfer material conveyance direction). G1 and G2 indicate a region of the toner image on the ITB 10 that is transferred to a transfer material and a region that is not transferred, that is, an inner region and an outer region, and other symbols are the same as those in FIG. FIG. 16 shows how the entire toner image is transferred from one end of the transfer material to the other end in the longitudinal direction without considering the curvature of the surface of the ITB 10 and the surface of the secondary transfer roller 8 in the secondary transfer portion. It shows.

  In the wide-area printing process, up to the primary transfer of the four color toner images onto the ITB 10 is the same as the normal printing process, and in the inner area G1, the processes after the secondary transfer are the same as the normal printing process. However, in the wide area printing process, the processes after the secondary transfer are different in the outer area G2. Hereinafter, steps after the secondary transfer in the outer region G2 will be described.

  In the secondary transfer process, the transfer bias is controlled so that the inner region G1 is transferred with high efficiency. Therefore, as shown in FIG. 16, since there is no transfer material in the outer region G2, the transfer material is naturally transferred to the transfer material. It is not transferred but transferred to the secondary transfer roller 8. Since the above-described transfer bias value is not suitable for this transfer, the efficiency is not high, and a lot of secondary transfer residual toner remains on the ITB 10, and most of it has the same charge amount as that before entering the secondary transfer portion. Subsequently, the secondary transfer residual toner is positively charged in the time zone indicated by Ch2 as in the normal printing process. However, since the amount is large, most of the secondary transfer residual toner adheres to the charging brush with a negative charge amount. Some of the toner with a small charge amount of 5 μc / g or less in absolute value is hidden by a large amount of toner and passes through the charging portion without being positively charged. Subsequently, when the positively charged secondary transfer residual toner and the toner of 5 μc / g or less enter the photosensitive drum 1Y in the time zone indicated by RET, the former is collected, while the latter is electrostatic. Force does not work sufficiently and is not collected and remains on the ITB 10 as a collected residual toner. After this, the collected residual toner sequentially proceeds to the primary transfer portions of the second to fourth colors, but is hardly electrostatically transferred to the photosensitive drums 1M to 1K due to the small amount of charge.

  Furthermore, the collected residual toner circulates on the ITB 10 and passes through the secondary transfer portion again in the time zone indicated by Tr2RN2. As described above, since the residual toner has a small charge amount, there is a small amount of toner that mechanically adheres to the secondary transfer roller 8, but most of the toner passes therethrough. Subsequently, the collected residual toner enters the charging portion, and a positive bias is applied to the charging brush 9b in the time zone indicated by Ch2R, so that a positive charge is uniformly applied. However, a small amount of toner mechanically adheres to the charging brush 9b during this charging. Subsequently, the collected residual toner proceeds to the primary transfer portion of the first color, and is electrostatically transferred to the photosensitive drum by applying a positive bias in the time zone indicated by RETR, and thereafter, the secondary toner in the inner region G1. The cleaning is completed in the same manner as the transfer residual toner.

  Even in the cleaning process of the secondary transfer residual toner in the wide-area printing process described above, the toner is accumulated in the gaps between the charging brushes 9b, and thus the cleaning process of the charging brush 9b is necessary. Hereinafter, with respect to the cleaning process of the charging brush 9b, Comparative Example 2 will be described in detail first, and then this example will be described.

<Charging Brush Cleaning Process of Comparative Example 2>
The toner accumulated in the charging brush 9b in the normal printing process and the wide-area printing process both causes image defects, and thus needs to be reliably cleaned. In this comparative example, when the accumulated toner reaches a predetermined amount, the same charging brush cleaning process is executed. Hereinafter, setting of conditions for recognizing that the accumulated toner has reached a predetermined amount will be described.

  FIG. 17 shows the execution conditions of the charging brush cleaning process. The charging brush cleaning process always counts the number of printed pages in the normal printing process and the wide-area printing process, and the count number of both counts the predetermined number of times in any one of the conditions A to E obtained by combining those predetermined numbers. It is executed when exceeded. Conditions A to E are for determining that the accumulated toner has reached a predetermined amount even under different use conditions. That is, in condition A with respect to condition B, the charging brush cleaning process is executed even if the number of pages in the wide area printing process is only two pages larger and the number of printed pages is less than 20 pages. This is because the accumulated toner amount in the wide-area printing process for one page is about 10 times that in the normal printing process for one page. This relationship holds for each of the conditions A to E. The reason for this will be described below.

