JP2011107172A - Image forming apparatus and image forming method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of accurately express density gradations by using a black high-density magnetic toner and a black low-density nonmagnetic toner, and giving little difference in gloss from an image expressed by using a nonmagnetic toner. <P>SOLUTION: An image forming section PK where a toner image is formed by using a high-density black toner, which is a magnetic toner, is disposed in the downstream side along an intermediate transfer belt 12 from an image forming section PG where a toner image is formed by using a low-density black toner, which is a nonmagnetic toner. The black image in the image data is divided into an image for the high-density black toner and an image for the low-density black toner, and the toner images are respectively formed and superimposed by the intermediate transfer belt 12 on a recording material, with the high-density black toner in the lower side. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an image forming apparatus that performs black gradation expression using both a two-component developer and a one-component developer, and more particularly to an arrangement configuration of a developing apparatus that reduces variation in glossiness in gradation expression of a fixed image. .

  A monochrome image forming apparatus that develops an electrostatic image using a one-component developer containing magnetic toner is widely used. A one-component developer is less expensive than a two-component developer containing a non-magnetic toner and a magnetic carrier. A developing device using a one-component developer is smaller than a developing device using a two-component developer, has a simple structure, is easy to control, and is free from performance deterioration due to deterioration of a magnetic carrier. .

  On the other hand, full-color image forming apparatuses that develop an electrostatic image using a two-component developer containing a nonmagnetic toner and a magnetic carrier are also widely used. The two-component developer uses a so-called magnetic brush development method, and can control the amount of toner applied more precisely than a one-component developer to output a natural halftone image, and even a fine electrostatic image can be accurately developed. .

  However, even in a full-color image forming apparatus, the black single-color mode for outputting characters and diagrams in monochrome is frequently executed, and if it is the black single-color mode, it is more economical to use a one-component developer. For this reason, even in a full-color image forming apparatus using a two-component developer containing a non-magnetic toner and a magnetic carrier, a magnetic toner may be used for developing a black toner image (Patent Document 1).

  Patent Document 1 discloses a tandem type intermediate transfer type full-color printer in which yellow, magenta, cyan, and black image forming units are arranged along an intermediate transfer belt. Here, the black image forming unit is equipped with a developing device using magnetic toner, and the black toner image is on the uppermost layer of the other color toner images on the intermediate transfer belt so that it is the lowermost layer on the recording material. Overlaid.

  However, as described in Patent Document 2, since the magnetic toner contains magnetic particles, there is a problem that the glossiness of the fixed image is low and the transparency is low. For this reason, the image forming apparatus described in Patent Document 2 is provided with a black developing device using a one-component developer and a black developing device using a two-component developer. A black developing device using a one-component developer is used in a solid black portion of a full color image and a black single color mode.

  Patent Document 3 discloses an image forming apparatus that outputs a full color image using a black developing device using a one-component developer and a yellow, magenta, and cyan developing device using a two-component developer. Since magnetic toner alone does not have sufficient glossiness of a fixed image, it is possible to improve the glossiness of a black fixed image by fixing it integrally on a recording material in a state of being superimposed on a magenta or cyan toner image. Yes.

  Patent Document 4 discloses an image forming apparatus that expresses a density gradation of an output image by using a high density toner and a low density toner having different fixing densities per applied toner amount. Since the granularity is conspicuous in the halftone fixed image only with the high density toner, the gradation of cyan and magenta is expressed only by the amount of applied toner of the low density toner in the density range below the predetermined gradation.

JP 2007-212595 A Japanese Patent Laid-Open No. 9-15925 JP 2006-209001 A JP-A-5-35038

  As in the case of cyan and magenta disclosed in Patent Document 4, it has been proposed to reduce the graininess of a gray halftone image by using a high density toner and a low density toner together in black. Then, it has been proposed to assign a magnetic toner to the black high density toner and assign a non-magnetic toner to the black low density toner. As a result, it has been expected to reduce the cost of black toner that is frequently used in the black monochrome mode or the like.

  However, it was found that when black high-density / magnetic toner and low-density / non-magnetic toner are overlapped to express black gradation, uneven density is conspicuous and gradation cannot be expressed accurately. .

  Further, as described in Patent Document 3, there is a difference in gloss between an image portion expressed using black high-density / magnetic toner and an image portion expressed using chromatic / non-magnetic toner. Turned out to be conspicuous.

  The present invention can accurately express the density gradation using black high density magnetic toner and low density nonmagnetic toner, and there is no difference in glossiness from the image portion expressed using nonmagnetic toner. An object of the present invention is to provide an image forming apparatus that can be small.

  An image forming apparatus according to the present invention includes a first image forming unit that forms a first toner image using black magnetic toner, and a non-magnetic toner having a black fixing density per toner amount lower than that of the magnetic toner. A mode in which an image is formed on a recording material using both the second image forming unit that forms a second toner image using the first image forming unit and the first image forming unit and the second image forming unit. And a control unit that controls the first and second image forming units so that the second toner image is formed on the recording material above the first toner image.

  In the image forming apparatus of the present invention, since the fixed image of the non-magnetic toner having high transparency is positioned on the fixed image of the magnetic toner having low transparency (see FIG. 8), the opacity of the fixed image of the magnetic toner is non-magnetic toner. The reflected light from the fixed image is not obstructed. Therefore, density unevenness and inaccurate gradation expression caused by the magnetic toner fixed image hindering the reflected light from the non-magnetic toner fixed image do not occur.

  In addition, the surface of the fixed image of the magnetic toner having a rough surface and a rough feeling is covered with a flat and smooth fixed image of the non-magnetic toner, so that the surface of the image is compared with the case where the fixed image of the magnetic toner is exposed. Approaches a flat and smooth object.

  Therefore, it is possible to accurately express the density gradation using the black high density / magnetic toner and the low density / non-magnetic toner, and the difference in glossiness between the image portion expressed using the non-magnetic toner is small. That's it.

