JP2009020365A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus for preventing an influence of the surface potential of a previous color toner image in imaging processes for the second and subsequent colors, and suppressing scumming in a non image-forming area for the third and subsequent colors. <P>SOLUTION: The image forming apparatus includes an image carrier 11, one or more charging means 12, a latent image forming means 4, and three or more developing means 14. The image forming apparatus charges the image carrier 11 for each color by use of the charging means 12, exposes the non-image-forming area on the image carrier 11 by use of the latent image formation means 4, thereby forming a latent image, develops the latent image with toner of reverse polarity to the charging polarity by use of the developing means 14, thereby forming toner images in the respective colors one after another on the single image carrier 11, thus forming a color toner image in three or more colors. In the image forming apparatus, the charging means 12 used for image formation for at least the third and subsequent colors has: a corona ion generating member that generates positive polarity corona ions and negative polarity corona ions by applying voltage thereto; and a charging potential control means for controlling the charging potential of the image carrier 11 to have a prescribed potential. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an image forming apparatus such as a copying machine, a facsimile machine, and a printer, and a process cartridge that is detachable from the image forming apparatus. More specifically, the present invention relates to a plurality of colors on one image carrier by multiple development. The present invention relates to an image forming a color image by superimposing the toner images.

  As a general electrophotographic method, a tandem method using a plurality of photoconductors is employed in order to obtain a full-color image at a high speed. However, the use of a plurality of photoreceptors increases the size, complexity, and cost of the image forming engine. For this reason, a multiple development method has been proposed in which a plurality of colors are sequentially developed and superimposed on one photoconductor (Patent Document 1, Patent Document 2, etc.).

  FIG. 15 is an image forming unit showing an example of an image forming method employed when performing multiple development. In this image forming method, a latent image is formed by exposing the image portion on the surface of the photosensitive member uniformly charged by the charging device to attenuate the potential, and charging the latent image to the surface of the photosensitive member. A toner image is formed by developing with toner having the same polarity as the polarity.

  When this negative-positive development method is employed for multiple development, first, when the first color toner image is formed, the charging device 21 uniformly charges the surface of the photoconductor 30 to the negative polarity. The surface is exposed to form a latent image, and the developing device 41 develops the latent image with the first color toner having a negative polarity, thereby forming a first color toner image. Next, the charging device 22 charges the surface of the photoconductor 30 and the toner image of the first color to a negative polarity, exposes the surface of the photoconductor 30 through the toner image, forms a latent image, and develops the latent image. The device 42 develops with the negative color second color toner and forms a second color toner image on the first color toner image. By performing such an image forming process for each color, a toner image of a plurality of colors can be formed on the photoreceptor 30.

  FIG. 16 is a schematic diagram relating to the potentials of the photoreceptor 30 and the toner image surface in the image forming process from the charging device 21 to the exposure unit 52 in FIG. 15 described above. First, the photosensitive member 30 is charged at −600 [V] by the charging device 21 as shown in FIG. Next, the exposure unit 51 exposes the surface of the photoreceptor 30 where an image is to be formed. The surface potential of the photoreceptor 30 exposed in this way becomes −50 [V] as shown in FIG. During the development of the first color, a predetermined amount of toner adheres to the photoconductor 30 with a development potential of about 300 [V], and a toner image of the first color is formed. This toner image generates a new surface potential component due to the charge held by itself, and here, as shown in FIG. 16C, about 100 [V] is generated as the toner image potential. In this state, when the photosensitive member 30 and the first color toner image are sufficiently recharged by the charging device 22, the surface potentials of the photosensitive member 30 and the first color toner image are ensured to be substantially the same. The toner image absorbs a part of the negative ions emitted from the charging device 22, so that the toner image potential is increased as shown in FIG. In this state, when the exposure unit 52 exposes the part where the toner image exists and the part where the toner image does not exist with the same light amount, the development potential of the part where the toner image exists is as shown in FIG. In contrast, the potential of the toner image potential becomes shallower than the development potential of the portion where the toner image does not exist. As a result, in the development of the second color, the toner adhesion amount greatly changes between the portion where the toner image of the first color exists and the portion where the toner image does not exist, and density unevenness occurs in the developed toner image. Occurs.

  That is, in order to form a high-quality image by the multiple development method, it is necessary to reduce the above-described influence on image formation of the next color due to the potential of the toner image of the previous color.

  In the image forming apparatus disclosed in Patent Document 3, the charge having the opposite polarity to the charge polarity that the image carrier and the toner are charged by the charging unit between the development of the toner image of the previous color and the charging of the next color is performed. The image carrier and the toner image are neutralized by the neutralizing means that provides the above.

  However, even if the toner image is neutralized before charging the next color as in the image forming apparatus of Patent Document 3, the toner is charged again with the same polarity as the charge polarity with which the toner is charged when the next color is charged. Charges up. For this reason, the toner image cannot be sufficiently neutralized, so that it is difficult to reduce the influence.

Japanese Patent Laid-Open No. 08-087179 Japanese Patent Laid-Open No. 10-003191 JP-A-8-286456

  In order to solve the above problems, the present applicant has proposed an image forming apparatus described in Japanese Patent Application No. 2007-055998 (hereinafter referred to as “prior application”). More specifically, in the image forming apparatus of the prior application, the charging unit applies the image carrier to the latent image formed by exposing the non-image portion on the image carrier charged by the charging unit by the latent image forming unit. An image forming process in which development is performed with toner having a polarity opposite to the charge polarity to be charged is performed for each color to form a color toner image in which the toner images of the respective colors are superimposed on the image carrier. Thus, for example, when forming a two-color toner image, the first-color toner image formed on the image carrier through the image-forming process is charged when the second-color image-forming process is performed. The toner is charged with a polarity opposite to that of the toner, as in the case of the image carrier. As a result, when the image carrier is charged to a predetermined potential (the same potential as that of the first color) by the charging unit, the toner image of the first color is also removed. That is, the toner image can be neutralized without providing a dedicated neutralization unit for neutralizing the toner image. Since the surface of the image carrier on which the toner image was formed during the first color image formation process is not exposed, the surface potential of that portion of the image carrier was charged during the first color image formation process. The potential remains. Therefore, when developing the latent image of the second color by eliminating the potential of the toner image of the first color by charging the second color and reducing the potential, there is no portion where the toner image of the first color is present in the latent image of the second color. The development potential is substantially the same for each portion, and a toner image of the second color can be formed without causing uneven toner density. Needless to say, when two or more color toner images are formed, the next color toner image can be formed without being affected by the potential of the previous color toner image.

  Therefore, according to the prior application, the toner image of the previous color can be easily removed and the charging potential of the image carrier can be set to a predetermined potential.

  However, in the image forming apparatus of the prior application, when a color image is formed by superimposing three or more colors on a photoconductor as an image carrier, the following problems occur. This will be described with reference to FIG. Here, the photoconductor is positively charged and the toner is negatively charged.