Similar to the description of the charging brush cleaning process of the comparative example 1, it is necessary to execute again at such a frequency that an image defect does not occur in the largest accumulation amount region.
Regarding the amount of accumulated toner in the normal printing process, since the image pattern is not determined, it may be considered uniform in the image area in the longitudinal direction. On the other hand, in the wide area printing process, as described above, the amount of accumulated toner is large in the outer region G2. In consideration of this, in the outer region G2, the region with the largest total amount of toner accumulation in the circumferential direction in the longitudinal direction is the most accumulated amount region. This will be described below.

  FIG. 18 shows the longitudinal position dependency of the accumulated toner amount in the wide area printing step. K1 and K2 indicate inner and outer regions of the transfer material in the longitudinal direction. G1 and G2 are the same as those in FIG. It can be seen that the region having the largest accumulation amount in the longitudinal direction is the region K2 having the maximum circumferential length of the outer region G2, and the region K2 is the most accumulated amount region. However, the maximum accumulation amount area here is the area where the accumulation amount is the largest in the wide-area printing process, but no toner exists in this area in the normal printing process. That is, the accumulated amount becomes 10 times as described above, not the value when the accumulated amount of the wide area printing process and the normal printing process is compared in this area, but the accumulated amount in each of the wide area printing process and the normal printing process. Is the value when the accumulated amounts of the regions with the largest number are compared.

In the wide-area printing process, the area K2 has an image longer than the image length in the circumferential direction. As described above, in the outer area G2, a large amount of negative toner is charged compared to the charged secondary transfer residual toner as described above. It adheres electrostatically. For example, in the normal printing process, the image on the ITB 10 is 297 mm (A4 size), the printing rate is 5% (loading amount 0.025 mg / cm ² ), and the transfer rate to the paper, that is, the transfer efficiency is 90%. In this case, in the image area in the longitudinal direction, the secondary transfer residual toner per 1 cm in the longitudinal direction is 29.7 × 0.025 × 0.1 = 0.07425 mg. If 40% of the toner adheres to the charging brush 9b, the accumulated toner per 1 cm in the longitudinal direction is 0.0297 mg. On the other hand, it is assumed that the outer area is 5 mm on the front, rear, left and right sides of the paper in the wide-area printing process, and the transfer efficiency in the outer area is 60% in the secondary transfer. In this case, the secondary transfer residual toner per 1 cm in the longitudinal direction in the region K2 is (29.7 + 0.5 × 2) × 0.025 × 0.5 = 0.38375 mg. If 80% of the toner adheres to the charging brush 9b, the accumulated toner per 1 cm in the longitudinal direction is 0.307 mg. For the above reasons, the amount of accumulated toner in the most accumulated amount area in the wide area printing process is much larger than that in the image area in the longitudinal direction of the normal printing process, and the amount is about 10 times. Therefore, as described above, conditions were set such that 2 pages in the wide printing process correspond to 20 pages in the normal printing process. It should be noted that more accurate conditions can be set for values such as the printing rate and transfer efficiency by using values that are more in line with actual usage conditions. Further, it is possible to perform the charging brush cleaning process more optimally by performing the control based on the printing information.

  As described above, all of the conditions A to E are conditions for accumulating the same amount of toner in the maximum accumulation amount region, and it is necessary to execute the charging brush cleaning process. Next, the charging brush cleaning process will be described.

  The sequence of the charging brush cleaning process as Comparative Example 2 has the same configuration as the charging brush cleaning process of Comparative Example 1 of Example 1. That is, the application time Tch in one bias application to the charging brush 9b is a value such that the time average of the discharged toner amount in the first negative bias application is increased on average in all the conditions A to E. 600 ms. The number of repetitions was set to 6 times.

  The toner accumulated on the charging brush 9b by cleaning the secondary transfer residual toner in the normal printing process and the wide area printing process is cleaned by the charging brush cleaning process as described above in the comparative technique.

<Charging Brush Cleaning Process of Example 2>
A method for shortening the time required for the charging brush cleaning process of Comparative Example 2 will be described.
Also in this embodiment, as described in the comparative example 2, it is possible to determine a single bias application time so that the amount of discharged toner per unit time in a single bias application to the charging brush 9b is maximized. is necessary. Further, in the present embodiment, in consideration of the fact that the polarity of the accumulated toner is different under the different conditions of the page number ratio in the wide-area printing process as shown in the conditions A to E, the positive and negative biases are applied in each condition. Individually, the bias application time is determined once. By doing so, the time of the charging brush cleaning process can be shortened compared with the comparative example 2.