1 is an explanatory diagram of a configuration of an image forming apparatus. It is explanatory drawing of a structure of the image development apparatus using a two-component developer. It is explanatory drawing of the structure of the image development apparatus using a one-component developer. FIG. 6 is an explanatory diagram of a relationship between a reflection density of a fixed image and a toner application amount. It is explanatory drawing of the color separation control in Example 1. FIG. It is explanatory drawing of the relationship between the gradation of image data, and the reflection density of a fixed image. FIG. 10 is an explanatory diagram of color separation control in the third embodiment. It is explanatory drawing of the glossiness of a fixed image.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the present invention, as long as black gradation is expressed using high density / magnetic toner and low density / non-magnetic toner, a part or all of the configuration of the embodiment is replaced with the alternative configuration. This embodiment can also be implemented.

  Therefore, it can be carried out without distinction between full color / monochrome, tandem type / 1 drum type, intermediate transfer system / recording material conveyance system. In the present embodiment, only main parts related to toner image formation / transfer will be described. However, the present invention includes a printer, various printing machines, a copier, a fax machine, a composite machine, in addition to necessary equipment, equipment, and a housing structure. It can be implemented in various applications such as a machine.

  In addition, about the general matter of the image forming apparatus shown by patent documents 1-4, illustration is abbreviate | omitted and the overlapping description is abbreviate | omitted.

<Image forming apparatus>
FIG. 1 is an explanatory diagram of the configuration of the image forming apparatus. As shown in FIG. 1, the image forming apparatus 100 includes a tandem type intermediate transfer system in which yellow, magenta, cyan, gray, and black image forming portions PY, PM, PC, PG, and PK are arranged along an intermediate transfer belt 12. This is a full color printer.

  In the image forming unit PY, a yellow toner image is formed on the photosensitive drum 1Y and is primarily transferred to the intermediate transfer belt 12. In the image forming unit PM, a magenta toner image is formed on the photosensitive drum 1M, and is primarily transferred onto the yellow toner image on the intermediate transfer belt 12. In the image forming units PC, PG, and PK, a cyan toner image, a low density black toner image, and a high density black toner image are formed on the photosensitive drums 1C, 1G, and 1K, respectively. Transcribed.

  The five-color toner images primarily transferred to the intermediate transfer belt 12, which is an example of an intermediate transfer body, are conveyed to the secondary transfer portion T2 and collectively transferred to the recording material P. The recording material P on which the five-color toner images are secondarily transferred is heated and pressed by the fixing device 17 to fix the toner images on the surface, and then discharged to the outside of the machine body.

  The intermediate transfer belt 12, which is an example of an intermediate transfer body, is supported around a tension roller 10, a drive roller 9, and a counter roller 13, and is driven by the drive roller 9 to rotate in a direction indicated by an arrow R 2 at a predetermined process speed. .

  Of the rollers forming the primary transfer surface of the intermediate transfer belt 12, transfer from the photosensitive drums 1 Y, 1 M, 1 C, 1 G, and 1 K is performed on the opposite side of a driven roller (not shown) on the downstream side in the rotation direction of the intermediate transfer belt 12. An optical sensor (not shown) for detecting the registered registration image is arranged. Based on the detection result of the registration misalignment image by the optical sensor, control is performed to correct the image writing timing and the image writing start position in the image forming units PY, PM, PC, PG, and PK as needed. . Further, control is performed to correct the image density and the toner replenishment amount in the image forming units PY, PM, PC, PG, and PK based on the density detection result of the patch image by the optical sensor.

  The recording material P drawn from the recording material cassette 14 is separated one by one by the separation roller 15 and sent to the registration roller 16. The registration roller 16 receives and waits for the recording material P in the stopped state, and sends the recording material P to the secondary transfer portion T2 in synchronization with the toner image on the intermediate transfer belt 12.

  The belt cleaning device 18 rubs the intermediate transfer belt 12 with a cleaning blade, escapes the transfer to the recording material P, passes through the secondary transfer portion T2, and collects the transfer residual toner remaining on the intermediate transfer belt 12. .

  The secondary transfer roller 11 abuts on the intermediate transfer belt 12 whose inner surface is supported by the opposing roller 13 to form a secondary transfer portion T2. The power source D <b> 2 applies a positive transfer voltage to the secondary transfer roller 11, thereby transferring the toner image carried on the intermediate transfer belt 12 to the recording material P by being charged negatively.

  The image forming units PY, PM, PC, and PG are substantially the same except that the color of toner used in the developing devices 4Y, 4M, 4C, and 4G is different from chromatic yellow, magenta, cyan, and achromatic low density black. Configured identically. The image forming unit PK is the same as the image forming units PY, PM, PC, and PG except that the separately provided developing device 4K is a one-component developer developing device that uses an achromatic high density black magnetic toner. Configured.

  In this embodiment, the photosensitive drums 1K and 1G correspond to the first photosensitive member and the second photosensitive member, respectively, and the developing devices 4K and 4G respectively correspond to the first developing device and the second developing device. It corresponds to. Further, a facing portion between the developing device 4K and the photosensitive drum 1K corresponds to the first image forming portion, and a facing portion between the developing device 4G and the photosensitive drum 1G corresponds to the second image forming portion.

  In the following, the achromatic image forming part PG and the developing device 4K will be described in detail. For the other chromatic image forming parts PY, PM, and PC, G at the end of the reference numerals attached to the constituent members being described is used. It shall be explained by replacing with Y, M, and C.

  In the image forming unit PG, a corona charger 2G, an exposure device 3G, a developing device 4G, a primary transfer roller 5G, and a cleaning device 6G are individually attached around a photosensitive drum 1G that is an example of a photoconductor. The photosensitive drum 1G has an organic semiconductor photosensitive layer on the surface of the base layer of the aluminum cylinder, the base layer is connected to the ground potential, and the charging polarity of the surface of the photosensitive layer is negative.

