JP2008107803A - Image forming apparatus and image forming method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus that uses dark toners and light toners which have the same hue and are different from each other in density, the image forming apparatus forming a high-quality image irrespective of the surface roughness of a recording material, and to provide an image forming method. <P>SOLUTION: The image forming apparatus is equipped with: an image bearing member; a toner image forming means forming a toner image on the image bearing member, using light toners and dark toners which have the same hue as that of the light toners and are darker than the light toners; a transfer means which electrostatically transfers a toner image on the image bearing member onto the recording material; and an adjusting means which adjusts the toner image forming means so that a rate between the dark toners and the light toners included in a toner image with predetermined density on the image bearing member is changed according to the roughness of the surface of the recording material onto which the toner is transferred. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an electrophotographic image forming apparatus (hereinafter simply referred to as “image forming apparatus”) and an image forming method, such as a copying machine, a printer, a facsimile apparatus, and a multifunction peripheral, which perform image formation using an electrophotographic system. In particular, the present invention relates to an image forming apparatus and an image forming method using dark toner and light toner having the same hue and different densities.

  In an image forming apparatus that forms a color image, a four-color toner image such as Y (yellow), M (magenta), C (cyan), and K (black) is formed on a photoconductor drum that is an image carrier.

  Each of these toner images is transferred in a superimposed manner, for example, on a sheet (transfer material) held on a transfer drum (transfer film). In this case, when an electrostatic latent image of, for example, cyan, which is the first color, is formed on the photosensitive drum based on the input signal obtained by reading the image information, it is obtained by developing the cyan latent image. The C toner image is transferred to a sheet on the transfer drum. Such a series of transfer processes is repeated for the other three colors of Y, M, and K toners in the order of second, third, and fourth, thereby obtaining a color image.

  In recent years, in an image forming apparatus using a digital image signal, a latent image is gathered and formed on the surface of a photosensitive drum carrying dots of a constant potential, and a solid portion, a halftone portion, and a halftone portion are changed by changing the density of the dots. Each line part is expressed. However, in this case, it is difficult for toner particles to be placed faithfully on the dots, and the toner particles protrude from the dots. Therefore, it is difficult to obtain the gradation of the toner image corresponding to the dot density ratio between the black part and the white part of the digital latent image. In addition, if the dot size is reduced to increase the resolution in order to improve the image quality, the reproducibility of the latent image formed by a collection of minute dots becomes more difficult, especially in the highlight area. There is a tendency that the gradation is deteriorated and the color tone of the image lacks sharpness. In addition, irregular dot irregularity gives rise to a feeling of graininess, and contributes to a decrease in image quality in the highlight portion. Image unevenness due to such graining is an unstable element of image quality that cannot be predicted.

  On the other hand, with regard to the ink jet recording method, for example, as seen in the dark and light ink process technology disclosed in Japanese Patent Application Laid-Open No. 58-39468, the system itself is simple and supported by the recent use of excellent high-quality dedicated paper. The above-mentioned problems that the electrophotographic system has do not occur. Furthermore, in the case of the ink jet recording method, there is no graininess which is a unique advantage of using dark and light inks. In particular, when light-colored ink is used, excellent performance is exhibited. Therefore, if such a light-colored color can be adopted in an electrophotographic system, it will be a great progress.

Furthermore, with regard to optical dot gain, which has been an obstacle to electrophotography when pursuing high-quality images, the idea of introducing light color toner is effective against the problems associated with the use of minute toners. . Based on this concept, light color toner (light toner) is used for the highlight area, dark toner (dark toner) is used for the solid area, and an image is formed by combining a plurality of toners having different densities. A forming apparatus has been proposed (see, for example, JP-A-11-84764 and JP-A-2000-305339). Further, an image forming apparatus has been proposed in which a light toner having a maximum reflection density less than half of the maximum reflection density of dark toner is combined (see, for example, JP-A-2000-347476). Further, there has been proposed an image forming apparatus that combines a dark toner having an image density of 1.0 or more and a light toner having a toner density of less than 1.0 when the toner amount on the sheet is 0.5 mg / cm ² . (For example, refer to JP-A-2000-231276). Further, there has been proposed an image forming apparatus in which a toner whose ratio of recording density of dark toner and light toner is adjusted to 0.2 to 0.5 is combined (for example, see JP-A-2001-290319).