  FIG. 17A shows a state after developing the first color toner image with negatively charged toner. Next, as shown in FIG. 17B, the surface of the photoreceptor is charged to 500 [V] in order to form a second color toner image. As a result, the first color toner image, which is positively charged and originally negatively charged, is neutralized, and the potential of the toner layer of the first color can be almost zero. Thereafter, the non-image portion of the second color is exposed as shown in FIG. 17C, and the toner image of the second color is developed with the negatively charged second toner as shown in FIG. Then, as shown in FIG. 17E, the surface of the photoreceptor is charged to 500 [V] in order to form a third color toner image. As in the case of forming the second color toner image with this charging, the photoconductor is positively charged and the second color toner image that is originally negatively charged is neutralized, so that the toner layer potential of the second color is substantially zero. be able to. However, the toner image of the first color in which the toner layer potential becomes almost zero at this charging is positively charged. Therefore, when the non-image portion of the third color is exposed, as shown in FIG. 17F, the potential difference between the non-image portion of the portion where the toner image of the first color is present and the developing bias is the portion of the portion where the toner image of the first color is not present. It becomes smaller than the potential difference between the non-image area and the developing bias. When the third color toner image is developed in such a state, there arises a problem that a so-called background stain occurs in which the toner adheres to a non-image portion where the first color toner image is present. Further, when developing the toner image of the fourth color, the problem of background contamination becomes more serious because the toner layer potential of the toner image of the first color is increasing to the positive side.

  The present invention is to improve the prior application in view of the above background, and its object is to suppress the influence of the surface potential of the toner image of the previous color in the image forming process for the second and subsequent colors, and An object of the present invention is to provide an image forming apparatus capable of suppressing the occurrence of background stains in the non-image portions of the third and subsequent colors.

In order to achieve the above object, the invention of claim 1 is directed to exposing an image carrier, one or more charging means for charging the image carrier, and the image carrier charged by the charging means. A latent image forming unit that forms a latent image for each color, and three or more developing units that visualize the latent image formed on the image carrier with toners of different colors. For each color, the image carrier is charged by the charging unit, the latent image forming unit exposes a non-image portion on the image carrier to form a latent image, and the developing unit converts the latent image to the developing unit. In an image forming apparatus in which a toner image of each color is sequentially formed on a single image carrier by developing with a toner having a polarity opposite to the charged polarity to form a color toner image of three or more colors, at least the third color The charging means used for subsequent image formation is applied with a voltage. A corona ion generating member for generating positive and negative corona ions, and a charging potential control means for controlling the charging potential of the image carrier to a predetermined potential. To do.
According to a second aspect of the present invention, in the image forming apparatus according to the first aspect, the corona ion generating member is a discharge electrode, and the charging means used for image formation for at least the third color is provided with the image carrier. One or more discharge electrodes are arranged inside a case having an opening on the side close to the image carrier and facing the image carrier.
According to a third aspect of the present invention, in the image forming apparatus according to the first or second aspect, the corona ion generating member further includes a voltage applying unit that applies an alternating voltage or a voltage in which a direct current is superimposed on the alternating current. Is.
According to a fourth aspect of the present invention, in the image forming apparatus of the second aspect, the discharge electrode includes a plurality of the discharge electrodes, a first voltage applying unit that applies a positive DC voltage to the discharge electrodes, and the discharge And a second voltage applying means for applying a negative DC voltage to the electrode.
According to a fifth aspect of the present invention, in the image forming apparatus of the fourth aspect, the positive DC voltage is made larger in absolute value than the negative DC voltage.
According to a sixth aspect of the present invention, in the image forming apparatus according to the first, second, third, fourth, or fifth aspect, the charging potential control means is a grid electrode, and the corona ion generation means and the image carrier are disposed between The grid electrode is provided on the surface.
According to a seventh aspect of the present invention, in the image forming apparatus according to the first, second, third, fourth, or sixth aspect, the developing unit is disposed so as to face the image carrier in a non-contact manner in a developing region. Further, the toner carrying body for carrying the toner and a plurality of electrodes provided on the toner carrying body along the surface of the toner carrying body and insulated from each other, the plurality of electrodes being respectively applied. It is characterized by having a hopping electric field generating means for hopping the toner because the relative polarities of the voltages are different between the adjacent electrodes.
The invention according to claim 8 is the image forming apparatus according to claim 7, wherein the toner carrier moves the toner carried on the surface of the toner carrier by moving the surface of the toner carrier. It is a surface moving member conveyed to a development area.
According to a ninth aspect of the present invention, in the image forming apparatus according to the eighth aspect, the hopping electric field generating means conveys the toner to the development area while hopping the toner carried on the surface of the toner carrying member. A traveling wave electric field for generating the electric field is generated on the surface of the toner carrier.

  Here, the “non-image portion” is a portion that does not constitute a toner image of that color for each color.

In the present invention, as described in the description of the previous application, when the second color latent image is developed by discharging the first color toner image by charging the second color and reducing the potential, the second color is developed. In the latent image, the development potential is substantially the same between the portion where the first color toner image is present and the portion where the first color toner image is absent. Similarly, in the third and subsequent colors, for example, the potential of the toner image of the second color can be reduced by removing the toner image of the second color by charging the third color. The development potential is substantially the same. As a result, it is possible to suppress the influence of the surface potential of the previous color toner image in the next color image forming process without providing a dedicated neutralizing means for eliminating the previous color toner image, and uneven toner density can be prevented. The toner images for the second and subsequent colors can be formed without any occurrence.
Further, at least the third and subsequent charging devices can generate positive and negative corona ions from the corona ion generating member. Thus, for example, when the third color image is formed, even if the first color toner image is charged to the same polarity as the charging polarity for charging the image carrier, the corona ion having the opposite polarity to the charging polarity for charging is used. Can be neutralized. Therefore, the potential of the toner image of the first color is made substantially zero by controlling the charging potential of the image carrier by the charging potential control means while supplying positive and negative corona ions from the corona generating member to the image carrier. In addition, the image carrier can be uniformly charged to a predetermined charging potential. Therefore, when the exposure of the third color is performed, the potential difference between the non-image portion of the portion where the toner image of the first color is present and the developing bias becomes appropriate, and the third color toner adheres to the non-image portion. Can be suppressed.

  As described above, according to the present invention, it is possible to suppress the influence of the surface potential of the previous color toner image in the image forming process for the second and subsequent colors, and to suppress the occurrence of background stains in the non-image portions for the third and subsequent colors. There is an effect.

Hereinafter, an embodiment in which the present invention is applied to a laser printer (hereinafter simply referred to as a printer 100) which is an electrophotographic image forming apparatus will be described.
FIG. 2 is a schematic configuration diagram of the printer 100 according to the present embodiment. The printer 100 shown in FIG. 2 includes a photoconductor belt 11 in which an organic photoconductor is configured in a belt shape, and is rotated in the direction of arrow E in the drawing by a rotation driving mechanism (not shown).

  Developing devices 14K, 14M, 14C, and 14Y, which are developing cartridges, and a corona charging device 12 to be described later are opposed to the photosensitive belt 11 for each color, and toner images are sequentially transferred to the photosensitive belt 11 as the photosensitive belt 11 moves. 11 are configured to be stacked on top of each other. This is a configuration called a one-pass color. Further, the developing device 14 as a developing cartridge is detachable from a space opened by retracting the photosensitive belt 11, and can be replaced by a user.