  The corona charger 2G irradiates the surface of the photosensitive drum 1G with charged particles accompanying corona discharge, and uniformly charges the surface of the photosensitive drum 1G to the negative dark potential VD. The exposure device 3G scans the scanning line image data obtained by developing the separation color image of low density black with a rotating mirror, and writes an electrostatic image of the image on the surface of the photosensitive drum 1G. The surface potential of the photosensitive drum 1G charged to the dark portion potential VD is exposed to light and the potential is lowered to the light portion potential VL, so that the toner is electrostatically attached to the exposed portion. The developing device 4G develops the electrostatic image on the photosensitive drum 1G into a toner image by using a two-component developer containing a low-density black nonmagnetic toner.

  The cleaning device 6G slides a cleaning blade on the photosensitive drum 1G to collect the transfer residual toner remaining on the photosensitive drum 1G by escaping from the transfer to the intermediate transfer belt 12. The primary transfer roller 5G presses the inner surface of the intermediate transfer belt 12 to contact the photosensitive drum 1G to form the primary transfer portion TG. The power source DG applies a positive transfer voltage to the primary transfer roller 5G, thereby causing the toner image carried negatively and carried on the photosensitive drum 1G to be primarily transferred to the intermediate transfer belt 12.

<Two-component developing device>
FIG. 2 is an explanatory diagram of a configuration of a developing device using a two-component developer. As shown in FIG. 2, the developing device 4G stirs a two-component developer obtained by mixing a low-density black non-magnetic toner and a magnetic carrier so that the non-magnetic toner becomes negative and the magnetic carrier becomes positive. Charge. The charged two-component developer is carried in the state of a magnetic brush on the developing sleeve 25 rotating around the fixed magnet 29 and rubs against the photosensitive drum 1G.

  The interior of the developing container 22 is divided into a developing chamber C1 and a stirring chamber C2 by a partition wall 19, and a toner storage chamber C3 that stores replenishing toner (nonmagnetic toner) is disposed above the stirring chamber C2. A replenishing port 21 is provided in the toner storage chamber C3, and an amount of replenishing toner commensurate with the toner consumed by development is dropped and replenished into the stirring chamber C2 through the replenishing port 21.

  In the developing chamber C1 and the stirring chamber C2, a two-component developer in which toner particles (nonmagnetic) and carrier particles (magnetic) are mixed is accommodated. In the developing chamber C <b> 1, a conveying screw 23 is disposed, and the two-component developer in the developing chamber C <b> 1 is conveyed in the longitudinal direction of the developing sleeve 25 by the rotational driving of the conveying screw 23. The agitating chamber C2 accommodates a conveying screw 24, and the two-component developer in the agitating chamber C2 is conveyed in the opposite direction to the two-component developer in the developing chamber C1 by the rotational driving of the conveying screw 24.

  The partition wall 19 is provided with openings that allow the developing chamber C <b> 1 and the stirring chamber C to communicate with each other on the near side and the far side in the longitudinal direction of the developing sleeve 25. The two-component developer conveyed by the conveying screw 23 is transferred to the conveying screw 24 through one opening, and the two-component developer conveyed by the conveying screw 24 is transferred to the conveying screw 23 through the other opening. In this way, the two-component developer circulates between the developing chamber C1 and the stirring chamber C2 while being mixed and stirred, and the toner and the carrier rub against each other through the stirring. The toner is charged to the negative polarity for developing the latent image through friction with the carrier, and the magnetic carrier is charged to the positive polarity for adsorbing the toner on the developing sleeve 25.

  An opening is provided in a portion of the developing container 22 near the photosensitive drum 1G, and a developing sleeve 25 formed of a nonmagnetic material such as aluminum or nonmagnetic stainless steel is rotatably disposed in the opening. The developing sleeve 25 rotates in the direction of the arrow R4 to carry and convey the two-component developer to the developing unit 26.

A roller-shaped magnet 29 is fixedly disposed in the developing sleeve 25, and the magnet 29 has a developing magnetic pole S <b> 1 facing the developing unit 26. The magnet 29 is a developing magnetic pole S1.
In addition, it has N1, N2, N3, and S2 poles.

  The two-component developer picked up at the N2 pole by the rotation of the developing sleeve 25 is regulated in layer thickness by the layer thickness regulating blade 28 while being conveyed from the S2 pole to the N1 pole to form a developer thin layer. The layer thickness regulating blade 28 regulates the layer thickness of the two-component developer carried and conveyed by the developing sleeve 25 to the developing unit 26.

  Then, the two-component developer spiked in the magnetic field of the developing magnetic pole S1 forms a magnetic brush and develops the electrostatic image on the photosensitive drum 1G. Thereafter, the two-component developer on the developing sleeve 25 falls into the stirring chamber C1 due to the repulsive magnetic field between the N3 pole and the N2 pole. The two-component developer that has fallen into the stirring chamber C1 is stirred and conveyed by the conveying screws 23 and 24.

  A developing magnetic field formed by the developing magnetic pole S1 of the magnet 29 in the developing unit 26 forms a magnetic brush before development, and the magnetic brush contacts the photosensitive drum 1 to develop the electrostatic image. The magnetic brush of the two-component developer carried on the developing sleeve 25 comes into contact with the photosensitive drum 1G that rotates in the direction of arrow R1 at the developing unit 26, and the electrostatic image on the photosensitive drum 1G is reversely developed at the developing unit 26. At that time, the toner adhering to the ear (magnetic brush) of the magnetic carrier and the toner adhering to the sleeve surface instead of the ear transfer to the region of the bright portion potential (exposure portion potential) VL of the electrostatic image. Involved in development.