  In the case of the image forming apparatus disclosed in the above patent document, the type of sheet as a transfer material is premised on the use of plain paper. As described above, the inkjet recording method realizes a high-level image forming technique by stable high-resolution binary recording by using high-quality dedicated paper having excellent printing performance. Even if such an image forming technique based on the ink jet recording method is introduced into a mainstream electrophotographic method in which plain paper is used as it is, it is very difficult to realize it. Conventionally, in order to fill in the difference in performance between high-quality dedicated paper and plain paper, the electrophotographic method uses a low-resolution screen used for printing to improve density gradation. Accordingly, the use of light toner in the low density portion can improve the problem of roughness and graininess peculiar to the electrophotographic method.

JP 58-39468 A Japanese Patent Laid-Open No. 11-84764 JP 2000-305339 A JP 2000-347476 A Japanese Patent Laid-Open No. 2000-231276 JP 2001-290319 A

  By the way, when trying to improve the image quality of an image such as a photograph, the smoothness of the highlight portion tends to be emphasized as an image using dark and light toner.

  However, as a result of smoothing the highlight portion, the amount of applied toner in the highlight portion increases. When a toner image is formed on a sheet (recording material) having a rough surface, the toner image having a large amount of applied toner tends to cause uneven transfer of light toner in a highlight portion.

  An object of the present invention is to provide an image forming apparatus and an image forming apparatus capable of forming a high-quality image without depending on the surface roughness of the recording material in an image forming apparatus using dark toner and light toner having the same hue and different densities. Is to provide a method.

  A typical means in the present invention for solving the above-described problem is that the image carrier is composed of an image carrier, a light toner, and a dark toner having the same hue as the light toner and a darker toner than the light toner. A toner image forming means formed on the body, a transfer means for electrostatically transferring the toner image on the image carrier to a recording material, and a roughness of a surface of the recording material on which the toner image is transferred And an adjusting means for adjusting the toner image forming means so that the ratio of the dark toner and the light toner contained in the toner image of a predetermined density on the image carrier is different.

  According to another exemplary means of the present invention, a toner image is formed on the image carrier using a light toner and a dark toner having the same hue as the light toner and a higher density than the light toner, A toner image on the image carrier is electrostatically transferred to a recording material, and a toner image of a predetermined density on the image carrier is formed according to the roughness of the surface of the recording material on which the toner image is transferred. The toner image forming means is adjusted so that the ratio of the dark toner and the light toner contained is different.

  In the present invention, when dark toner and light toner having the same hue and different densities are used, it is possible to form a high-quality image regardless of the surface roughness of the recording material.

  Hereinafter, an electrophotographic full-color image forming apparatus and an image forming method as preferred embodiments of an image forming apparatus according to the present invention will be described in detail with reference to the drawings.

  As shown in FIG. 1, the electrophotographic full-color image forming apparatus of the present embodiment has a digital color image reader unit (hereinafter simply referred to as “reader unit”) 1R in the upper part, and a digital color image printer unit (hereinafter referred to as “reader part”). 1P) (hereinafter simply referred to as “printer unit”) is arranged. In this case, the printer unit 1P can be configured to operate based on the read signal output from the reader unit 1R.

  The reader unit 1 </ b> R irradiates and scans the original 30 placed on the original table glass 31 with light from the exposure lamp 32, and condenses the light image reflected from the original 30 onto the full-color CCD sensor 34 by the lens 33. As a result, a color separation image signal is obtained. Each of the color-separated image signals is subjected to image processing by a video processing unit through an amplifier circuit (not shown), and is sent to the printer unit 1P via an image memory. In addition to the image reading signal output from the reader unit 1R, the printer unit 1P also receives an image signal from a computer, an image signal by FAX communication, and the like.