  The exposure devices 4K, 4M, 4C, and 4Y are devices for writing latent images corresponding to black, magenta, cyan, and yellow, respectively, on the charged photosensitive belt 11 according to image information. Various devices such as an optical scanning device using a polygon and an LED array can be used.

  Below the photosensitive belt 11, there is provided a paper feeding device 5 for storing a transfer material such as the transfer paper P and for starting transfer of the transfer paper P during image formation. A fixing device 17 including a heating roller 17a and a pressure roller 17b facing the heating roller 17a for fixing an unfixed toner image formed on the transfer paper P is provided above the photosensitive belt 11. It is.

  At the time of image formation, the transfer paper P sent from the paper feeding device 5 is conveyed to a contact portion between the photosensitive belt 11 and the transfer device 16, and a full color image formed on the photosensitive belt 11 is transferred at this contact portion. The image is transferred onto the transfer paper P by the voltage applied to the device 16. Thereafter, when the transfer paper P reaches the fixing device 17, the toner image on the transfer paper P is heated while being sandwiched between the heating roller 17a and the pressure roller 17b to be fixed on the transfer paper P. A visible image is formed.

  Toner that remains on the photosensitive belt 11 without being transferred (transfer residual toner) is cleaned by the cleaning device 18, and the surface of the photosensitive belt 11 after cleaning is used for next image formation.

  FIG. 3 is an enlarged schematic configuration diagram in the vicinity of the photosensitive belt 11. In FIG. 3, the photosensitive belt 11 is disposed in the center, and on the circumference thereof, corona charging devices 12Y, 12M, 12C, and 12K and four-color developing devices 14Y, 14M, 14C, and 14K for each of the four colors. Then, a transfer device 16 and a cleaning device 18 are arranged. Although the photosensitive belt 11 is used here, a photosensitive drum may be used.

  In this embodiment, four color toner images of yellow (Y), magenta (M), cyan (C), and black (K) are superimposed on the photosensitive belt 11 to form a color image. This mechanism will be described. In FIG. 3, a corona charging device 12 and a developing device 14 are arranged for each color in the order of YMCK from the upstream side to the downstream side in the circumferential direction (arrow direction E) of the photosensitive belt 11, and the toner images in the order of YMCK. Are superimposed. Note that the superposition order is not limited to this.

  The photosensitive belt 11 is rotated at a linear speed of 100 [mm / sec]. In the corona charging device 12Y, the photosensitive belt 11 is uniformly charged at 500 [V], and a non-image portion is exposed with the writing light 13Y output from the exposure device 4Y to form a yellow electrostatic latent image. The electrostatic latent image is formed as a toner image on the photosensitive belt 11 with a negatively charged yellow toner by the developing device 14Y having a developing bias of 250 [V].

  Next, the magenta toner image is superimposed on the photosensitive belt 11 on which the yellow toner image is formed by the same charging, exposure, and development processes as those for the yellow color. The same applies to cyan and black colors. In this manner, four color toner images of yellow, magenta, cyan, and black are superimposed on the photoconductor.

  Then, the four-color toner images are collectively transferred by the transfer device 16 to the transfer paper P transported by the paper transport path 15. Finally, the toner image is fixed on the transfer paper by the fixing device 17 and an image is output.

  The exposure devices 4Y, M, C, and K that output the writing light 13Y, 13M, 13C, and 13K use laser diode elements having a wavelength of 780 [nm] that are shifted from the center of the visible light region that is highly absorbed by the toner. However, the present invention is not limited to this.

[Example 1]
The corona charging devices 12Y, 12M, 12C, and 12K used in this embodiment are AC corotron charging devices. However, the present invention is not limited to the AC (alternating voltage) corotron charging device, and may be an AC scorotron charging device using a grid. Furthermore, it is not necessary to use AC for all corona charging devices, and a normal DC (direct current voltage) scorotron charging device or other methods may be partially used. As described above, when the toner layers of the first color and the second color are superimposed on the photosensitive belt 11, the problem of the toner layer potential can be solved. It is effective to use an AC corona charging device at 12C and 12K). Furthermore, since it is important to supply both positive ions and negative ions simultaneously with the charging device, there is no particular limitation as long as it is such a charging device.

  As shown in FIG. 4, the corona charging device 12, which is an AC corotron charging device, includes a single AC power source 54 that applies an AC voltage to the corona electrode via a corona electrode 51, a shield (metal case) 52, and a capacitor 53. And it is comprised from one DC power supply 55 which applies a DC voltage to the shield 52. FIG.

  The corona electrode 51 is a wire made of tungsten and having a diameter of 60 [μm]. An AC voltage is applied to the corona electrode 51 via a capacitor 53. The waveform of the AC voltage can be a sine wave or a rectangular wave, and is not particularly limited. When square waves are used, ions can be generated with the highest efficiency. The shield 52 is made by processing a stainless steel plate having a thickness of 1 [mm], and the side of the shield 52 facing the photosensitive belt 11 is open. The material constituting the shield 52 may be any material as long as it has conductivity, but stainless steel is adopted from the viewpoint of ozone resistance. A DC voltage is applied to the shield 52.

  Note that the AC voltage is applied to the corona electrode 51 via the capacitor 53 for the following reason. When an AC voltage is applied directly to the corona electrode 51, negative discharge is stronger than positive discharge, and therefore more negative ions are generated. If negative ions are always generated in the same amount as positive ions, there is no problem. However, when various conditions are changed, the amount of negative ions generated changes, which makes it difficult to control the charged potential. When an AC voltage is applied to the corona electrode 51 via the capacitor 53, the capacitor 53 does not flow a DC current, and the amount of current that can flow is limited by the capacitance of the capacitor, so that a positive discharge current and a negative discharge current are applied. And the equivalent amount of positive ions and negative ions can always be generated from the corona electrode 51, and the control becomes easy.

  When an AC voltage is applied to the corona electrode 51, positive ions and negative ions are generated simultaneously. Of the ions generated from the corona electrode 51, relatively many ions having a polarity opposite to the voltage applied to the shield 52 are attracted to the shield. Further, among the ions generated from the corona electrode 51, relatively many ions having the same polarity as the voltage applied to the shield reach the surface of the photosensitive belt and charge the photosensitive belt 11. If the voltage applied to the shield 52 is equal to the potential of the photosensitive belt 11, the movement of positive ions and negative ions generated from the corona electrode 51 is stopped and equal amounts of positive ions and negative ions are converted into the photosensitive member. The belt surface will be reached. Therefore, an equilibrium state where the potential on the surface of the photoreceptor belt does not change apparently is obtained. In this way, the voltage applied to the shield 52 and the potential of the photosensitive belt 11 become the same, so that the charging potential of the photosensitive belt 11 can be easily controlled to be a predetermined potential.

  Here, an AC voltage is applied to the corona electrode 51 via the capacitor 53 for the controllability of the charging potential of the photosensitive belt 11, but the DC voltage is superimposed on the AC voltage or the voltage applied to the shield 52 is reduced. Since the charging potential can be controlled by setting, the capacitor 53 is not necessarily used.