  The power source D4 applies an oscillating voltage obtained by superimposing an AC voltage on the DC voltage Vdc to the developing sleeve 25. The dark part potential (non-exposed part potential) VD and the bright part potential (exposed part potential) VL of the electrostatic image are located between the maximum value and the minimum value of the oscillating voltage. As a result, an alternating electric field whose direction changes alternately is formed in the developing unit 26, and the toner and the carrier vibrate vigorously in the alternating electric field. The toner adheres to the region of the bright portion potential (exposure portion potential) VL of the photosensitive drum 1G by shaking off the electrostatic binding force on the carrier.

  The difference between the maximum value and the minimum value of the oscillating voltage (peak-to-peak voltage Vpp) is preferably 1 to 5 kV, and the frequency is preferably 1 to 10 KHz. As the waveform of the oscillating voltage, a rectangular wave, a sine wave, a triangular wave, or the like can be used. The DC voltage Vdc of the oscillating voltage is set to a value between the dark part potential VD and the bright part potential VL of the electrostatic image, but the dark part potential is closer to the bright part potential VL than the dark part potential VD. This is preferable in preventing fog toner from adhering to the potential VD region.

  The developing sleeve 25 has a diameter of 20 mm, a length of 340 mm, a thickness of 1 mm, and a peripheral speed of 200 mm / sec. The minimum gap between the developing sleeve 25 in the developing unit 26 and the photosensitive drum 1G is preferably 0.2 to 1 mm. The developer amount regulated by the layer thickness regulating blade 28 is such that the free height on the surface of the developing sleeve 25 of the magnetic brush formed by the magnetic field of the developing magnetic pole S1 is the minimum gap between the developing sleeve 25 and the photosensitive drum 1G. The amount of the developer is preferably 1.2 to 2 times that of the developer.

  The magnetic pole is used such that the strength of the developing magnetic field by the developing magnetic pole S1 on the surface of the developing sleeve 25 (the magnetic flux density in the direction perpendicular to the sleeve surface) is 100 mT.

<Two-component developer>
As the toner of the two-component developer, a non-magnetic toner having high permeability is used in consideration of color mixing after fixing. As the non-magnetic toner, a known toner obtained by adding a colorant or a charge control material to a binder resin can be used, and a toner having a volume average particle size of 5 to 15 μm can be preferably used. In this embodiment, toners having an average particle diameter of 6 μm are used for gray, yellow, cyan, and magenta. The mixing ratio (weight ratio) of the toner in the two-component developer is preferably 5 to 12% by weight for all colors.

  Various types of conventionally known magnetic carriers can be used. A resin carrier formed by dispersing magnetite as a magnetic material in a resin and dispersing a conductive substance such as carbon black for conductivity and resistance adjustment can be used. It is also possible to use a material in which the resistance is adjusted by oxidizing and reducing the surface of a magnetite simple substance such as ferrite, or a resistance adjusted by coating the surface of a magnetite simple substance such as ferrite with a resin.

  As described above, the two-component developer of gray, yellow, magenta, and cyan is a developer configuration that is generally used in a full-color image forming apparatus. High definition image quality is obtained.

  The two-component developer is excellent in reproducibility of high-definition images as compared with the one-component developer. The reason is as follows. In the first place, if a high-definition electrostatic image on the photosensitive drum 1G can be faithfully reproduced with a toner image, a so-called rough feeling does not occur. When the toner image is slightly disturbed with respect to the electrostatic image, minute density unevenness is generated in a portion where the density is originally uniform, and a rough feeling is generated.

  In the two-component development method, since the magnetic brush of the two-component developer contacts and develops the photosensitive drum 1G, toner very close to the electrostatic image contributes to the development. As a result, the electric field due to the electrostatic image is generated. Since the toner strongly acts on the toner, it is possible to develop it faithfully to an electrostatic image, that is, an electric field.

Since the toner (non-magnetic) used in the developing device 4G is used to reduce the granularity of the low-to-medium density black halftone image, the fixing per toner load is higher than the toner (magnetic) used in the developing device 4K. The black density of the image is low. That is, the low density black toner (gray toner) is a non-magnetic toner in which the fixing density of black per toner load is lower than that of the high density black toner (black magnetic toner). Specifically, as shown in FIG. 4, the low density black toner has a black fixing density of 0.7 per toner applied amount of 0.5 mg / cm ² , whereas the high density black toner has 1 .4.

As described above, the toner (nonmagnetic) used in the developing device 4G has the pigment content adjusted so that the reflection density is less than 1.0 per 0.5 mg / cm ² of toner applied on the recording material. . In contrast, the toner (magnetism) used in the developing device 4K is adjusted in pigment content so that the reflection density is 1.0 or more per 0.5 mg / cm ² of toner applied on the recording material. .

<Single component development device>
FIG. 3 is an explanatory diagram of a configuration of a developing device using a one-component developer. As shown in FIG. 1, the image forming unit PK that forms a high-density black toner image is equipped with a developing device 4K that develops an electrostatic image on the photosensitive drum 1K using high-density black magnetic toner. . The developing device 4K employs a so-called known magnetic one-component developing configuration.

  As shown in FIG. 3, the developing container 53 is filled with magnetic toner as a one-component developer, and the stirring blade 60 stirs the magnetic toner in the developing container 53. The developing sleeve 58 rotates in the direction of arrow R4, and the photosensitive drum 1K rotates in the direction of arrow R1. The magnetic blade 52 carries the magnetic toner on the developing sleeve 58 by applying it very thinly. The developing sleeve 58 conveys the carried magnetic toner to the developing unit 26 formed by the photosensitive drum 1K and the developing sleeve 58 as it rotates.

  The magnetic toner particles are negatively charged by friction between the magnetic toner particles, friction between the developing sleeve 58 and the magnetic toner particles, and friction between the magnetic blade 52 and the magnetic toner particles. A charge having a polarity opposite to that of the electrostatic image formed on the photosensitive drum 1K with respect to the DC voltage Vdc of the oscillating voltage applied to the developing sleeve 58 is given to the magnetic toner particles.