  In the printer unit 1P, for example, six photosensitive drums 1a to 1f as image carriers are supported rotatably in the direction of the arrow in the figure. Hereinafter, in the description, the six photosensitive drums 1a to 1f will be described by being represented by reference numeral "1". Similarly, other members are also described by representative symbols. Around each photosensitive drum 1, a pre-exposure lamp 11, a corona primary charger 2, a laser exposure optical system 3, a potential sensor 12, six developing devices 40 loaded with toners having different spectral characteristics, a transfer device 5A, and A cleaning device 6 is arranged. Each of these devices forms six image forming portions (toner image forming means) Pa to Pf, but of course, the number is not limited to six, and an arbitrary number of four or more image forming portions are installed. be able to.

  Therefore, in the six developing devices 40, a thin magenta (LM) toner is loaded in the developing device 41 which is one of them. The developing device 42 is loaded with light cyan (LC) toner. The developing device 43 is loaded with yellow (Y) toner, the developing device 44 is loaded with magenta (M) toner, the developing device 45 is loaded with cyan (C) toner, and the developing device 46 is loaded with black (K) toner. Has been. Each toner is negatively charged in the developing device.

  The magenta (M) toner loaded in the developing device 44 is granulated using a dark M toner (hereinafter referred to as “dark toner”) and a light M toner (hereinafter referred to as “light toner”). The goal is to reduce it. Similarly, for the cyan (C) toner loaded in the developing device 45, dark toner and light toner are used.

  The dark magenta toner and the light magenta toner are used simultaneously in one image, and the dark magenta toner and the light magenta toner are overlapped at the same pixel that forms the image.

  The method of using the dark cyan toner and the light cyan toner is the same.

  In addition, an image forming unit including a developing device loaded with a metallic toner such as gold or silver or a toner containing a fluorescent agent can be provided. The developing device 40 is loaded with a two-component developer that uses a mixture of toner and carrier, but the technical idea of the present invention can be achieved even when only a single-component developer is used.

  In the scanner 3 of the laser exposure optical system, an image reading signal output from the reader unit 1R is converted into an optical signal by a laser output unit (not shown), and the laser beam converted into the optical signal is reflected by a polygon mirror. The The reflected laser light is projected one by one on the drum surfaces of the six photosensitive drums 1 through the lens and each reflecting mirror.

  With the above configuration, image formation is performed in the printer unit 1P as follows.

  The photosensitive drum 1 is rotated in the direction of the arrow, and the photosensitive drum 1 after being neutralized by the pre-exposure lamp 11 is uniformly charged by the primary charger 2 and exposed for each separated color to be exposed to the photosensitive drum 1. An electrostatic latent image is formed thereon.

  Subsequently, the developing device 40 is operated to develop the latent image on the photosensitive drum 1, and a visible image (toner image) using a resin and a pigment as a base is formed on the photosensitive drum 1. The toner in the developing device 40 is replenished as needed at a desired timing so as to keep the toner ratio (or toner amount) in the developing device 40 constant from the toner container (hopper) 60 for each color adjacent to the scanner 3. The The toner image formed on the photosensitive drum 1 is primarily transferred to an intermediate transfer belt 5 as an intermediate transfer body in each transfer device 5, and the respective toner images are sequentially superimposed on the intermediate transfer belt 5.

  The intermediate transfer belt (image carrier, intermediate transfer member) 5 is wound between each of the driving roller 51, the driven roller 52, and the roller 54, and transmits the rotational power from the rotational driving source to the driving roller 51. The intermediate transfer belt 5 is rotated by the roller 51 being rotationally driven. A transfer cleaning device 50 is disposed at a position facing the intermediate transfer belt 5 so as to be able to contact and separate from the driving roller 51. The transfer cleaning device 50 pressurizes the drive roller 51 after superimposing images of necessary colors on the intermediate transfer belt 5, transfers the toner onto a sheet as a transfer material (recording material), and then residual toner on the intermediate transfer belt 5. Clean and remove.