  In this embodiment, the AC voltage applied to the corona electrode 51 is a sine wave having a peak-to-peak voltage of 12 [kV] and a frequency of 2 [kHz], and the DC voltage applied to the shield 52 is 500 [V]. The capacitor 53 has a capacitance of 1000 [pF]. If the capacitance of the capacitor 53 is small, the amount of alternating current decreases and the amount of discharge at the corona electrode 51 also decreases. Therefore, the capacitance of the capacitor 53 is preferably 100 [pF] or more.

  Next, the effect of the present invention will be described with reference to FIG. Here, the photosensitive belt 11 is positively charged and the toner to be developed is negatively charged.

First, as shown in FIG. 1A, the photosensitive belt 11 is positively charged to 500 [V] by the corona charging device 12Y. In this embodiment, an AC corotron charging device is used. However, since charging is performed without a toner image on the photosensitive belt 11, the AC corotron charging device is not necessarily used. Other charging devices such as a DC scorotron charging device may be used. Next, as shown in FIG. 1B, the non-image portion is exposed with the writing light 13Y. Next, as shown in FIG. 1C, the negatively charged toner image of the first color is developed by the developing device 14Y. In this embodiment, the photosensitive belt 11 having a thickness of about 20 [μm] and a toner charge amount of about −30 [μC / g] is used, and the toner is about 0.45 [mg / cm ² ]. When the toner is developed with an adhesion amount of approximately 100 V, a potential of about −100 [V] is induced on the surface of the photoreceptor belt by the charge of the toner, and the potential due to the thickness of the toner layer (this is the toner layer potential) is about −40 V. occured. Note that the magnitude of the toner layer potential varies depending on the toner charge amount and the adhesion amount. Next, as shown in FIG. 1 (d), the corona charging device 12M causes the outermost surface (the portion without the toner layer to be the surface of the photoreceptor belt and the portion with the toner layer to be the surface including the toner layer) to 500 [V]. Charge. Since positive ions are supplied at this time, the photosensitive belt 11 is positively charged, and the negatively charged toner is neutralized. Therefore, the charge of the toner layer of the first color can be efficiently removed while charging the surface of the photosensitive belt, and the toner layer potential can be reduced to approximately 0 [V] (approximately −several [V] at most).

  In other words, the toner layer potential can be almost 0 [V] regardless of the magnitude of the toner layer potential. The breakdown of the outermost surface potential of the portion where the first color toner image is developed is that the photoreceptor is originally positively charged, the photoreceptor belt surface potential is 500 [V], and the potential due to the toner charge is −140. In [V], the outermost surface potential is 360 [V]. In other words, if the charge on the toner layer can be completely eliminated, the outermost surface potential returns to 500 [V], which is the potential at which the surface of the photoreceptor belt is positively charged. When positive ions are supplied, the surface of the photoreceptor belt is positively charged and the toner is negatively charged. Therefore, most of the positive ions are selectively attracted to the toner to remove the charge of the toner. Accordingly, the toner layer can be neutralized, the toner layer potential can be set to approximately 0 [V], and the outermost surface potential can be set to 500 [V].

  In this embodiment, an AC corotron charging device is used. However, since all that is necessary (for charging the second color) is to supply positive ions, the AC corotron charging device is not necessarily used. In some cases, other charging devices such as a general DC scorotron charging device may be used.

Next, as shown in FIG. 1E, the non-image portion is exposed with the writing light 13M. At this time, since the toner layer potential of the first color is almost 0 [V], an appropriate latent image potential can be obtained. Next, as shown in FIG. 1F, the negatively charged toner image of the first color is developed by the developing device 14M. Similarly to the first color, the photosensitive belt 11 having a thickness of about 20 [μm] and a toner charge amount of about −30 [μC / g] is used, and the toner is about 0.45 [mg / cm ^2]. ], An electric potential of about −100 [V] is induced on the surface of the photoreceptor due to the toner charge, and a toner layer potential of about −40 V is generated. Next, as shown in FIG. 1G, the outermost surface is charged by the corona charging device 12C for charging the third color (the surface without the toner layer is the surface of the photosensitive belt, and the portion with the toner layer is the surface including the toner layer). Is charged to 500 [V]. The potential of the toner layer of the second color can be set to approximately 0 [V] (at most, about −several V) similarly to the charging of the second color.

  Here, when only the positive ions are supplied to the outermost surface at the time of charging of the third color, the toner layer of the first color which has already been neutralized and the toner layer potential is almost 0 [V] is positively charged. As a result, a positive toner layer potential is generated, resulting in a problem of background contamination.

  Therefore, in this embodiment, an AC corotron charging device is used. The AC corotron charging device can supply both positive ions and negative ions simultaneously, and undergoes the following charging process. First, since the positive ions reach the surface of the photosensitive belt relatively more than the negative ions at the beginning of charging, the surface of the photosensitive belt is charged with positive ions. When the surface of the photosensitive belt is charged with positive ions, the amount of positive ions reaching the surface of the photosensitive belt gradually decreases, and when the surface potential of the photosensitive belt becomes equal to the potential applied to the shield 52, Equal amounts of positive ions and negative ions reach the surface of the photoreceptor belt, and the potential of the surface of the photoreceptor belt apparently changes to an equilibrium state, and charging is completed.

  When a portion having the second color toner layer is charged by the corona charging device 12C which is an AC corotron charging device, the following phenomenon occurs. First, at the beginning of charging, positive ions reach relatively more than negative ions, so that the photosensitive belt 11 and the second color toner layer are positively charged. However, negative ions have also reached at the same time, so that equal amounts of positive ions and negative ions arrive as charging progresses. At this time, since the charge on the surface of the photosensitive belt is more stable than the toner surface, the positively charged second color toner layer is neutralized with negative ions and the surface of the photosensitive belt with positive ions. It happens to be charged. Therefore, the surface of the photosensitive belt can be charged to 500 [V] while the second color toner layer is maintained at approximately 0 [V] (approximately several [V] at most).

  For comparison, when charging was performed using a normal DC scorotron charging device, the toner layer potential of the second color was about 40 [V].

  Further, when the peak-to-peak voltage and frequency of the AC voltage applied to the corona electrode 51 of the corona charging device 12C, which is an AC corotron charging device, are increased, and when a rectangular wave is used rather than a sine wave, positive ions and negative ions Since the amount of generation increases, the above effect is great. Under the conditions in this embodiment (the sine wave having a peak-to-peak voltage of 12 [kV] and a frequency of 2 [kHz]), the toner layer potential of the second color cannot be reduced to almost 0 [V], and to some extent. A positive toner layer potential of a magnitude is generated (can be made smaller than in the case of a DC scorotron charging device).

  In the corona charging device 12C, since it is important to supply both positive ions and negative ions at the same time, such a charging device may be used, and the invention is not limited to the AC corotron device.

  Next, as shown in FIG. 1H, the non-image portion is exposed with the writing light 13C. As described above, since the toner layer potential of the second color toner layer is substantially 0 [V], an appropriate latent image potential can be obtained.

  Although not shown in the drawings, since an appropriate latent image potential of the third color toner image can be obtained, the third color toner image can be developed without background contamination.