  The power source D4 is a developing bias unit that applies a vibration voltage to the developing sleeve 58 during development. In the developing unit 26, the magnetic toner carried on the developing sleeve 58 is caused to fly by the action of the magnetic field by the magnet 55 fixed in the developing sleeve 58 and the alternating voltage of the oscillating voltage, and the static on the photosensitive drum 1 </ b> K. Reverse development of the image.

  The developing sleeve 58 has a diameter of 20 mm, a length of 340 mm, a thickness of 1 mm, and a peripheral speed of 200 mm / sec. The minimum gap between the developing sleeve 58 in the developing unit 26 and the photosensitive drum 1K is 0.2 mm. The developing magnetic field generated by the developing magnetic pole S1 has a magnetic flux density of 100 mT on the developing sleeve 58.

  The amplitude of the alternating voltage of the oscillating voltage (peak-to-peak voltage Vpp) is 1.0 kV, the frequency is 3 KHz, and the waveform is a rectangular wave. The DC voltage Vdc of the oscillating voltage is set between the dark part potential VD = −500V and the bright part potential VL = −200, and is −400V. However, these numerical values are all representative values and are not limited thereto.

  The developing device 4K is a magnetic one-component developing device, and does not require the stirring of the carrier and the toner and the management of the toner ratio (toner concentration) in the developer as in the two-component developing method, so that the configuration is simple. Control is also simple. Since the developing device 4K does not use a carrier, the developer is not changed due to carrier deterioration, and the developing device has an excellent running cost. However, since the developing device 4K is a non-contact developing method, there is an aspect inferior to the two-component developing method in faithful reproducibility with respect to a high-definition electrostatic image as described above.

<Single component developer>
In recent years, the level of image quality required with the expansion of applications of image forming apparatuses has increased, and image forming apparatuses that increase the number of colors compared to the conventional four-color mixed type of three primary colors + black have been put into practical use. . Low to medium density by adding two color toner images of low density (light) cyan and low density (light) magenta in addition to the conventional four color toner images of cyan, magenta, yellow and black. This reduces the graininess of halftone images.

  By the way, as a toner developing method in electrophotographic image formation, a two-component developing method using a two-component developer containing a non-magnetic toner and a magnetic carrier, and a magnetic one-component developing method using a one-component developer containing a magnetic toner, There is. In an image forming apparatus that reproduces a full-color image by superimposing a plurality of color toner images, a two-component development method is usually employed.

  This is because, in order to perform color reproduction by mixing colors by superimposing toner images, the fixed image is required to have high transparency, and in this respect, the magnetic toner contains magnetite particles and has low light transmittance. Also, in an image quality-oriented image forming apparatus that expresses density gradation by superimposing a high-density toner image and a low-density toner image, the two-component development method is also used from the viewpoint of high definition and toner loading stability. It is used.

  However, in the two-component development method, since a two-component developer of toner and carrier is used, a configuration for stirring the carrier and toner and a configuration and control for managing the toner ratio (toner concentration) in the developer are required. For this reason, compared with the magnetic one-component development system, there are problems such as complication of the developing device and an increase in running cost due to replacement of carrier deterioration.

  On the other hand, an image forming apparatus dedicated to monochrome images often employs a magnetic one-component development method. The reason for this is maintenance-free and running cost reduction because there is no need for carrier replacement. In an image forming apparatus dedicated to monochrome images, it is not necessary to consider color mixing due to superposition of toner images, so magnetic toner having low light transmittance can be used.

  Even in the case of an image forming apparatus for full color output, the ratio of the number of full color / monochrome prints investigated by general users is overwhelmingly about 9 to 7 for monochrome for full color 1 to 3 for office use. The monochrome ratio is high. For this reason, in order to reduce the total cost of maintenance of the image forming apparatus, there is a case where a magnetic one-component development method is adopted to reduce the running cost and extend the maintenance interval in monochrome printing.

  In other words, conventionally, the two-component development method emphasizing image quality and the magnetic one-component method emphasizing the reduction in maintenance frequency and running cost are classified, and can be selected according to the point that the user places importance on.

  However, in recent years, the image quality of black-and-white images as well as the image quality of full-color images has been regarded as important. In order to reduce the graininess of the gray halftone image, an image forming apparatus that expresses the density gradation of black by using high-density black toner and low-density black toner separately or by superimposing them is also proposed. ing. For example, the low to medium density region is mainly expressed by low density black toner, and the medium to high density region is expressed by high density black toner. As a result, both high reproducibility of high density black and high image quality with less graininess are achieved in the highlight region.

  In this case, it is difficult to adopt a magnetic one-component development method and to design with a focus on reducing the number of maintenance and running costs. However, adopting a two-component development method capable of high-definition reproducibility and high transparency to design with an emphasis on image quality pushes up the cost of monochrome printing that is frequently executed in the black monochrome mode, etc. The total cost of maintenance becomes a problem.

  Therefore, in the image forming apparatus 100, the magnetic toner and the non-magnetic toner as described above are properly used to improve the image quality of the monochrome print and reduce the running cost. It achieves both high image quality in a full-color image forming apparatus, maintainability, and reduced running costs.

  In the image forming apparatus 100, a known magnetic toner is used as a black one-component developer used in the developing device 4K. The magnetic toner has an average particle diameter of 5 to 8 μm, magnetite is used as the magnetic material, and the magnetic material content of the toner is about 30 to 60 wt%.

  In the image forming apparatus 100 that reproduces the density by mixing the colors of the toner layers, it is necessary to increase the transparency of the toner alone. Usually, magnetite is contained inside the toner, such as magnetic toner, and the toner having low permeability is used. It is not preferable to use it.

  However, in the image forming apparatus 100, an image forming process is performed such that when each color toner is superimposed on the recording material P, a high-density black toner image, which is a magnetic toner, is placed on the recording material P at the bottom. (See FIG. 8). For this reason, it is possible to ensure good color mixing while using magnetic toner as black toner.

