JP2020140087A - Image forming apparatus and image forming method - Google Patents

Abstract

To provide an image forming apparatus that can prevent dirt on a recording medium due to granular scumming.SOLUTION: The image forming apparatus forms a negative electrostatic latent image on a photoconductive photoreceptor 1, performs reverse development in a dry two-component developing system to obtain a visible image, and transfers and fixes the visible image to a sheet-like recording medium with a transfer device 12, and comprises: imaging means 14 that acquires image data of a non-image part in a visible image of a ground staining pattern formed on a recording medium 1; image data processing means that converts the image data acquired by the imaging means into a monochrome image having gradation properties, obtains a binary black and white image with a threshold set to the gradation properties of the monochrome image, and counts the number of black pixels in the obtained black and white image; and image formation condition adjustment means that adjusts an image formation condition according to the number of black pixels counted by the image data processing means.SELECTED DRAWING: Figure 1

Description

The present invention relates to an image forming apparatus and an image forming method.

A negative electrostatic latent image is formed on a photoconductive photoconductor, and the negative electrostatic latent image is inverted and developed by a dry two-component developing method to obtain a visible image, and the visible image is transferred into a sheet by a transfer device. An electrophotographic image forming apparatus that transfers and fixes to a recording medium is widely known as various monochrome and color copying apparatus, printer, composite apparatus, and the like.
It has been conventionally known that various adjustments are made to an image transferred / fixed to a sheet-shaped recording medium and output in connection with an image forming apparatus (Patent Documents 1 to 3 and the like).
The electrophotographic image forming apparatus has a phenomenon called "ground stain".
That is, it is a phenomenon in which toner adheres to the background potential portion, which is a non-image portion of the photoconductor, and causes image stains, and "as if the toner was scattered" on a white background portion of a visible image formed on a sheet-shaped recording medium. It is a phenomenon of adhesion.
As a typical background stain, "reverse charging toner" that is charged in the opposite polarity to the normal charging polarity is generated due to deterioration of the toner over time, and this reverse charging toner is the non-image part of the electrostatic latent image. There is known "skin fogging" that adheres to the skin potential portion and is transferred to a recording medium to become stains on the recording medium and stain the recording paper.

On the other hand, the electric charge charging the background portion of the negative electrostatic latent image formed on the photoconductive layer of the photoconductor leaks the photoconductive layer into a hole shape toward the conductive portion side, lowering the background portion potential and causing a “hole shape”. There is also ground stain that occurs when the "leakage portion" is developed with an appropriate charged toner and visualized.
Since this ground stain occurs in "granular" depending on the hole-shaped leak portion, it is referred to as "granular ground stain" in this specification. Granular ground stains are characterized by "large and conspicuous grains" compared to so-called ground stains and skin fog.
Conventionally, various techniques for reducing or preventing "skin fogging" have been known, but no technique for dealing with "granular background stains" has been known.

An object of the present invention is to realize an image forming apparatus capable of suppressing stains on a recording medium due to granular background stains.

The image forming apparatus of the present invention forms a negative electrostatic latent image on a photoconductive photoconductor, reverse-develops the negative electrostatic latent image by a dry two-component developing method to obtain a visible image, and obtains the visible image. An image forming apparatus for transferring an image to a sheet-shaped recording medium by a transfer device and fixing the image, and an imaging means for acquiring image data of a non-image portion in a visible image of a background stain pattern formed on the recording medium. The image data acquired by the imaging means is converted into a monochrome image having gradation, and a binary black-and-white image is obtained by a threshold set for the gradation of the monochrome image, and the obtained black-and-white image is obtained. It has an image data processing means for counting the number of black pixels in an image, and an image forming condition adjusting means for adjusting the image forming condition according to the number of black pixels counted by the image data processing means.

According to the present invention, it is possible to realize an image forming apparatus capable of suppressing stains on a recording medium due to granular background stains.

It is a figure which shows one embodiment of the image forming apparatus. It is a figure for demonstrating the background dirt. It is a figure explaining the relationship between the granular ground dirt rank and the number of black pixels. It is a figure explaining the relationship between the charge bias and the granular dirt rank. It is a figure for demonstrating change of the target value of development γ. It is a figure which shows the processing process by image data processing means, image formation condition adjustment means, and adjustment process as a flow chart.