  Further, the charging, exposure and development processes of the fourth color toner image are performed in the same manner. If an AC corotron charging device is used as the corona charging device 12K, the surface of the photoreceptor belt can be charged while maintaining the toner layer potential of the first color and the toner layer potential of the second color at approximately 0 [V] based on the same principle as described above. An appropriate latent image potential can be obtained by exposure. When a normal DC scorotron charging device is used with the corona charging device 12C for the third color and the corona charging device 12K for the fourth color, the toner layer of the first color is charged twice and the toner layer potential becomes about 100 [V]. In other words, the problem of background contamination becomes serious, but this can be avoided in the present invention.

  Next, the developing devices 14Y, 14M, 14C, and 14K used in this embodiment will be described. In this embodiment, since the next color toner image is developed from the presence of the previous color toner image, a non-contact developing device is used so as not to disturb the previous color toner image.

  The developing device 14Y of this embodiment is shown in FIG. Since the other developing devices 14M, 14C, and 14K are the same, here, the developing device 14Y will be representatively described.

  The developing device 14Y includes a case that accommodates a toner carrying roller 81, a mag roller 82, a two-component developer, and stirring screws 83 and 84. Except for the toner carrying roller 81, it is the same as the normal two-component developing system. The two-component developer is obtained by mixing toner in magnetic carrier powder so that the weight ratio is about 6 [wt%]. The two-component developer is conveyed to the toner carrying member by a mag roller 82 containing a permanent magnet, and a part of the toner is transferred to the toner carrying roller by a bias potential applied thereto. The toner transferred to the toner carrying roller 81 forms a cloud state (a state in which the toner is floating) according to the principle described below, and the developing portion (a portion facing the image carrier) is rotated by the rotation of the toner carrying roller 81. Carried to.

  A toner image is formed by the difference between the average potential on the surface of the toner carrying roller 81 and the image carrying body potential, and unnecessary toner that has not contributed to the development returns to the mag roller 82 again. Since the cloud is formed, the adhesion force of the toner is very low, and the toner returned from the developing portion by the toner carrying roller 81 is easily scraped by the spikes of the two-component developer following the rotation of the mag roller 82. It is averaged. By repeating this, a substantially constant amount of toner is always carried on the toner carrying roller 81 in a cloud state. Although the two-component developing method is adopted as a method for supplying toner to the toner carrying roller 81, the configuration of the developing device is not limited to this.

  The toner carrying roller 81 will be described. FIG. 6 shows an enlarged surface of the toner carrying roller 81. A spatially periodic aluminum vapor deposition electrode 76 is disposed on the support base 75, and the surface is covered with a resin coat 77.

  As shown in FIG. 7, voltages Va and Vb having different waveforms are applied alternately to the periodic electrodes. As shown in FIG. 8, Va and Vb are opposite in time (phase is shifted by 180 degrees). Then, an oscillating electric field is formed between the electrode to which Va is applied and the electrode to which Vb is applied. Therefore, the toner hops between the electrode to which Va is applied and the electrode to which Vb is applied to form a cloud state (a state where the toner is floating). In this way, the toner can be carried in a cloud state on the toner carrier. In FIG. 8, Va and Vb are shown as rectangular waves, but normal AC voltages formed by sine waves may be used. Here, the periodic electrodes are divided into two and voltages having different waveforms are applied alternately. However, if the conditions are such that a vibrating electric field is formed and the toner hops to form a cloud state, the periodic electrodes are divided into three or more. You may comprise so that the voltage of a different waveform may be applied to each.

  Here, a voltage in which an AC component is a rectangular wave having a peak-to-peak voltage of 600 [V] and a frequency of 1 [kHz] and a DC component of 250 [V] is superimposed is applied to Va and Vb. The development bias that triggers the development of the latent image with toner in the development region is the time average value of this voltage. That is, the developing bias is 250 [V].

  Since this developing method forms and develops a cloud state, it is possible to reduce the influence of the adhesive force between the toner carrier and the toner. Therefore, the toner can respond to a small development electric field in the development region, and a sufficient toner adhesion amount can be obtained even with such a small development bias.

  If the developing potential may be small, the potential difference between the developing bias and the non-image portion can be increased without changing the charging potential of the photosensitive belt 11. The effect of the AC corotron charging device increases as the generation amount of positive ions and negative ions increases, but there is a problem that increasing the amount of ion generation increases the amount of ozone. Therefore, the effect of the AC corotron charging device is reduced when it is desired to suppress the generation of ozone. In such a case, for example, even if the toner layers of the first and second colors are positively charged and a toner layer potential of about 50 [V] is generated, the development bias is set to 300 [V] and the development potential is set to 200. Since [V] can be sufficiently developed, the problem of background stains can be avoided.

  Furthermore, if the amount of adhesion between the toner and the toner carrier is very small, development can be performed faithfully with respect to the latent image potential, so that an image with high dot reproducibility and excellent quality can be obtained.

[Example 2]
The overall configuration of the printer 100 that is the image forming apparatus of the second embodiment is the same as that of the first embodiment and is shown in FIG. The same effect as that of the first embodiment can be realized by using the same corona charging device 12 as that of the first embodiment shown in FIG.

  The second embodiment is different from the first embodiment in the developing devices 14Y, 4M, 4C, and 4K. Since all the developing devices 14 are the same, the developing device 14Y will be representatively described.

  The developing device 14 of the second embodiment has substantially the same shape as the developing device 14 of the first embodiment, and is shown in FIG. The difference is the toner carrying roller 81. FIG. 9 shows an enlarged surface of the toner carrying roller 81 of the second embodiment. A spatially periodic aluminum vapor deposition electrode 76 is disposed on the support base 75, and the surface is covered with a resin coat 77.

  In Example 2, the periodic electrode is divided into three as shown in FIG. 9 (here, it may be divided into three or more), and voltages Va and Vb having different waveforms are applied to the respective electrodes as shown in FIG. , Vc is applied. Then, as in Example 1, the toner hops between the electrode to which Va is applied, the electrode to which Vb is applied, and the electrode to which Vc is applied to form a cloud state (a state where the toner is floating). .

  Furthermore, as shown in FIG. 11, a traveling wave electric field for conveying toner is generated by appropriately shifting the phases of Va, Vb, and Vc. As a result, the toner can be conveyed. Therefore, the toner in the cloud state can be conveyed to the developing portion (the portion facing the image carrier) without mechanically rotating the toner carrying roller 81 itself.

  Here, Va, Vb, and Vc are applied with a voltage in which an AC component is a rectangular wave having a peak-to-peak voltage of 600 [V] and a frequency of 1.5 [kHz] and a DC component of 250 [V] is superimposed. The development bias that triggers the development of the latent image with toner in the development region is the time average value of this voltage. That is, the developing bias is 250 [V].

  In this developing method, a cloud state is formed and developed, so that the amount of adhesion between the toner and the toner carrier is very small. Therefore, under the condition that the toner is sufficiently conveyed to the development area, the development is completed when the photosensitive belt potential with the toner attached on the photosensitive belt 11 becomes equal to the development bias. That is, a sufficient toner adhesion amount can be obtained even with a small development potential.