<Example 1>
FIG. 4 is an explanatory diagram of the relationship between the reflection density of the fixed image and the applied toner amount. FIG. 5 is an explanatory diagram of the color separation control in the first embodiment. FIG. 6 is an explanatory diagram of the relationship between the gradation of the image data and the reflection density of the fixed image. In FIG. 5, (a) is a low density black toner, and (b) is a high density black toner.

  As shown in FIG. 1, the image forming apparatus 100 uses a tandem-type intermediate transfer system, and a high-density black image forming unit PK is disposed at the most downstream position in the rotation direction of the intermediate transfer belt 12. In the image forming unit PK arranged at the most downstream position, the high-density black toner image is transferred to the uppermost surface of the other color toner image on the intermediate transfer belt 12. However, after that, when secondary transfer is performed on the recording material P at the secondary transfer portion T2, the lowermost position is reached on the recording material P.

  Using such a configuration, a magnetic toner image with low light transmission (turbidity) is placed on the bottom of the recording material P, so that other color toner images are laid under the high-density black toner image. There will be no damage. Even if the permeability of the high-density black toner is low, it does not block the light of the other color toner image, so that there is no problem in the color mixing performance.

  In addition, when a low density black toner image is superimposed on the upper side of the high density black toner image to express black of a predetermined density gradation, the low density black toner itself uses a highly transmissive nonmagnetic toner. Therefore, it is possible to mix colors with the high density black toner image in the lower layer. That is, a good color mixing property can be obtained by fixing the second toner image or the like on the first toner image on the recording material. Similarly, other yellow, magenta, and cyan toner images are also non-magnetic toners, and therefore, there is no problem with color mixing if they are superimposed on the high density black toner image.

As shown in FIG. 4, the covering powers (density expression power) of the low density black toner (broken line) and the black toner (solid line) evaluated by measuring the reflection density of the fixed image are shown. The low density black toner increases the reflection density of the fixed image as the toner amount on the recording material P increases. When the toner amount is 0.5 mg / cm ² , the reflection density of the fixed image becomes 0.7. The reflection density of the fixed image increases as the toner amount on the recording material P increases, and the reflection density of the fixed image becomes 1.4 when the toner amount is 0.5 mg / cm ² .

  As shown in FIG. 1, when the image data of each color is received, the control unit 110 forms an image exposure signal corresponding to the image data of each color and controls the exposure devices 3Y, 3M, 3C, 3G, and 3K. The control unit 110 expresses the black gradation of the output image by superimposing the applied toner amount developed by the first developing device (4K) and the applied toner amount developed by the second developing device (4G). Thus, the exposure means (3K, 3G) is controlled.

  The control unit 110 distributes the density gradation of each pixel of the black image data to the two exposure images using the look-up table shown in FIGS. 5A and 5B, and uses the image in the exposure apparatus 3G. An exposure signal and an image exposure signal used in the exposure apparatus 3K are generated. In this way, the exposure device 3G is controlled based on the low density black toner image look-up table (LUT) shown in FIG. 5A, and the high density black toner image look shown in FIG. 5B. The exposure apparatus 3K is controlled based on the up table (LUT).

  In FIG. 5, the horizontal axis represents the black tone value (0) in the image data before color separation into a high-density black toner image (first toner image) and a low-density black toner image (second toner image). ~ 255 levels). The vertical axis represents the gradation values (0 to 255 levels) assigned to both when high-density black toner image and low-density black toner image are separated. Here, separation means that image data of a certain color (also referred to as a plate or a channel) is divided into two image data for a high density (dark color) toner image and a low density (light color) toner image. Say.

  As shown in FIGS. 5A and 5B, the toner application amount (exposure gradation) of the high density black toner and the toner application amount (exposure gradation) of the low density black toner with respect to the density gradation of the image. Is allocated. The control unit 110 superimposes the high density black toner image and the low density black toner image and forms an image by adding the densities of both. In the image forming unit PG and the image forming unit PK, electrostatic image formation and development are performed in parallel to form divided toner images of respective gradations, which are primary-transferred onto the intermediate transfer belt 12 and superimposed integrally. Adapted.

  According to the control of the first embodiment, unlike image formation with a normal high density black toner image alone, the low density black toner is actively used in the low density part to the middle density part of the image signal, so that per unit dot The concentration of becomes lower. For this reason, even if the development is somewhat disturbed with respect to the electrostatic image of the unit dots, the granularity of the toner image is less noticeable.

  In addition, since the two-component development method is used for developing the low-density black toner, it is possible to reproduce the toner image faithfully to a high-definition latent image. For this reason, in the low density gradation region of black where the graininess is particularly conspicuous, the graininess can be overwhelmingly reduced as compared with the conventional black and white image forming apparatus.

  On the other hand, since the graininess is hardly noticeable in the high density portion of the black image, there is no problem even if the magnetic one-component development method, which is slightly inferior in high-definition reproducibility, is used intensively. Further, in terms of density gradation reproducibility, a necessary color mixing property is ensured by superimposing a non-magnetic and highly transmissive low density black toner image on the recording material above the high density black toner image.

  Therefore, as shown in FIG. 6, faithful black density gradation reproduction can be performed for all gradation stages of image data. In addition, reduction in graininess in the highlight portion and accurate reproduction of density gradation are achieved.

  Further, as shown in FIG. 8, the magnetic toner, which is a high-density black toner, contains magnetic particles as a toner pigment as compared with the non-magnetic toner. It tends to be glossy. On the other hand, since the low density black toner is a non-magnetic toner, it is easily smoothed by pressurization after being melted in the fixing process, resulting in a high gloss image finish. For this reason, when a black image is formed with the high-density black toner alone, an apparent difference in glossiness is formed with the surrounding non-magnetic toner image. However, in Example 1, a smooth fixed image made of non-magnetic toner is superimposed on a rough fixed image made of magnetic toner for all gradation levels except the maximum density of the image data. Less gloss unevenness.