Hereinafter, embodiments will be described.
FIG. 1 shows one embodiment of an image forming apparatus.
The image forming apparatus 100 is a full-color image forming apparatus of a quadruple tandem type intermediate transfer method in which a dry two-component developing method is adopted as a developing method. This image forming apparatus is an example, and the apparatus configuration and the image forming method are not limited except that the developing method is a dry two-component developing method.

The image formation operation will be described.
When a "print start command" is input to the controller CT, the photoconductive photoconductor 1, the intermediate transfer belt 2, the rollers in the paper feed transport path, etc. start to rotate, and the paper feed tray 3 at the bottom of the device starts to rotate. Feeding of recording paper, which is a "sheet-shaped recording medium", is started.
The surface of the photoconductor 1 is charged to a uniform potential by the charging unit 4, and the surface is exposed by light scanning with the writing light emitted from the writing unit 5 as an exposure means to form an electrostatic latent image. To. Since the writing light exposes a portion corresponding to the image to be formed, the portion corresponding to the image of the electrostatic latent image is potential-attenuated. The electrostatic latent image formed in this way is a "negative electrostatic latent image".

The photoconductor 1 carrying the formed negative electrostatic latent image is reverse-developed by a dry two-component developing device 18, and the negative electrostatic latent image is visualized in a specific color.
In the image forming apparatus shown in FIG. 1, the photoconductor 1 has four colors, and these four photoconductors are for the color image components of yellow, magenta, cyan, and black in order from the left of the figure. Toner images of these colors are formed on the photoconductor 1.
The four types of toner images formed are the transfer bias applied to the primary transfer roller 6 installed facing each photoconductor 1 via each photoconductor 1 and the intermediate transfer belt 2 and the primary transfer roller 6 A "full-color toner image" is formed as a visible image by being sequentially superposed on the intermediate transfer belt 2 and transferred by the pressing force from.
The full-color toner image formed on the intermediate transfer belt 2 is transferred to the recording paper which is conveyed at the same timing by the resist roller 7. This transfer is performed by the secondary transfer bias and pressing force applied by the secondary transfer roller 8.

The recording paper on which the full-color toner image is transferred is fixed by heating and pressurizing the full-color toner image supported on the surface by the fixing unit 9 which is a “fixing device”.

In the case of "single-sided printing", the recording paper is directly conveyed and discharged onto the output tray 10.
In the case of "double-sided printing", the transport direction of the recording paper is changed downward, the transport direction is reversed by the switchback roller 11, and the recording paper proceeds from the rear end of the paper to the refeed path 30. At this time, the front and back of the recording paper are reversed. The front-back inverted recording paper passes through the re-feeding path 30 and joins the original feeding path, the full-color toner image is transferred to the back surface, and the paper is discharged onto the tray 10 after being fixed by the fixing unit 9. Toner.
When the toner image is transferred onto the recording paper, the surface of each photoconductor 1 is removed of residual toner remaining on the surface by the cleaning unit 12, and then uniformly discharged by the quenching lamp 13.
Toner remaining after transfer remains on the intermediate transfer belt 2, and these are also removed by the intermediate transfer belt cleaning unit 15.
By repeating the above operation, single-sided printing or double-sided printing is performed.

The above-mentioned "background fog" and "granular background stain" may occur on the background portion (white background portion) of the toner image formed on the recording paper which is a sheet-like recording medium in such an image forming process.
As described above, the background fog is generated by the change in the charging polarity of the toner used for development, and the granular background stain is caused by the charge charging the photoconductive layer of the photoconductor on the conductive part (photoconductive layer) of the photoconductor. It is generated by the adhesion of toner to the portion where the potential of the leaked portion is attenuated by "leaking" to the carrying portion).
The image forming apparatus shown in the first embodiment shown in FIG. 1 has an image pickup device 14 as an “imaging means” on the discharge side of the fixing device 9. As the image sensor 14, a known appropriate one such as a CCD area sensor or a CMOS area sensor can be used, but here, for the specificity of the description, it is referred to as a “CCD area sensor”.
In the present invention, a "skin stain pattern" is formed on the non-image forming portion (the portion that does not come into contact with the recording paper) of each photoconductor. The background stain pattern is an area region in which the non-image forming portion is uniformly charged by each charging unit 4, and has an area having a size imaged by the CCD area sensor 14. This background stain pattern is a "white background portion" as a developed image, and if there is no background stain, it is "visualized as a white background portion". If there is a background fog or a granular background stain, these become a visible image visualized as a background stain.