  The sheets are conveyed one by one from the storage units 71, 72, 73 or the manual sheet feeding unit 74 by sheet feeding means 81 to 84 provided respectively. The sheet whose skew has been corrected by the registration roller 85 is sent to the secondary transfer portion T at a desired timing, and a positive voltage is applied from the transfer power source 561 to the secondary transfer roller (transfer means) 56. The toner image on the intermediate transfer belt 5 is electrostatically transferred. The sheet on which the toner image has been transferred by the secondary transfer unit 56 passes through the conveyance unit 86, is fixed by the heat roller fixing device 9, and is discharged to a paper discharge tray or a post-processing device. On the other hand, the intermediate transfer belt 5 after the secondary transfer is cleaned with the transfer residual toner by the transfer cleaning device 50 and again used for the primary transfer process of each image forming unit.

  Further, when performing so-called double-sided printing in which images are formed on both the front and back sides of a sheet, the conveyance path guide 91 is driven immediately after the sheet passes through the fixing device 9. After the sheet is guided to the reversing path 76 through the conveying path 75, the reversing roller 87 is rotated in the reverse direction, the trailing edge of the fed sheet is set at the top, and the sheet is withdrawn in the direction opposite to the fed direction. Send to 77. The sheet that has passed the double-sided conveyance path 77 is skew-corrected and timed by the double-sided conveyance roller 88, and conveyed to the registration roller 85 at a desired timing. Transfer the image to the surface.

  Next, image formation for each image formation mode will be described.

  As described above, for the magenta (M) toner and the cyan (C) toner, dark toner and light toner belonging to the same hue and having different densities are prepared. Those belonging to the same hue are those having the same spectral characteristics of the coloring components (pigments), but in general they are four colors such as Y, M, C, K even if they are not exactly the same. In the normal color concept, the range can be called the same color. Further, toners belonging to the same hue and having different concentrations are usually toners having the same spectral characteristics and different amounts of color components (pigments) contained in a toner based on resin and color components (pigments). I mean.

Here, the light toner is defined. A combination of several kinds of toners belonging to the same hue and having different densities and having a relatively low density is called a light toner. In this embodiment, the light and low density light toner has an optical density after fixing of less than 1.0 when the toner amount on the sheet is 0.5 mg / cm ² . On the other hand, the dark toner having a high density has an optical density after fixing of 1.0 or more per 0.5 mg / cm ² of toner on the sheet. In that case, the amount of the pigment of the dark toner is adjusted so that the optical density after fixing becomes 1.6 when the amount of the toner on the sheet is 0.5 mg / cm ² . Further, the light toner is designed such that the applied density is 0.5 mg / cm ² and the optical density after fixing is 0.8. In this way, by mixing appropriate amounts of two types of dark toner and light toner for magenta and cyan, respectively, target gradation reproducibility of cyan and magenta is obtained.

  As shown in FIG. 2, the full-color image forming apparatus of the present embodiment is equipped with a sheet reading detection unit (recording material detection unit) 123. The sheet reading detection unit 123 irradiates light onto the surface of the sheet 32 fed and conveyed from the paper cassette 102 by the paper feed roller 103, collects the reflected light, and forms an image to form a specific area of the sheet 32. Is detected. The sheet reading detection unit 123 also includes an LED 33 as a light irradiation unit, and includes a CMOS sensor 34 as a reading unit, lenses 35 and 36 as imaging units, and the like.

  Therefore, the light emitted from the light source LED 33 irradiates the surface of the sheet conveyance guide 31 or the surface of the sheet 32 on the sheet conveyance guide 31 through the lens 35. The reflected light from the sheet 32 is collected through the lens 36 and imaged by the CMOS sensor 34. The image is detected by the CMOS sensor 34 to read the surface of the sheet conveyance guide 31 or the sheet 32. The light from the LED 33 is arranged to be irradiated from an oblique direction with a predetermined angle with respect to the surface of the sheet 32.

  3A to 3C, the CMOS sensor 34 in the sheet reading detection unit 123 reads the surface of three types of sheets 32 (hereinafter referred to as “recording sheets A, B, and C”), and these recording sheets. It is a figure which shows the example which digitally processed each surface of A, B, and C with the output from the CMOS sensor 34 by 8x8 pixel. Such digital processing is performed by converting the analog output from the CMOS sensor 34 into 8-bit pixel data by an A / D converter (not shown) which is a conversion means.