  If the developing potential may be small, the potential difference between the developing bias and the non-image portion can be increased without changing the charging potential of the photosensitive belt 11. The effect of the AC corotron charging device increases as the generation amount of positive ions and negative ions increases, but there is a problem that increasing the amount of ion generation increases the amount of ozone. Therefore, the effect of the AC corotron charging device is reduced when it is desired to suppress the generation of ozone. In such a case, for example, even if the toner layers of the first and second colors are positively charged and a toner layer potential of about 50 [V] is generated, the development bias is set to 300 [V] and the development potential is set to 200. Since [V] can be sufficiently developed, the problem of background stains can be avoided.

  Furthermore, if the amount of adhesion between the toner and the toner carrier is very small, development that is faithful to the latent image potential can be performed, so that an image with high dot reproducibility and excellent quality can be obtained.

[Example 3]
The overall configuration of the printer 100 that is the image forming apparatus of the third embodiment is the same as that of the first and second embodiments, and is shown in FIG. The developing devices 14Y, 14M, 14C, and 14K can achieve the same effect by using the developing device 14 described in the first or second embodiment.

  The third embodiment differs from the first and second embodiments in corona charging devices 12Y, 12M, 12C, and 12K.

  As shown in FIG. 12, the corona charging device 12 which is the corotron charging device of this embodiment is a direct current that applies a direct current voltage to the two corona electrodes 61 and 62, the shield (metal case) 63, the grid 64, and the corona electrode 61. The power source 65 includes a DC power source 66 that applies a DC voltage to the colon electrode 62, and a DC power source 67 that applies a DC voltage to the shield 63 and the grid 64. In this embodiment, the shield 63 and the grid 64 are electrically connected and the same DC voltage is applied. However, the shield 63 and the grid 64 are electrically insulated and a DC voltage is applied from different DC power sources. Also good.

  The corona electrode 61 and the corona electrode 62 are wires having a diameter of 60 [μm] made of tungsten. The shield 63 is made by processing a stainless steel plate having a thickness of 1 [mm], and the side of the shield 63 facing the photosensitive belt 11 is open. The grid 64 is a mesh made of a stainless steel plate having a thickness of 0.1 [mm] by edging. When the grid 64 is provided, the surface of the photosensitive belt can be uniformly charged to the voltage applied to the grid 64, but the grid 64 may not be provided depending on circumstances. The material constituting the shield 63 and the grid 64 may be any material as long as it has conductivity, but stainless steel is adopted from the viewpoint of ozone resistance.

  As shown in FIG. 6, a positive ion and a negative ion can be generated simultaneously by applying a DC voltage of opposite polarity to the corona electrode 61 and the corona electrode 62. In this embodiment, a DC voltage of 5 [kV] is applied to the upstream corona electrode 61 in the photosensitive belt rotation direction, and −4 [kV] is applied to the downstream corona electrode 63 in the photosensitive belt rotation direction. Apply DC voltage. A DC voltage of 500 [V] is applied to the shield 63 and the grid 64. As a result, the surface of the photosensitive belt can be charged to 500 [V]. Although it is desirable to apply the same DC voltage polarity as the charging polarity of the photosensitive belt 11 (that is, the polarity of the DC voltage applied to the shield 63 and the grid 64) to the upstream corona electrode 61 in the photosensitive belt rotation direction. The reverse is also possible. Further, although the photosensitive belt 11 is positively charged here, the same applies to the case where the photosensitive belt 11 is negatively charged.

  Further, the magnitudes of the reverse polarity DC voltages applied to the corona electrode 61 and the corona electrode 62 are set so that the generation amounts of positive ions and negative ions are approximately the same. In general, in the case of the same voltage, the negative discharge generates more ions than the positive discharge, so it is desirable to increase the positive DC voltage with an absolute value as in this embodiment.

  Here, the principle of charging the surface of the photoreceptor belt can be explained as follows. Of the positive ions and negative ions generated from the corona electrode 61 and the corona electrode 62, ions having a polarity opposite to the voltage applied to the shield 63 and the grid 64 (negative ions in this embodiment) are relatively large. 64, ions having the same polarity as the voltage applied to the shield 63 and the grid 64 (positive ions in this embodiment) reach the surface of the photoconductor belt and charge the photoconductor belt 11 positively. If the potential of the photosensitive belt 11 is equal to the voltage applied to the shield 63 and the grid 64 (grid voltage), the movement of the positive ions and negative ions is stopped, and equal positive ions and negative ions are generated. It reaches the surface of the photoconductor belt, and it is in an equilibrium state where the potential on the surface of the photoconductor belt apparently does not change. In this way, the voltage applied to the shield 63 and the grid 64 and the potential of the photosensitive belt 11 become the same, so that the charging potential of the photosensitive belt 11 can be easily controlled to be a predetermined potential. .

  As in the case of the AC corotron using the capacitor described in the first embodiment, the charging potential of the photosensitive belt 11 can be controlled only by the voltage applied to the shield 63. However, the ion potential can be controlled under conditions such as charging uniformity and temperature and humidity. Considering that the amount of generation changes, it is desirable to install the grid 64.

  Further, in the case of the AC corotron charging apparatus using the capacitor described in the first embodiment, the positive ions and the negative ions can be generated only alternately, and there is a problem in the chargeability at high speed. The charging property at high speed means whether or not the target charging potential can be obtained when the rotational speed of the photosensitive belt 11 is high. More ions are required to ensure charging at high speed. In order to solve the toner layer potential problem, it is better that the amount of generated ions is large. In the corona charging device 12 of the present embodiment, both positive ions and negative ions can be generated at the same time, so that more positive ions and negative ions can be supplied than in the AC corotron charging device.

  In this embodiment, since it is important to generate both positive ions and negative ions simultaneously, such a charging device may be used, and the configuration of the corotron charging device shown in FIG. 12 is not limited. .

  Further, the effect obtained by using the corona charging device 12 of the present embodiment is large when the corona charging device 12C for the third color and the corona charging device 12K for the fourth color are used. The charging device 12Y and the second color corona charging device 12M may use other charging devices such as a general DC scorotron charging device.

  The effects of the third embodiment will be described using FIG. 1 as in the first and second embodiments. Here, the photosensitive belt 11 is positively charged and the toner to be developed is negatively charged.

  Note that the contents from the first color charging to the second color charging shown in FIG. 1A to FIG.

  As shown in FIG. 1 (g), the outermost surface (the surface without the toner layer is the surface of the photosensitive belt and the portion with the toner layer is the surface including the toner layer) is charged by 500 [ V]. The toner layer potential of the second color can be set to approximately 0 [V] (at most, about −several [V]) similarly to the charging of the second color.

  Here, when only the positive ions are supplied to the outermost surface at the time of charging of the third color, the toner layer of the first color which has already been neutralized and the toner layer potential is almost 0 [V] is positively charged. As a result, a positive toner layer potential is generated, resulting in a problem of background contamination.

  Therefore, in this embodiment, the corona charging device 12C shown in FIG. 12 is used as the corona charging device 12C. This can supply both positive ions and negative ions at the same time, and undergoes the following charging process. First, since the positive ions reach the surface of the photosensitive belt relatively more than the negative ions at the beginning of charging, the surface of the photosensitive belt is charged with positive ions. As the photosensitive belt surface is charged with positive ions, the amount of positive ions reaching the photosensitive belt surface gradually decreases, and the photosensitive belt surface potential and the voltage applied to the shield 63 and the grid 64 become equal. In this case, equal amounts of positive ions and negative ions reach the surface of the photoreceptor belt, and the potential on the surface of the photoreceptor belt is apparently changed, and charging is completed.