The upper figure of FIG. 2 is an example of a “CCD image” obtained by capturing a “visible image of a background stain pattern” with the CCD area sensor 14. In this example, "ground cover (dots having a low density and a small size)" and "granular stains (a relatively high density and a large size. Illustrated by reference numerals Q1 and Q2)" are mixed.
Such a CCD image is converted into a "monochrome image having tonality", and is converted into a binary black-and-white image by a threshold value set for the tonality of the monochrome image.
The lower figure of FIG. 2 shows a state in which the captured CCD image is converted into a “256 gradation monochrome image” and then converted into a “binary black and white image” with the gradation level 128 of this monochrome image as a threshold value. ing.
In the black-and-white image thus obtained, in the upper figure of FIG. 2, a gray image having a gradation level of 128 or higher is a "black image", and a gray image having a gradation level of less than 128 is a "white image". Become.

Comparing the upper and lower figures of Fig. 2, for example, as you can see from the "inside the dashed circle", the converted black-and-white image shows almost no "background fog", but "granular background stains". "Remains.

In this black-and-white image, when the number of pixels in the black image portion, that is, the "number of black pixels" is counted, the count number represents the degree of "granular background stain".

Therefore, the charging bias of each photoconductor 1 is changed to form a plurality of types of "skin stain patterns with different charging potentials", and the above black-and-white image is formed with respect to the visible images of these skin stain patterns, and black pixels are formed. I counted the number. Multiple types of "visible images of background stain patterns" were divided into eight samples according to the number of black pixels.
Table 1 shows the results of visually "ranking" the degree of "granular background stain" in the "visible image of the background stain pattern" corresponding to each of these eight samples A to H.

表１に見られるように、目視順位が１から８に向かうにつれて「粒状地汚れ」が目視で顕著になる。即ち、目視順位が良いもの（順位数が小さいもの）は黒ピクセル数が少ない傾向があり、上記黒ピクセル数は「粒状地汚れを数値化」したものとして妥当であると考えられる。

As can be seen in Table 1, "granular background stains" become more noticeable visually as the visual order goes from 1 to 8. That is, those having a good visual ranking (those with a small number of rankings) tend to have a small number of black pixels, and the number of black pixels is considered to be appropriate as "quantifying granular background stains".

Next, image samples with different degrees of granular background stains (visible images of background stain patterns) are ranked as "5 ranks" of rank 1 to rank 5 to be "granular background stain rank", and the number of black pixels is used. As a result of investigating the correspondence, it became as shown in FIG.
The higher the number of ranks, the better the granular background stain rank (horizontal axis) (the granular background stain is less noticeable visually), and it is difficult to "visualize the granular background stain" at rank 4 or higher.
When the number of black pixels is 500 or more, the granular background stain rank gradually decreases, and the granular background stain becomes easy to see.
Therefore, in this case, the number of black pixels: 500 is set as the "black pixel number threshold value", and the image formation conditions are adjusted with the number of black pixels exceeding 500.
That is, the image formation conditions are adjusted so that the number of black pixels is less than 500.
Granular background stains tend to worsen as the charging bias increases (the absolute value of the charging potential increases). That is, it is considered that when the development bias becomes high and the absolute value of the surface potential of the photoconductor becomes large, the voltage applied in the thickness direction of the photoconductive layer becomes high, and the surface charge leaks easily.
As a result of examination by the inventors through experiments, in order to obtain an image of a level higher than the granular background stain rank 4 (an image of a level in which granular background stains are generated but hardly visible to the naked eye), a charging bias is applied. It was found that the value should be -650V or higher (absolute value smaller than 650).
That is, as shown in FIG. 4, when the charging bias is −650 V or more (on the left side of the figure), the granular background stain rank is 4 or more, and the granular background stain at a visually visible level does not occur.
In short, the degree of granular background stain is quantified as the number of black pixels from the "visible image of the background stain pattern" captured by the CCD area sensor 14, and when the number of black pixels exceeds the threshold value, the image formation condition is adjusted to obtain the granular background. Improve dirt and make it invisible to the naked eye.
By the way, assuming that the photoconductor surface potential: -600V and the exposed part potential: -100V as one of the criteria for image formation conditions, the development potential is -500V or less (absolute) in order to make the charging bias -650V or more. It is necessary to keep it to a small value).
When the development potential becomes larger than −500 V in absolute value, the electric force that pushes the toner to the electrostatic latent image increases, and the toner adheres to the portion that should be a white background, and the background stain increases.
As described above, by adjusting the charge bias as one of the image formation conditions according to the number of black pixels, granular background stains can be improved.
By the way, the target value of "development γ" under the standard image-forming conditions exemplified above is 0.8 mg / cm ² / kV. In order to obtain the toner adhesion amount of 0.4 mg / cm ² required to obtain a "solid image with sufficient image density" under such conditions, -625 V is required as the development potential, but in the above example As described above, at "development potential: -500 V", granular background stains are effectively improved, but the density of the solid image becomes lighter.
Therefore, the target value of the developed γ under the image forming condition is changed from the standard 0.8 mg / cm ² / kV to 1.0 mg / cm ² / kV.
In FIG. 5, the horizontal axis represents the developing potential (-V) and the vertical axis represents the amount of toner adhered to show the relationship between the two. The slope of the approximate straight line obtained from the plot is a numerical value called "development γ". The approximate straight line of the reference development γ target value: 0.8 is shown by a broken line, and the approximate straight line in which the development γ is adjusted to 1.0 with respect to the occurrence of granular ground stain is shown by a solid line.