  FIG. 3A schematically shows the surface of a recording paper A such as a so-called rough paper in which the fiber on the surface of the sheet is relatively rough. Similarly, FIG. 3B schematically shows the surface of a recording paper B such as a commonly used so-called plain paper. FIG. 3C schematically shows the surface of a recording paper C such as smooth paper (gross paper) in which paper fiber is sufficiently compressed. The images of the surfaces read by the detection by the CMOS sensor 34 are digitally processed to become images 43, 44, and 45 shown in FIGS. 4 (a) to 4 (c), respectively. As shown in the figure, the image of the surface read differs depending on the material and type of each sheet such as recording paper A, B, C, which is a phenomenon caused by the different fibrin states on the paper surface. That is, it is possible to discriminate according to the surface state of the paper fiber based on the video read and digitally processed by the detection by the CMOS sensor 34.

  With reference to the flowchart of FIG. 5, the reading operation of the surface of the recording sheets A, B, and C will be described.

  First, in steps S50 and S51, the CMOS sensor 34 reads the surface several times at several positions on the sheet 32, that is, the recording papers A, B, and C by irradiation with the LED 33. Subsequently, after the LED is turned off (step: S52), the gain calculation of the filter calculation means (not shown) and the constant for the filter calculation are adjusted (step: S53). The gain calculation and the filter calculation are processed in a programmable manner by the control processor. The gain calculation is performed by adjusting the gain of the analog output from the CMOS sensor 34, for example. That is, when the amount of reflected light reflected on the sheet surface is excessive or small, the sheet surface cannot be read well, so that the gain is adjusted when the signal change cannot be derived. As for the filter operation, the analog output from the CMOS sensor 34 is A / D converted. For example, in the case of 8-bit 256-gradation digital data, 1/32, 1/16, 1/4, etc. An arithmetic operation is performed to remove a noise component output from the CMOS sensor 34.

  Next, it is determined whether or not the filter and gain have been adjusted to such an extent that it can be determined whether the sheet corresponds to the recording material A, B, or C (step: S54).

  When it is determined that the adjustment of the filter and gain has been performed to such an extent that it can be determined (Yes), the surface information is compared (step: 55). If it is determined that the adjustment has not been made to the extent possible (No), the process returns to step S51 and tries to read the surface of the sheet again. The sheet type is determined from the result of the surface information comparison calculation (step: S56), and an image processing method corresponding to the surface roughness of the sheet is determined (step: S57).

  As a method of surface information comparison calculation, the highest density pixel Dmax and the lowest density pixel Dmin are derived from the result of reading several areas on the sheet surface. Execute for each read area and average the pixels. In a sheet such as recording paper A shown in FIGS. 3 and 4, if the paper fiber element on the sheet surface is rough, many shadows of the fiber element are generated. For this reason, the difference between the bright part and the dark part is large, and the pixel maximum and minimum densities Dmax and Dmin are large. Incidentally, similarly in FIGS. 3 and 4, Dmax−Dmin becomes small on the surface of the sheet such as the recording paper C because the shadow of the fiber element is small. Thus, the comparison calculation is performed to determine the sheet type of the sheet, that is, the surface roughness.

  FIG. 6 schematically shows a method of determining the paper type of the recording papers A, B, and C based on the subtraction value of Dmax−Dmin. In this case, threshold values, for example, X and Y, which are used in advance as a standard for determining the paper type, are stored in a nonvolatile memory in the DC controller. However, the threshold value is not limited to two of X and Y, and can be set by a number larger than that. In order to see the paper type, the number of sheets of three recording papers A, B, and C is merely an example.

Therefore, the value of Dmax−Dmin is larger than the threshold value Y,
Dmax-Dmin> Y
If so, the sheet is determined to be recording paper A. Similarly, when the value of Dmax−Dmin is smaller than the threshold value Y and larger than the threshold value X, the sheet is determined as recording paper B, and when the value of Dmax−Dmin is smaller than the threshold value X, recording is performed. Judged as paper C.

