42 is not sufficient, toner supply failure will occur, increasing the likelihood that blurring or the like will occur. In view of this, the CPU 150 analyzes the image density in a direction orthogonal to a rotation axis of the developing roller 41, for image information for each color printed in the image forming apparatus 100. Then, based on the analysis results, the CPU 150 detects and determines in advance whether or not there is a high likelihood of a toner supply shortage.

FIG. 3 is a schematic diagram showing the state of analysis in image processing. Image information 65 is two-dimensional data that holds image density information for each color, which is defined by the transport direction of the recording material R and the rotation axis direction of the developing roller 41. In this embodiment, the CPU 150 divides the image information 65 into a plurality of image regions 66a to 66c in a direction parallel to the rotation axis direction of the developing roller 41. Then, consecutive rendering in a direction orthogonal to the rotation axis direction is analyzed for each image area, and the print percentage is detected. In this example, when a solid black image is considered to have a 100% print percentage and a solid white (no printing) image is considered to have a 0% print percentage, the CPU 150 considers a printing rate of 80% or more to be a high print percentage. However, the threshold value is not limited to this, and may be determined as appropriate. When the CPU 150 calculates the above-described print percentage, it preferably calculates the average print percentage based on image information in the range in which the developing roller 41 makes one revolution.

The determination of whether or not the average print percentage within the circumferential length of the developing roller 41 is greater than or equal to a threshold value (80%) will be described while showing the relationship with the print percentage of the image data. That is, in this embodiment, a case where image data with an average print percentage of 80% or more continues for the circumferential length of the developing roller 41 or longer will be described by way of example. Note that although image data for one revolution of the developing roller 41 is considered as a unit of determination here, there is no limitation to this.

(1) As a first example, it is envisioned that the average print percentage in the first half of the first revolution of the developing roller 41 is 80%, and the average print percentage in the second half of the first revolution is 79%. In this case, considering the image data for one revolution, the average print percentage is 79.5%. Accordingly, the first example is not subject to the control of this embodiment. (2) As a second example, it is envisioned that the average print percentage in the first revolution of the developing roller 41 is 80%, and the average print percentage in the second revolution is 79%. In this case, the state in which the average print percentage is 80% or more continues for the circumferential length or longer, and therefore this case is subject to the control of this embodiment. (3) As a third example, a case is envisioned in which there is consecutive image data in which the average print percentage in the first half of the first revolution of the developing roller 41 is 90% and the average print percentage in the second half of the first revolution is 79%. In this case, considering the image data for one revolution, the average print percentage is 84.5%. Accordingly, this case is subject to the control of this embodiment. (4) As a fourth example, a case is envisioned in which, in the first revolution of the developing roller 41, the average print percentage is 0% (solid white image) for the first ⅕ of a revolution, and the average print percentage for the next ⅘ of a revolution is 100% (solid black image). In this case, when considering image data for one revolution, the average print percentage is 80%. Accordingly, this case is subject to the control of this embodiment.

In FIG. 4, time is indicated on the horizontal axis and print percentage is indicated on the vertical axis, and the change in print percentage over time when rendering is performed with a print percentage of 100% is indicated in section T1-T2. In such a case, a comparison will be made between the comparative examples (FIGS. 5 and 6) and the examples (FIGS. 7 and 8). Here, time T1 is the timing on the photosensitive drum 1 corresponding to the leading edge of the image forming region on the recording material R excluding blank spaces and the like. Time T2 is the timing on the photosensitive drum 1 corresponding to the trailing edge of the image forming region on the recording material R. In this embodiment, the section before time T1 is called a pre-processing section, the section T1-T2 is called an image forming section, and the section after time T2 is called a post-processing section.

FIG. 5 shows supply bias control in a comparative example. In the pre-processing section, the supply bias is V0. At time T1, the CPU 150 instantaneously changes the supply bias from V0 to V0−200V in the direction of the same polarity as the normal charging polarity of the toner T (negative polarity in this embodiment). During the image forming period, that is, from time T1 to time T2, the supply bias V0−200V is maintained. At time T2, the bias is instantaneously changed in the direction of the polarity different from that of the toner T. Here, the original supply bias V0 is returned to.

In FIG. 6, the horizontal axis indicates time and the vertical axis indicates the toner supply amount, the toner amount in the supply roller 42 in the comparative example is indicated by a broken line, and the toner supply amount to the developing roller 41 is indicated by a solid line. In the pre-processing section, the toner supply amount is a constant toner supply amount according to the supply bias V0. When the supply bias is changed in the direction of the same polarity as the toner T at time T1, the toner amount in the supply roller 42 decreases, and the toner T is supplied from inside the foam elastic member of the supply roller 42 to the surface of the developing roller 41. Conversely, when the supply bias is changed in the direction of the polarity different from that of the toner T at time T2, the toner amount in the supply roller 42 increases, and the toner T is accumulated in the foam elastic member of the supply roller 42.

Here, as time elapses from the time when the bias is changed (time T1), the toner amount in the supply roller 42 balances with the electrostatic potential (time T11). As a result, the flow of toner T from the supply roller 42 becomes steady, and the increase in the toner supply amount supplied to the developing roller 41 due to the supply bias disappears (time T11). That is, in the case of the comparative example, since the toner supply amount decreases as time elapses from time T1, there is a high likelihood that a toner supply shortage will occur, and image defects such as blurring are likely to occur. In the illustrated example, the toner supply amount St1 is sufficient at time T1, but the supply amount gradually decreases and falls below a toner supply amount Sts required for a high print image at a certain timing. Thereafter, the supply amount further decreases and becomes steady at an insufficient toner supply amount St2.