By changing the target value of development γ from 0.8 to 1.0, a sufficient solid image can be obtained even with a development potential of −500V.
Therefore, when the number of black pixels becomes 500 or more, the charging bias is adjusted to -650V and the developing potential is set to -500V to improve granular background stains and set the target value of developing γ to 1.0. By adjusting to, an image having a sufficient solid image density can be obtained.

In the image forming apparatus described above, the "imaging means" is the CCD area sensor 14.
The image data processing means and the image forming condition adjusting means are stored as a computer or a CPU as a program in the controller CT shown in FIG.
The image data processing means, the processing process by the image forming condition adjusting means, and the adjusting process in the embodiment described above are shown in FIG. 6 as a flow chart.

Step: Starting in S0, image data acquisition is executed in step: S1.
In order to acquire image data, a "skin stain pattern" is formed on the photoconductor, this pattern is developed to obtain a "visible image of the skin stain pattern", and the obtained visible image is captured by an imaging means (CCD). An image is taken by the area sensor 14) and converted into image data.
In the embodiment described above, the “visible image of the background stain pattern” is fixed on the recording paper which is a sheet-shaped recording medium, and then the image is taken by the imaging means 14, but the image is not limited to this. The visible image transferred to the image may be imaged before fixing and converted into image data.

Then, in step: S2, "monochrome image conversion" is executed.

In the monochrome image conversion, the image data captured by the imaging means 14 is sent to the image data processing means of the controller CT, and the image processing means into which the image data is input is a monochrome image having gradation, that is, "gray with shading". This is done by converting to an "image".
In the above-described embodiment, the number of gradations of the monochrome image is set to "256 gradations", but of course, it is not limited to 256 gradations. For example, 128 gradations, 512 gradations, 1024 gradations and the like may be used.
When a monochrome image having gradation is obtained, the "black and white image acquisition" of step S3 is performed by the image data processing means. In the above description, the gradation level 128 is used as the threshold value used for acquiring the black-and-white image, but of course, this threshold value is not limited to this.
Assuming that the maximum value of gradation is 100%, for example, 30 to 70%, preferably 40 to 60% is appropriate, and 128 gradations shown as a specific example above is 50%, which is preferable 1. This is an example.

Then, in step: S4, "counting the number of black pixels" is performed. This step: S4 is also performed by the image data processing means of the controller CT.
In the black pixel count, the number of black pixels in the black-and-white image acquired in step S3: NBP is counted.
"Black pixel number threshold value: SNBP" is stored in advance in the image data processing means.
Black pixel number threshold: The SNBP is "500" in the example described above, and is the number of black pixels at which "granular background stains" become visible when the image formation process is performed above this value, which is experimental in advance. Is set to.