  In general, a monochrome image often used in an office has an overwhelmingly large number of text documents and a high ratio of binary solid black characters. Therefore, an image is formed mainly by high-density black toner using a magnetic one-component developing method. As a result, maintenance performance and running cost, which are indispensable for an image forming apparatus dedicated to monochrome images, are not significantly impaired.

  By using the configuration and control as described above, it is possible to obtain a high image quality without the graininess of the highlight portion as compared with the conventional image forming apparatus dedicated to monochrome images. At the same time, faithful gradation reproduction is possible by considering the order in which the fixed image of opaque magnetic toner and the fixed image of transparent nonmagnetic toner are superimposed. Further, by adopting a magnetic one-component development method for developing a high density portion where the graininess is hardly noticeable, it is possible to maintain maintainability and low running cost.

  As shown in FIGS. 5 (a) and 5 (b), the control means (110) allows the black toner to be developed using only the toner applied amount developed by the second developing device (4G) in the density range of a predetermined gradation or less. Express gradation. In the high brightness to halftone region, the density gradation is expressed exclusively by the low density black toner. When expressing the same black density gradation, the toner image of the low density black toner can increase the area or the number of toner images per unit area as compared with the toner image of the high density black toner. The area or number of electrostatic images can be increased as compared with the case of using a high-density black toner image. For this reason, the amount of applied toner for each dot constituting a halftone image with a low density gradation can be controlled more precisely than when a toner image with a high density black toner is used, thereby reducing variation from place to place. This principle is the same as in the case where high density and low density toners are used for cyan and magenta, as shown in Patent Document 4.

  Also, in the halftone area to the high density area, combining the high density black toner reduces the total amount of applied toner combined with the low density black toner, and the secondary transfer efficiency due to the large amount of applied toner. Is suppressed.

  According to the configuration and control of the first embodiment, high image quality without graininess can be obtained in the highlight portion as compared with the conventional monochrome image forming apparatus. Further, by forming an image by placing a magnetic toner on the recording material on the lower layer again, color mixing is ensured, and faithful density gradation reproduction of black is possible. Furthermore, by adopting a magnetic one-component development system for developing high-density areas where graininess is not noticeable in the first place, maintenance performance and running costs that are the merits of the magnetic one-component development system are reduced without sacrificing image quality. Can be realized.

  According to the configuration and control of the first embodiment, fixing is performed in a state where the non-magnetic toner is superposed on the magnetic toner, so that when the magnetic toner and the non-magnetic toner are superposed, the transparency and the flatness after the fixation are fixed. The problem that occurs due to the difference in degree is solved. That is, a problem specific to a configuration in which a black image is formed by superimposing magnetic toner and non-magnetic toner is solved.

<Example 2>
In the first embodiment, a five-color full-color image forming apparatus in which yellow, magenta, and cyan toners are combined in addition to high-density black toner and low-density black toner has been described. On the other hand, the second embodiment is an image forming apparatus dedicated to monochrome images using high density black toner and low density black toner. That is, in FIG. 1, the image forming portions PY, PM, PC, and PK are removed, and only the image forming portions PG and PK are disposed along the intermediate transfer belt 12. The color separation control described in the first embodiment can also be applied to such a monochrome two-color image forming apparatus using both high density black toner and low density black toner.

  In the first embodiment, the image forming apparatus of the tandem type intermediate transfer method has been described. However, the application of the color separation control in the first embodiment is not limited to this method. For example, even in a tandem direct transfer type image forming apparatus configured by arranging the image forming units PY, PM, PC, PG, and PK shown in FIG. Similar effects can be obtained by applying similar control. However, in this case, in order to transfer the toner image of the magnetic toner to the bottom on the recording material, the high density black image forming portion PK is disposed on the most upstream side of the recording material conveyance belt, The image forming unit PG needs to be arranged on the downstream side of the image forming unit PK.

<Example 3>
FIG. 7 is an explanatory diagram of color separation control in the third embodiment. FIG. 8 is an explanatory diagram of the glossiness of a fixed image.

  In Example 1, the maximum density of black is expressed by only the magnetic toner. However, in Example 2, the glossiness is increased by superimposing a nonmagnetic toner image on the magnetic toner image even at the maximum density of black.

  In Example 1, as shown in FIGS. 5A and 5B, color separation was performed to form an image of a high-density black toner and a low-density black toner. In Example 3, FIG. ) And (b) to form an image with a color separation pattern as shown in FIG. That is, in the third embodiment, the color separation control shown in FIGS. 5A and 5B is replaced with the color separation control shown in FIGS. 7A and 7B. Since the other control and the configuration of the image forming apparatus 100 are the same as those of the first embodiment, the third embodiment will be described with reference to FIGS.

  In the image forming apparatus 100, a magnetic toner is used as the high-density black toner. However, since the magnetic toner uses a magnetic material as a toner pigment as compared with a non-magnetic toner, the toner is generally smoothed even after melting in the fixing process. Hateful. That is, the fixed image tends to be low gloss. On the other hand, the low-density black toner is a non-magnetic toner, and thus tends to become high gloss by being smoothed by pressurization after being melted in the fixing process. Due to the difference in smoothness of the fixed image, if there is an image portion made of only magnetic toner, the difference in glossiness from the surrounding image portion made of only non-magnetic toner becomes conspicuous. Even in monochrome print output, unnatural glossy non-uniformity occurs and the apparent image quality deteriorates in an image in which a portion developed with high-density black toner and a portion developed with low-density black toner coexist. End up.