  Even in this embodiment, as in the comparative example, when the number of counts of both the normal printing step and the page number of the wide area printing step exceeds a predetermined number in any of the conditions A to E in which those predetermined numbers are combined, A charging brush cleaning process is executed. Before explaining this process in detail, how the polarity of the accumulated toner changes depending on the frequency of execution of the wide-area printing process, that is, the ratio of the number of printed sheets, under conditions A to E will be described.

  As described above, in the wide area printing process, when the secondary transfer residual toner in the outer region G2 first enters the charging portion, a large amount of minus toner is electrostatically attached to the charging brush 9b. Further, as described above, the amount of toner that adheres to the charging brush 9b when the collected residual toner enters the charging portion again is the amount of toner that adheres when the secondary transfer residual toner in the outer region G2 first enters the charging portion. Very small compared to the amount. That is, the toner accumulated in the charging brush 9b in the two bias application states Ch2 and Ch2R at the charging portion in the wide area printing process is a large amount of negative toner and a very small amount of toner, and the polarity is almost negative. On the other hand, in the normal printing process, as shown in the first embodiment, the toner that mechanically adheres to the charging brush 9b during charging includes plus toner, and the ratio of minus toner is not particularly high.

  That is, the accumulated toner in the wide-area printing process has a negative polarity distribution compared to the accumulated toner in the normal printing process. For this reason, the accumulated toner during the charging brush cleaning process shows the same tendency as shown in FIG. 11 of the first embodiment, and the accumulated amount of minus toner increases as the execution ratio of the wide area printing process increases. , The amount of accumulated plus toner is reduced. In the longitudinal direction, the accumulation amount is particularly large in the region K2 in FIG. The charging brush cleaning process in the present embodiment in consideration of the above will be described.

FIG. 19 shows a sequence of the charging brush cleaning process in this embodiment. ChP2 and ChN2 represent a state in which the toner accumulated on the charging brush 9b is discharged onto the ITB 10 by applying a positive and negative bias for discharging the toner to the charging brush 9b, respectively. TchP2 and TchN2 represent application times per time of ChP2 and ChN2, respectively. RETP2 and RETN2 represent a state in which the discharged toner on the ITB 10 is reversely transferred to the photosensitive drums 1Y and 1M by applying a primary transfer plus and minus bias for collecting discharged toner in the first and second colors, respectively. Yes. The process is the same as the charging brush cleaning process of the first embodiment shown in FIG.

  Next, the time required for the charging brush cleaning process will be described. As in the comparative example 2, a method of determining a single bias application time is also applied here so that the time average of the amount of toner discharged with a single bias application to the charging brush 9b becomes large. However, in consideration of the fact that the amount of accumulated negative toner is larger and the amount of accumulated positive toner is smaller toward the above-described condition A side, the negative and positive discharge bias application time TchN2, in each of the conditions A to E, The optimum values of TchP2 are all different. Therefore, when the optimum values of TchN2 and TchP2 in each case of conditions A to E were obtained from experiments, the values shown in FIG. 20 were obtained. It should be noted that the number of repetitions was 6 times.

  In order to compare with the time required for the charging brush cleaning process in Comparative Example 2, when the amount of discharged toner in Condition A was examined, the same tendency as in FIG. 14 of Example 1 was shown. That is, compared to the method of Comparative Example 2, in this example, there was no generation of useless application time, and sufficient discharge could be performed in a short time. This tendency was the same in the conditions B, C, D, and E.

  As described above, in the charging brush cleaning process of Comparative Example 2, 600 ms that is TchN2 in the condition A, which is the longest as one bias application time according to the present embodiment, in all of the conditions A to E, and Both plus and minus were applied for a common time. According to the present example, the charging brush cleaning process time can be shortened compared with Comparative Example 2 under all conditions. Specifically, (600 × 2−140−600) × 6 = 2760 ms could be shortened under condition A. When calculated in the same manner, the conditions B, C, D, and E could be shortened by 2940 ms, 3120 ms, 3360 ms, and 3600 ms, respectively.