  When the corona charging device 12C charges a portion where the second color toner layer is present, the following phenomenon occurs. First, at the beginning of charging, positive ions reach relatively more than negative ions, so that the photosensitive belt 11 and the second color toner layer are positively charged. However, negative ions have also reached at the same time, so that equal amounts of positive ions and negative ions arrive as charging progresses. At this time, since the charge on the surface of the photosensitive belt is more stable than the toner surface, the positively charged second color toner layer is neutralized with negative ions and the surface of the photosensitive belt with positive ions. It happens to be charged. Therefore, the surface of the photosensitive belt can be charged to 500 [V] while the second color toner layer is maintained at approximately 0 [V] (approximately several [V] at most).

  For comparison, when charging was performed using a normal DC scorotron charging device, the toner layer potential of the second color was about 40 [V].

  In the corona charging device 12C, since it is important to supply both positive ions and negative ions at the same time, such a charging device may be used, and the invention is not limited to the AC corotron device.

  Next, as shown in FIG. 1H, the non-image portion is exposed with the writing light 13C. As described above, since the toner layer potential of the second color toner layer is substantially 0 [V], an appropriate latent image potential can be obtained.

  Although not shown in the drawings, since an appropriate latent image potential of the third color toner image can be obtained, the third color toner image can be developed without background contamination.

  Further, the charging, exposure and development processes of the fourth color toner image are performed in the same manner. If the corotron charging device shown in FIG. 12 is used as the corona charging device 12K, the photosensitive belt is maintained while maintaining the toner layer potential of the first color and the toner layer potential of the second color at approximately 0 [V] on the same principle as described above. The surface can be charged, and an appropriate latent image potential can be obtained by exposure. When a normal DC scorotron charging device is used with the corona charging device 12C for the third color and the corona charging device 12K for the fourth color, the toner layer of the first color is charged twice and the toner layer potential becomes about 100 [V]. In other words, the problem of background contamination becomes serious, but this can be avoided in the present invention.

As described above, according to the present embodiment, the photosensitive belt 11 that is an image carrier, the charging devices 12Y, 12M, 12C, and 12Bk that are one or more charging units that charge the photosensitive belt 11, and the charging device 12Y. , 12M, 12C, and 12Bk, the exposure device 4 that is a latent image forming unit that exposes the photosensitive belt 11 charged by each color to form a latent image for each color, and the latent image formed on the photosensitive belt 11 Developing devices 14Y, 14M, 14C, and 14Bk, which are three or more developing means for visualizing with different color toners, are charged by the charging device 12 for each color and the photosensitive belt 11 is charged. The exposure device 4 exposes a non-image portion on the body belt 11 to form a latent image, and the developing device 14 develops the latent image with toner having a polarity opposite to the charging polarity. In a printer which is an image forming apparatus that sequentially forms a toner image of each color on a single photosensitive belt 11 to form a color toner image of three or more colors, a charging device 12 used for image formation for at least the third color and thereafter. Is a corona electrode that is a corona ion generating member that generates positive and negative corona ions by applying a voltage, and a charging potential control that controls the charging potential of the photosensitive belt 11 to be a predetermined potential. It has high voltage power supplies 55 and 67 and shields 52 and 53 or a grid 64 constituting the means. As a result, when the next color image is formed after the previous color toner image is formed, ions having the same polarity as the charged polarity of the photosensitive belt 11 discharged from the charging devices 12Y, 12M, 12C, and 12Bk are used. In addition, the surface potential of the photoreceptor belt can be made uniform as much as possible while eliminating the influence of the charge of the reverse polarity which the toner image of the previous color has. That is, between the development of the previous color toner image and the formation of the next color latent image, the charge of the previous color toner image and the initialization of the potential of the photosensitive belt 11 are performed by one charging device 2. Can be performed simultaneously. Therefore, the next color toner image can be formed without being affected by the potential of the previous color toner image. Therefore, it is possible to suppress the influence of the potential of the previous color toner image in the image forming process of the next color without providing a dedicated neutralizing unit for neutralizing the previous color toner image, and the above neutralizing unit is provided. The device body can be reduced in size.
Further, at least the third and subsequent charging devices 12 can generate positive and negative corona ions from the corona electrodes. Thus, for example, when the third color image is formed, even if the first color toner image is charged to the same polarity as the charging polarity to be charged, the charge is eliminated by corona ions having a polarity opposite to the charging polarity to be charged. be able to. Therefore, by controlling the charging potential of the photosensitive belt 11 by the charging potential control means while supplying positive and negative corona ions from the discharge electrode to the photosensitive member, the potential of the first color toner image is made substantially zero, In addition, the photosensitive belt 11 can be uniformly charged to a predetermined charging potential. Therefore, when the third color exposure is performed, the potential difference between the potential of the non-image portion of the portion where the toner image of the first color is present and the developing bias becomes appropriate, so that the third color toner adheres to the non-image portion. Can be suppressed. Therefore, it is possible to suppress the occurrence of background stains in the non-image portions after the third color.
In addition, according to the present embodiment, the corona ion generating member is a corona electrode that is a discharge electrode, and the charging device 12 used for image formation for at least the third color is close to the photosensitive belt 11 and the photosensitive belt. One or more corona electrodes are disposed inside shields 52 and 63 which are cases having an opening on the side facing 11. Thereby, positive and negative corona ions can be generated simultaneously without using a means for generating large corona ions. Accordingly, it is possible to obtain an image forming apparatus such as a small-sized and low-cost high-quality printer.
Further, according to the present embodiment, the corona electrode 51 has the AC power source 54 that is a voltage applying unit that applies an AC voltage or a voltage in which a DC is superimposed on the AC. As a result, both positive and negative corona ions can be generated from one corona electrode 51, so that the charging device 12 can be reduced in size and an image forming apparatus such as a smaller and lower cost printer can be obtained. be able to.
In addition, according to the present embodiment, a plurality of corona electrodes are provided, and a DC power source 65 that is a first voltage applying unit that applies a positive DC voltage to the corona electrode 61 and a negative polarity to the corona electrode 62. And a DC power supply 66 which is a second voltage applying means for applying a DC voltage. Thereby, since both positive and negative corona ions can be generated efficiently, high-speed charging properties can be ensured.
In addition, according to the present embodiment, the amount of positive and negative ions generated can be made comparable by increasing the positive DC voltage in absolute value than the negative DC voltage, It becomes easy to control the charging potential.
Further, according to the present embodiment, the grid 64 is used as the charging potential control means, and the grid 64 is provided between the corona electrodes 61 and 62 and the photosensitive belt 11, so that the charging potential can be controlled by the grid 64. Therefore, the controllability of the charging potential is more excellent and the charging uniformity is also improved.
Further, according to the present embodiment, the developing device 14 includes a toner carrying roller 81 that is a toner carrying body that carries toner and is disposed so as to face the photosensitive belt 11 in a non-contact manner in the development region, and a toner carrying The plurality of electrodes are provided on the toner carrying roller 81 so as to be along the surface of the roller 81 and insulated from each other, and the plurality of electrodes are adjacent to each other in the direction of relative polarity of the applied voltage. Since the electrodes are different from each other, hopping electric field generating means for hopping the toner is provided. Thus, since the toner on the toner carrying roller 81 is in a floating state (referred to as toner cloud), the amount of adhesion between the toner and the toner carrying roller 81 is very small. Therefore, even with a small development potential (difference between the photoreceptor belt potential and the development potential in the image area), sufficient development can be performed without contact. If the development potential is small, the potential difference between the development bias and the non-image portion can be increased without changing the charging potential of the photosensitive belt 11, so that the occurrence of background stains can be suppressed. Furthermore, if the amount of adhesion between the toner and the toner carrying roller 81 is very small, development can be performed faithfully with respect to the latent image potential, so that an image with high dot reproducibility and excellent quality can be obtained.
Further, according to the present embodiment, the toner carrying roller 81 is a surface moving member that conveys the toner carried on the surface of the toner carrying roller 81 to the development area by moving the surface of the toner carrying roller 81. is there. Thus, the toner carried on the surface of the toner carrying roller 81 can be easily conveyed to the development area.
Further, according to the present embodiment, the hopping electric field generating means generates a traveling wave electric field for transporting the toner to the development area while hopping the toner carried on the surface of the toner carrying roller 81. It is generated on the surface of the carrying roller 81. Accordingly, since the toner carried on the toner carrying roller 81 can be conveyed to the developing region by the traveling wave electric field without rotating the toner carrying roller 81, a driving unit for driving the toner carrying roller 81 is not necessary. In addition, the apparatus body can be further downsized.