  Therefore, in the third embodiment, as shown in FIG. 7A, even when the black image input signal has the maximum value, the amount of low density black toner that is placed on the high density black toner by a certain amount or more is set. It is a plate pattern. The control means (110) secures a predetermined amount or more of the amount of toner developed by the second developing device (4G) in all black gradations including the highest gradation. At the same time, the higher the black gradation, the larger the applied amount of toner developed by the first developing device (4K).

  As a result, as shown in FIG. 7B, the consumption of the non-magnetic toner (two-component developer) is slightly increased as compared with the case where the maximum density is expressed only by the high density black toner. However, the glossiness of the entire image increases uniformly, and the glossiness difference due to the black density difference is almost eliminated, and a fixed image having a uniform glossiness can be obtained. That is, in the past, in order to make the glossiness of the entire image uniform, the transparent toner image was overlaid on the entire surface. However, a fixed image having a uniform glossiness can be obtained without depending on the transparent toner image. can get.

  As shown in FIG. 8A, a nonmagnetic toner layer is overlaid on the magnetic toner layer. As shown in FIG. 8B, since the magnetic toner layer has a large image surface roughness after fixing, the glossiness is lower than that of the nonmagnetic toner layer. However, by fixing the non-magnetic toner layer on top, the concave portion of the surface roughness is filled with the non-magnetic toner and the fixing surface becomes smooth, so that the glossiness comparable to that of the non-magnetic toner layer alone is achieved. Is obtained.

  According to the color separation control of the third embodiment, image formation with only magnetic toner is eliminated, and a non-magnetic toner image is superimposed and fixed on the magnetic toner image at any density gradation. For this reason, the remarkable gloss change in the image surface due to the exposure of the fixed image surface of the magnetic toner is suppressed.

  According to the color separation control of the third embodiment, the same effect as that of the first embodiment can be obtained, and uniform glossiness in the image plane can also be realized.

  The present invention can also be applied to a configuration in which a first developing device (dark) and a second developing device (light) are attached to a single photoconductor (including a rotary type). In this case, the first image forming unit is in a state of developing an electrostatic image using the first developing device, and the second image forming unit is developing the electrostatic image using the second developing device. It is a state to do.

  In the first to third embodiments, when a black image is formed, control is performed in which image formation is performed using both high density black toner and low density black toner. However, a mode in which image formation is performed using only high-density black toner may be selected by user settings through the operation panel. This is because, in the case of an image that does not require gloss, a line image, or a character image, the running cost can be significantly reduced by using only high-density black toner.

  Similarly, a mode may be provided in which image formation is performed using only low-density black toner. This is because even if the cyan, magenta, and yellow are superimposed on the low density toner, a considerable part of the gradation realized by the high density black toner can be covered.

  The setting of these modes may be switchable within the main body, or may be switchable by the user according to the designation data of the image forming job transmitted from the external terminal.

1Y, 1M, 1C Photosensitive drum 1K Photosensitive drum (first photoconductor) 1G Photosensitive drum (second photoconductor)
2Y, 2M, 2C, 2K Corona charger 3Y, 3M, 3C, 3K Exposure device 4Y, 4M, 4C Development device 4K Development device (first development device) 4G Development device (second development device)
5Y, 5M, 5C, 5K Primary transfer rollers 25, 58 Development sleeve, 29, 55 Magnet 11 Secondary transfer roller, 12 Intermediate transfer belt (intermediate transfer member)
13 counter roller, 14 recording material cassette 16 registration roller, 17 fixing device, 110 control unit PY, PM, PC image forming unit PK image forming unit (first image forming unit), PG image forming unit (second image forming unit) Part)

Claims (7)

A first image forming unit that forms a first toner image using black magnetic toner;
A second image forming unit for forming a second toner image using a non-magnetic toner having a black fixing density per toner applied amount lower than that of the magnetic toner;
In a case where a mode for forming an image on a recording material using both the first image forming unit and the second image forming unit is executed, the image is above the first toner image on the recording material. An image forming apparatus comprising: a control unit that controls the first and second image forming units so that the second toner image is formed. A first photosensitive member provided with a first developing device for developing the first toner image;
A second photosensitive member provided with a second developing device for developing the second toner image;
The first photoconductor and the second photoconductor are exposed so that the first toner image and the second toner image are overlapped to express the black gradation in the fixed image. Exposure means for forming an electrostatic image;
An intermediate transfer member to which a toner image is transferred from the first photosensitive member and the second photosensitive member;
The image forming apparatus according to claim 1, wherein the second photosensitive member is disposed upstream of the first photosensitive member in the rotation direction of the intermediate transfer member.   A plurality of photoconductors individually provided with developing devices using different chromatic color non-magnetic toners are arranged upstream of the second photoconductor in the rotation direction of the intermediate transfer body. The image forming apparatus according to claim 2.   The toner developed by the first developing device is increased as the black gradation is higher, while the toner applied amount developed by the second developing device is maintained at a predetermined amount or more in all the gradations of black including the highest gradation. The image forming apparatus according to claim 1, further comprising a control unit configured to control the exposure unit so as to increase a loading amount.   The image forming apparatus according to claim 4, wherein the control unit expresses a black gradation only by a toner applied amount developed by the second developing device in a density range of a predetermined gradation or less. The non-magnetic toner is less than the reflection density of the fixed images amount of applied toner per 0.5 mg / cm ² is 1.0, the magnetic toner, the amount of applied toner of the fixed image per 0.5 mg / cm ² 6. The image forming apparatus according to claim 1, wherein the reflection density is 1.0 or more. In an image forming method for forming a fixed image by heating and pressing a recording material onto which a toner image has been transferred,
A first toner image developed using a black magnetic toner and a second toner image developed using a non-magnetic toner having a black fixing density per toner amount lower than that of the magnetic toner are formed. A first step of:
A second step of transferring the first toner image and the second toner image to the recording material such that the second toner image is positioned on the first toner image on the recording material; Have
An image forming method, wherein the first toner image and the second toner image are superimposed to express a black gradation in the fixed image.

Images