Subsequently, the magnitude relationship between the number of black pixels counted in step: S4: NBP and the threshold number of black pixels: SNBP is determined in step: S5.
If it is determined in step: S5 that "NBP ≥ SNBP", the process proceeds to step: S6 to adjust the charging bias. In the above example, the charge bias is adjusted to -650V and the development potential is set to -500V. By executing this step: S6, it is possible to prevent the occurrence of granular ground stains that can be visually recognized.
When it is determined in step: S5 that "NBP <SNBP", granular ground stains that can be visually recognized do not occur. In this case, the process proceeds to step: S8 without proceeding to step: S6.

When proceeding to step: S7 after adjusting the charging bias, the target value of the developed γ is adjusted to increase the density of the solid portion.
When "adjusting the charging bias" in step: S6, if the shading of the solid portion is not a problem, the process may proceed to step: S8 without proceeding to step: S7.
Although the preferred embodiment of the invention has been described above, the present invention is not limited to the specific embodiment described above, and the invention described in the claims unless otherwise limited in the above description. Various modifications and changes are possible within the scope of the above.
The effects described in the embodiments of the present invention merely list suitable effects arising from the invention, and the effects according to the invention are not limited to "the ones described in the embodiments".

100 image forming device
1 Photoreceptor
9 Fixing device (fixing unit)
2 Intermediate transfer belt
14 Imaging means (CCD area sensor)
CT controller

JP-A-2017-107056 Japanese Unexamined Patent Publication No. 2000-122353 Japanese Unexamined Patent Publication No. 2008-96735

Claims (11)

A negative electrostatic latent image is formed on a photoconductive photoconductor, and the negative electrostatic latent image is inverted and developed by a dry two-component developing method to obtain a visible image, and the visible image is formed into a sheet by a transfer device. An image forming apparatus that transfers and fixes to a recording medium.
An imaging means for acquiring image data of a non-image portion in a visible image of a background stain pattern formed on the recording medium, and
The image data acquired by the imaging means is converted into a monochrome image having gradation, and a binary black-and-white image is obtained by a threshold value set for the gradation of the monochrome image, and the obtained black-and-white image is obtained. Image data processing means for counting the number of black pixels in
An image forming apparatus having an image forming condition adjusting means for adjusting an image forming condition according to the number of black pixels counted by the image data processing means. The image forming apparatus according to claim 1.
An image forming apparatus in which the image forming condition adjusting means reduces the charging bias for the photoconductor in absolute value with respect to the black pixel number threshold value or more set for the counted number of black pixels. The image forming apparatus according to claim 2.
An image forming apparatus in which the image forming condition adjusting means adjusts a target value of development γ with respect to the black pixel threshold value or higher. The image forming apparatus according to any one of claims 1 to 3.
The image data processing means converts the image data into a monochrome image having 256 gradations, converts the monochrome image into a black-and-white image with the threshold value set to 128, and counts the number of black pixels in the converted black-and-white image. apparatus. The image forming apparatus according to claim 4.
An image forming apparatus in which the image forming condition adjusting means adjusts the image forming condition for the number of black pixels: 500 or more. The image forming apparatus according to any one of claims 1 to 5.
An image forming apparatus in which the imaging means is arranged on the outlet side of the transfer device and images the visible image of the background stain pattern transferred to the sheet-shaped recording medium. An image forming method using the image forming apparatus according to any one of claims 1 to 6.
An image data acquisition step of capturing the visible image of the background stain pattern formed on the recording medium by the imaging means and acquiring the image data.
The acquired image data is converted into a monochrome image having gradation by the image data processing means, and a binary black-and-white image is obtained by a threshold set for the gradation of the monochrome image. A data processing step of performing data processing for counting the number of black pixels in the black and white image, and
An image forming method including an image forming condition adjusting step of adjusting an image forming condition according to the number of black pixels counted by the image data processing means. The image forming method according to claim 7.
An image forming method in which the image formation condition adjusting step reduces the charging bias for the photoconductor in absolute value with respect to the black pixel number threshold value or more set for the counted number of black pixels. The image forming method according to claim 8.
An image forming method including the step of adjusting the target value of the development γ with respect to the black pixel number threshold value or more set for the counted number of black pixels. The image forming method according to any one of claims 7 to 9.
A step in which the image data processing means converts the image data into a monochrome image having 256 gradations, converts the monochrome image into the black and white image with the threshold value set to 128, and counts the number of black pixels in the converted black and white image. The image forming method. The image forming method according to claim 10.
An image forming method in which the image forming condition adjusting step is a step of adjusting the image forming condition with respect to the counted number of black pixels: 500 or more.

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