  In this embodiment, the printer using the photosensitive belt 11 has been described. However, as described above, an image forming apparatus such as a printer using the photosensitive drum may be used. That is, as shown in FIG. 13, the corona charging device 12 shown in the first embodiment is provided to face the photosensitive drum 1, or the corona charging device 12 shown in the third embodiment is used as the photosensitive drum as shown in FIG. By providing it so as to oppose 1, the effects as described above can be obtained as in the case of using the photosensitive belt 11.

FIG. 6 is a schematic diagram relating to a potential of a photosensitive drum and a toner image surface showing a characteristic part of the present invention. 1 is a schematic configuration diagram of a printer according to an embodiment. FIG. 2 is an enlarged schematic configuration diagram in the vicinity of a photosensitive belt. 1 is a schematic configuration diagram of a corona charging device according to Embodiment 1 and Embodiment 2. FIG. FIG. 2 is a schematic configuration diagram of a developing device. FIG. 3 is an enlarged view of a surface portion of a toner carrying roller in Embodiment 1. The figure which showed the toner cloud state at the time of applying the voltage of the waveform from which an electrode which is periodically arranged alternately differs. The wave form diagram of the voltage applied to the electrode arranged periodically. FIG. 6 is an enlarged view of a surface portion of a toner carrying roller in Embodiment 2. The figure which showed the toner cloud state at the time of applying the voltage of the waveform from which an electrode which is periodically arranged alternately differs. The wave form diagram of the voltage applied to the electrode arranged periodically. FIG. 6 is a schematic configuration diagram of a corona charging device according to a third embodiment. FIG. 2 is a schematic configuration diagram in the vicinity of a corona charging device when the corona charging device of Example 1 is used in an image forming apparatus using a photosensitive drum. FIG. 6 is a schematic configuration diagram in the vicinity of a corona charging device when the corona charging device of Example 3 is used in an image forming apparatus using a photosensitive drum. Schematic configuration diagram of a conventional image forming unit. FIG. 6 is a schematic diagram relating to a potential of a photosensitive drum and a toner image surface in a conventional image forming apparatus. FIG. 6 is a schematic diagram relating to a potential of a photosensitive drum and a toner image surface in a conventional image forming apparatus showing a problem to be solved by the present invention.

Explanation of symbols

4 Exposure Device 11 Photosensitive Belt 12 Charging Device 13 Writing Light 14 Developing Device 15 Paper Conveying Path 16 Transfer Device 17 Fixing Device 18 Cleaning Device 51 Corona Electrode 52 Shield 53 Capacitor 54 AC Power Supply 55 DC Power Supply 61 Corona Electrode 62 Corona Electrode 63 Shield 64 Grid 65 DC power supply 66 DC power supply 67 DC power supply 75 Support base 76 Aluminum vapor deposition electrode 77 Resin coat 81 Toner carrying roller 82 Mag roller 83 Stir screw 84 Stir screw

Claims (9)

An image carrier;
One or more charging means for charging the image carrier;
Latent image forming means for forming a latent image for each color by exposing the image carrier charged by the charging means;
Three or more developing means for visualizing the latent image formed on the image carrier with toners of different colors,
For each color, the image carrier is charged by the charging unit, the latent image forming unit exposes a non-image portion on the image carrier to form a latent image, and the latent image is developed by the developing unit. In an image forming apparatus that forms three or more color toner images by sequentially forming toner images of each color on a single image carrier by developing with toner having a polarity opposite to the charging polarity.
The charging means used for image formation for at least the third color is a corona ion generating member that generates positive and negative corona ions by applying a voltage, and the charging potential of the image carrier is a predetermined potential. An image forming apparatus, comprising: a charging potential control unit that controls the image forming apparatus to be The image forming apparatus according to claim 1.
The corona ion generating member is a discharge electrode,
The charging means used for image formation for at least the third color is provided with one or more discharge electrodes in a case having an opening on the side close to the image carrier and facing the image carrier. An image forming apparatus. The image forming apparatus according to claim 1 or 2,
An image forming apparatus comprising voltage applying means for applying an alternating voltage or a voltage obtained by superimposing a direct current on an alternating current to the corona ion generating member. The image forming apparatus according to claim 2.
A plurality of the discharge electrodes; a first voltage applying means for applying a positive DC voltage to the discharge electrodes; and a second voltage applying means for applying a negative DC voltage to the discharge electrodes. An image forming apparatus comprising the image forming apparatus. The image forming apparatus according to claim 4.
An image forming apparatus, wherein the positive DC voltage is larger in absolute value than the negative DC voltage. The image forming apparatus according to claim 1, 2, 3, 4, or 5.
The charging potential control means is a grid electrode,
An image forming apparatus, wherein the grid electrode is provided between the corona ion generating means and the image carrier. The image forming apparatus according to claim 1, 2, 3, 4, 5 or 6.
The developing means is disposed so as to face the image carrier in a non-contact manner in a development region, and a toner carrier that carries the toner;
A plurality of electrodes provided on the toner carrier and insulated from each other along the surface of the toner carrier, and the plurality of electrodes are adjacent to each other in the direction of relative polarity of the applied voltage. An image forming apparatus comprising: a hopping electric field generating means for hopping toner by being different from each other by a plurality of electrodes. The image forming apparatus according to claim 7.
The toner carrier is a surface moving member that conveys the toner carried on the surface of the toner carrier to the development area by moving the surface of the toner carrier. . The image forming apparatus according to claim 8.
The hopping electric field generating means generates a traveling wave electric field on the surface of the toner carrying member for hopping the toner carried on the surface of the toner carrying member and conveying the toner to the developing region. What is claimed is: 1. An image forming apparatus comprising:

Images