FIG. 7 shows supply bias control in this embodiment. In this embodiment, the supply bias is gradually changed in the direction of the same polarity as the toner T (negative polarity in this embodiment) in the image forming section T1-T2. In FIG. 8, the toner amount in the supply roller 42 of this embodiment is indicated by a broken line, and the toner supply amount supplied to the developing roller 41 is indicated by a solid line. In this embodiment, as the supply bias gradually changes, the toner amount in the supply roller 42 also continues to decrease gradually. For this reason, the toner T continues to be supplied from the supply roller 42 to the developing roller 41. That is, in this embodiment, the toner supply amount supplied to the developing roller 41 is maintained throughout the image forming section T1-T2, and therefore the likelihood of insufficient toner supply occurring is reduced.

The toner supply amount St3 supplied to the developing roller 41 in the image forming section T1-T2 in FIG. 8 according to this embodiment is a stable value and exceeds the toner supply amount Sts required for a high print image. For this reason, even under conditions where high print images are consecutive, the likelihood that image defects such as blurring will occur is reduced.

Note that in FIG. 7, the supply bias was changed so that the slope was constant, but the method for changing the supply bias is not limited to this, as long as the toner required for a high print image can be supplied. For example, in the section T1-T2 of FIG. 7, the supply bias may also be reduced in a stepwise manner.

Also, after time T2, post-processing operations after the end of image formation are shown. By returning the supply bias to the value before time T1 at time T2, the supply roller 42 can be replenished with the toner T.

On the other hand, if excessive toner is supplied to a region where the toner T is not used, such as a white background area, the developing blade 43 may not be able to regulate the toner layer thickness to an appropriate thickness. In this case, image defects such as “fogging” occur, in which the toner T is developed on the white background. In view of this, it is necessary to determine whether the toner T is being supplied excessively and to appropriately control the supply bias.

FIG. 9 shows the relationship between the print percentage and the change amount of the supply bias (V/mm) in the control of this embodiment. In the control of this embodiment, in order to supply the toner T for a high print image, in an image region determined as being a high print image, the supply bias is changed by −0.4 V/mm with respect to the length in the rotational direction of the developing roller 41 of the image region (reference sign 75b). On the other hand, in a region determined as not being a high print image, the supply bias is controlled to be constant (here, 0.0 V/mm, reference sign 75a). Here, in this embodiment, the change amount of the supply bias is a negative value, but it may take a positive value as long as the toner T is in the direction of being supplied to the developing roller 41.

Also, the supply roller 42 of this embodiment has a configuration in which an open-cell foam elastic member rotates while being pressed by the developing roller 41. For this reason, it is compressed upstream of the contact portion and expanded downstream of the contact portion. This change in the state of compression and expansion prompts replacement of the toner T in the foam elastic member of the supply roller 42, and smooths changes in the toner amount caused by the supply bias.

Processing Flow

Specific control in this embodiment will be described with reference to the flow in FIG. 10. (Step S101) A print command is issued to the image forming apparatus 100 from a terminal such as the personal computer 200.

- (Step S102) The CPU 150 converts the image information 65 to a color space for the image forming apparatus 100. For example, an RGB system is converted to a CMYK system suitable for handling by the image forming apparatus 100.
- (Step S103) The CPU 150 determines whether or not a high print image is to be printed within each image region. First, the CPU 150 divides the image information 65 into a plurality of image regions 66. Then, in each image region 66, density information for each color is acquired. Then, for each divided image region 66 and for each color, it is determined whether or not a high print image that would result in insufficient toner supply will be printed in the direction parallel to the rotation axis of the developing roller 41.
- (Step S104) The CPU 150 determines the supply bias and the change amount thereof for the image region determined as being a high print image. The change amount of the supply bias at this time is determined based on the print percentage with reference to FIG. 9.
- (Step S105) The CPU 150 determines whether the determination of high print images has been performed for all image regions of the image. If there is a region that has not been subjected to determination yet, the processing returns to step S103 and continues. On the other hand, when the determination is performed for all image regions, the processing moves to step S106.
- (Step S106) The image forming apparatus 100 performs an actual image forming operation based on the determined supply bias. (Step S107) After image formation is complete, a standby state is transitioned to.

By doing the above, a sufficient amount of toner T can be supplied even when printing a high print image, and therefore image defects such as blurring will not occur. Also, even when printing a low print image, excessive toner T is not supplied, and therefore fogging can be suppressed.

Here, the method of this example is applicable even if the image forming apparatus 100 is a monochrome printer that can render only monochrome images, or a full-color printer such as a CMYK four-color printer.

Example 2

Next, Example 2 will be described. Configurations that are the same as those as in Example 1 described above are denoted by the same reference numerals, and detailed description thereof is omitted.

In Example 1 above, a case where high print images are consecutive was considered. In this example, a case will be considered in which a low print image such as a halftone image is printed after a high print image in a direction orthogonal to the rotation axis of the developing roller 41. Note that here, an image that has a lower print percentage than a high print image and is not a white image (no printing) is referred to as a low print image. Comparing this case with a case where a high print image is not printed before a low print image, it is found that because the amount of toner T used on the surface of the developing roller