  Next, as shown in the functional block diagram of FIG. 7, the image signal output from the full-color sensor 100 is input to the analog signal processing unit 101, and the gain and offset are adjusted. After the adjustment, the image signal is converted into an RGB digital signal of, for example, 8 bits (0 to 255 level: 256 gradations) for each color component by the A / D conversion unit 102. The shading correction unit 103 uses a signal obtained by reading a reference white plate (not shown) for each color and corresponds to each CCD sensor cell in order to eliminate variations in sensitivity for each of the sensor cell groups of the arranged CCDs. Optimize the gain and correct shading.

  The line delay unit 104 corrects a spatial shift included in the image signal output from the shading correction unit 103. This spatial shift is caused by the fact that the line sensors of the full color sensor 100 are arranged at a predetermined distance from each other in the sub-scanning direction. Specifically, the R (red) and G (green) color component signals are line-delayed in the sub-scanning direction with the B (blue) color component signal as a reference, and the phases of the three color component signals are synchronized.

  The input masking unit 105 converts the color space of the image signal output from the line delay unit 104 into the NTSC standard color space using the matrix arithmetic expression shown in FIG. That is, the color space of each color component signal output from the full color sensor 100 is determined by the spectral characteristics of the filter of each color component, but this is converted into the NTSC standard color space.

  The LOG conversion unit 106 includes a reference table (LUT) made of, for example, a ROM, and converts the RGB lightness signal output from the input masking unit 105 into a CMY density signal. The line delay memory 107 is an image output from the LOG conversion unit 106 during a period (line delay) in which a black character determination unit (not shown) generates control signals such as UCR, FILTER, and SEN based on the output from the input masking unit 105. Delay the signal.

  The masking / UCR unit 108 extracts the black component signal K from the image signal output from the line delay memory 107. Further, the masking / UCR unit 108 performs a matrix operation for correcting color turbidity of the recording color material of the printer unit on Y, M, C, and K, and in order of M, C, Y, and K for each reading operation of the reader unit 1R. For example, an 8-bit color component image signal is output. Note that the matrix count used for the matrix calculation is set by the CPU 200 (not shown).

  In addition, the γ correction unit 109 performs density correction on the image signal output from the masking / UCR unit 108 in order to match the image signal with the ideal gradation characteristics of the printer unit. The output filter (spatial filter processing unit) 110 performs edge enhancement or smoothing processing on the image signal output from the γ correction unit 109 based on a control signal from the CPU.

  The LUT (adjustment means) 111 is used to match the density of the original image and the density of the output image, and is composed of, for example, a RAM and its conversion table is set by the CPU. The pulse width modulator (PWM) 112 outputs a pulse signal having a pulse width corresponding to the level of the input image signal, and the pulse signal is input to a laser driver 113 that drives a semiconductor laser (laser light source). Based on the image signal input from the image reading unit 21, the scanner 3 scans and exposes the surface of the photoreceptor 1 with laser light to form an electrostatic latent image.

(Determination of recording ratio of light and light toner)
When it is determined whether the sheet corresponds to the recording paper A, B, or C, the mixing (distribution) ratio that is the ratio of the dark toner and the light toner for both magenta and cyan suitable for the sheet paper type, that is, the toner “Recording rates Rn, Rt”, which can be restated as a dot formation rate, are determined.

  FIG. 9 is a characteristic line diagram showing the dark and light toner recording rates of the dark toner indicated by the solid curve and the light toner indicated by the broken curve. Based on the 8-bit color component image signal Data that is data of the cyan component and the magenta component, processing for determining the recording rates Rn and Rt of the dark toner and the light toner is performed. The recording rate can be interpreted as indicating the amount of dot formation when the ratio is large and the density is high, and when the ratio is small, the density is low.

  For example, if the input gradation data is 100/255, the light toner recording rate Rt is determined to be 255/255, and the dark toner recording rate Rn is determined to be 40/255. The recording rate is shown as an absolute value with 100 percent being 255.

  Further, FIG. 9 shows the following. In a place where the amount of applied toner is large with light toner, the surface roughness of the sheet is affected, and transfer defects are likely to occur. On the other hand, since it is difficult to cause a transfer failure in a dark toner where the amount of applied toner is small, it is desirable to increase the dark toner recording rate in order to reproduce the low density portion. Therefore, the recording ratios Rn and Rt are changed according to the recording papers A, B, and C based on the color component image signal Data.