  Here, in the wide area printing process, when the secondary transfer residual toner on the ITB 10 first enters the charging portion, a method of applying a positive bias to the charging brush 9b to charge the toner is shown. On the other hand, a negative bias may be applied to the charging brush 9b to repel the secondary transfer residual toner outside the transfer material region, and then may be recovered with a negative primary transfer bias when entering the primary transfer portion. Also in this case, since the amount of negative secondary transfer residual toner outside the transfer material region is large, a part of the toner adheres to the charging brush 9b. Therefore, as a result, when the secondary transfer residual toner first enters the charging unit, the accumulated toner is the toner accumulated by charging the secondary transfer residual toner, as in the case of applying a positive bias to the charging brush 9b. Since the polarity distribution is biased to the negative side as compared with FIG.

  As described above, the amount of accumulated minus toner increases as the ratio of the number of pages in the wide-area printing process in the number of printed pages increases. In consideration of this, in this embodiment, the higher the page number ratio in this process, the more biased application time ratio in the charging brush cleaning process is biased to the minus side. By doing so, the charging brush cleaning process could be completed in a short time without taking an extra margin.

  In addition, when paper left in a low-temperature and low-humidity environment is used, the secondary transfer bias becomes high due to high paper resistance, and the toner in the region G is reversed to a positive polarity by discharge, and the secondary transfer residual toner Many can have a positive polarity. In this case, the accumulated toner also has a positive polarity. Contrary to the above configuration, the higher the execution ratio of the wide-area printing process, the smaller the negative and negative bias application time ratio in the charging brush cleaning process becomes smaller. By doing so, the same effect can be obtained.

  1Y, 1M, 1C, 1K ... photosensitive drum, 2Y, 2M, 2C, 2K ... charging roller, 6Y, 6M, 6C, 6K ... primary transfer roller, 7Y, 7M, 7C, 7K ... high voltage power supply, 8 ... secondary Transfer roller, 9a ... high voltage power supply, 9b ... charge brush, 10 ... ITB, 14Y, 14M, 14C, 14K ... cleaner, P ... transfer material

Claims (3)

An image carrier for carrying a toner image;
A rotatable intermediate transfer body onto which a toner image is transferred from the image carrier;
A primary transfer member that primarily transfers a toner image from the image carrier to the intermediate transfer body by applying a voltage;
A secondary transfer member that secondarily transfers a toner image from the intermediate transfer member to a transfer material by applying a voltage;
A charging member that charges the toner on the intermediate transfer member by applying a voltage downstream from the secondary transfer position in the rotation direction of the intermediate transfer member and upstream from the primary transfer position;
A cleaning member capable of removing toner on the image carrier;
With
A positive voltage is applied to the charging member to move the accumulated toner accumulated on the charging member to the intermediate transfer member, and then to move the accumulated toner from the intermediate transfer member to the image carrier and collect the accumulated toner from the cleaning member. In an image forming apparatus capable of executing a charging member cleaning process in which a negative voltage and a negative voltage are alternately applied at a predetermined time ratio,
In the charging member cleaning step, the time during which the voltage having the same polarity as the polarity having the high content ratio among the charging polarities of the accumulated toner is applied is longer than the time during which the voltage having the same polarity as the polarity having the low content ratio is applied. The time ratio is set so as to be longer. A first image forming step for performing primary transfer and secondary transfer;
A second image forming step in which only primary transfer is performed and secondary transfer is not performed;
Is possible and
The number of times of the first image forming step and the number of times of the second image forming step are any of a plurality of predetermined combinations among combinations of the number of times of the first image forming step and the number of times of the second image forming step. When the number of times in each combination is exceeded, it is determined that the amount of accumulated toner has reached a predetermined amount,
As the number of times of the second image forming step among the plurality of combinations is increased, the time ratio is set so that the ratio of the time during which the voltage having the same content and the polarity with the high content ratio is applied is larger. The image forming apparatus according to claim 1, wherein the image forming apparatus is set. A third image forming step of providing a blank area that does not form an image on the edge of the transfer material;
A fourth image forming step of forming an image on a transfer material without providing the blank area;
Is possible and
The number of times of the third image forming step and the number of times of the fourth image forming step are any of a plurality of predetermined combinations among combinations of the number of third image forming steps and the number of fourth image forming steps. When the number of times in each combination is exceeded, it is determined that the amount of accumulated toner has reached a predetermined amount,
As the number of times of the fourth image forming step among the plurality of combinations is larger, the time ratio is set so that the ratio of the time during which a voltage having the same polarity and the same polarity as the content ratio is applied is larger. The image forming apparatus according to claim 1, wherein the image forming apparatus is set.

Images