  As shown in FIG. 10, when the sheet 32 corresponds to, for example, the recording paper A based on the above determination processing result, the recording ratios of the light toner and the dark toner are changed so as to adapt to the recording paper A. 3 and FIG. 4, the recording paper A in this case has a lot of shading on the surface, and a lot of shadows of the fiber are generated, resulting in a large difference between bright and dark areas. This is because the pixel maximum and minimum densities Dmax and Dmin are increased. Similarly, in the case of the recording sheet B from the determination result, the recording rate between the recording sheet A and the recording sheet C is set.

  The change in the recording rate of the light toner and the dark toner shown in FIG. 10 will be described in detail. As shown in FIG. 10, the amount of dark toner used increases as the image data of the toner image to be formed increases. In this embodiment, the density of the thinnest toner image among the toner images formed using the dark toner is changed according to the surface roughness of the sheet. That is, the thinnest toner image among the density toner images formed using the dark toner and the light toner is changed depending on the surface roughness of the sheet. As shown in FIG. 10, when the sheet corresponds to the recording paper A, dark toner is used from the image data 20, and thereafter, dark toner is used up to 255. When the sheet corresponds to recording paper B, dark toner is used in the range of image data 50 to 255. When the sheet corresponds to the recording paper C, it is used in the range of image data 75 to 255.

  As described above, according to the present embodiment, the recording rates of light toner and dark toner of magenta and cyan are changed according to the type of the surface 32 of the sheet 32 such as recording sheets A, B, and C, respectively. As a result, it is possible to form a high-quality image without image unevenness according to the sheet type.

  In the image forming apparatus according to the present embodiment, a pattern generator (not shown) is mounted so that gradation patterns can be registered, and a signal can be directly transmitted to the pulse width modulator 62. ing.

  In this embodiment, the surface roughness of the sheet is detected by the sheet reading detection unit 123. However, as shown in FIG. 11, it is also possible to provide an operation panel 210 capable of inputting the surface roughness of the sheet used by the user, and adjust the image control method according to the input information. .

  In this embodiment, the apparatus for transferring the toner image formed on the photosensitive drum 1 once to the intermediate transfer belt and then to the sheet has been described. However, the present invention can also be applied to an image forming apparatus that directly transfers a toner image formed on the photosensitive drum 1 to a sheet.

(Second Embodiment)
In the second embodiment, in the same image forming apparatus as in the first embodiment, after changing the content ratio of the light and dark toner according to the sheet type (surface roughness) executed in the first embodiment, Depending on the sheet type (surface roughness), the resolution in the low density portion is lowered.

  Although the details will be described below, the configuration and operation of the main body are the same as those in the first embodiment, and therefore only the different configuration and control will be described.

  Usually, in the low density portion, there are instability differences of the electrostatic latent image and accompanying development instability factors, and there are also instability factors in the process of transferring the toner image onto the sheet 32. If the resolution of the low-density part is lowered in order to eliminate such instability and improve it, it is effective to suppress the influence of the surface condition of the sheet.

  11 (a) and 11 (b) show the sheet surface type on the image carrier such as the photosensitive drum 1 if the sheet paper type is expressed by the unevenness of the surface of the sheet, that is, "surface roughness". FIG. 6 schematically shows how the formed toner image is affected when transferred onto a sheet.

  In FIG. 11A, the toner image before transfer is formed in a uniform thin layer on the image carrier, but discharge occurs in the concave portion due to the surface roughness of the sheet during electrostatic transfer. Insufficient transfer. As a result, image defects due to transfer unevenness (image unevenness) occur for each degree of sheet surface roughness. On the other hand, in the case of FIG. 11B, if the toner image before transfer is concentrated (reduced in the number of lines) in a lump on the image carrier, image formation is affected by unevenness of the sheet surface roughness. Hateful. As a result, it is possible to reproduce a low density portion having regularity schematically shown by a checkered image, and to prevent image defects due to transfer unevenness.

  When the sheet type is determined in accordance with the flowchart of FIG. 5, the pulse width modulator (resolution changing means) 112 in the block diagram of FIG. 7 uses the toner of the image data 100 or less according to the sheet type (surface roughness). The number of lines of the image, that is, the resolution in the rotation axis direction of the photosensitive drum 1 is changed according to the type of sheet (surface roughness). In the low density portion where the image data is 100 or less, the resolution when using the sheet corresponding to the recording paper A having a rough surface is higher than the number of lines (resolution) when using the sheet corresponding to the recording paper C having a smooth surface. make low. At this time, the number of lines of an image having more than 100 image data related to the sheet type is 200.

  The number of lines is changed as shown in Table 1. In Table 1, [lpi] is the number of lines per inch.

  In the above, several embodiments of the image forming apparatus of the present invention have been described. However, the present invention is not limited to these embodiments, and other embodiments and application examples are within the scope not departing from the gist of the present invention. Variations and combinations thereof are also possible.

  For example, in each embodiment, assuming that a color image forming apparatus is used, it has been described that optimum image adjustment is performed by adjusting the recording ratio of dark and light toners using magenta and cyan. However, light toner is applied to gray toner. The same effect can be obtained even when dark toner is applied to black toner.

1 is a diagram showing an embodiment of an image forming apparatus according to the present invention. The figure which shows the structure of an image reading detection part. FIGS. 4A to 4C schematically show images formed on recording sheets A, B, and C as examples of sheet types in the first embodiment. FIGS. 4A to 4C are diagrams schematically showing images after pixel data conversion. 7 is a flowchart showing a series of operations from image processing to sheet paper type determination. The figure which shows typically the standard which determines recording paper A, B, and C by a threshold value. The functional block diagram which shows a structure. Matrix of basic three primary colors RGB. FIG. 6 is a characteristic diagram showing a recording ratio of dark and light toner. FIG. 6 is a characteristic diagram for determining the recording rate of dark and light toner. The figure which shows an operation panel. In the second embodiment according to the present invention, FIGS. 9A and 9B show image unevenness in the case where the line number reduction processing before and after the transfer is not performed and when the toner image is transferred from the photosensitive drum to the sheet. FIG.

Explanation of symbols

Pa to Pf ... Image forming section 111 ... Adjustment means 1a to 1f ... Photoconductor drum 56 ... Secondary transfer roller

Claims (7)

An image carrier;
A toner image forming means for forming a toner image on the image carrier using a light toner and a dark toner having the same hue as the light toner and darker than the light toner;
Transfer means for electrostatically transferring a toner image on the image carrier to a recording material;
In accordance with the roughness of the surface of the recording material onto which the toner image is transferred, the toner is set so that the ratio of the dark toner and the light toner contained in the toner image of a predetermined density on the image carrier is different. Adjusting means for adjusting the image forming means;
An image forming apparatus comprising:   The density of the toner image having the lightest density among the toner images formed using the light toner and the dark toner is lighter when the surface is rough than when the surface is smooth. The image forming apparatus according to claim 1, wherein the adjusting unit adjusts the ratio.   And a resolution changing unit that adjusts the toner image forming unit so that the resolution of the toner image formed by the toner image forming unit is lower when the surface is rough than when the surface is smooth. The image forming apparatus according to claim 2.   The image forming apparatus according to claim 3, further comprising a detection unit configured to detect a surface of the recording material.   The image forming apparatus according to claim 2, further comprising an input unit configured to input information relating to a roughness of a surface of the recording material onto which the toner image is transferred. Forming a toner image on the image carrier using a light toner and a dark toner having the same hue as the light toner and a darker density than the light toner;
Electrostatically transferring the toner image on the image carrier to a recording material;
In accordance with the roughness of the surface of the recording material onto which the toner image is transferred, the toner is set so that the ratio of the dark toner and the light toner contained in the toner image of a predetermined density on the image carrier is different. An image forming method comprising adjusting an image forming unit.   The density of the thinnest toner image among the density toner images formed using the light toner and the dark toner is thinner when the surface is rough than when the surface is smooth. The image forming method according to claim 6, wherein the ratio is adjusted.

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