JP3376055B2 - Image forming device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus having a charging member to which an oscillating voltage is applied in order to charge an image carrier such as a photoconductor or a dielectric.

[0002]

2. Description of the Related Art Conventionally, a corona charger has been widely used as a charging means for an image carrier in an image forming apparatus, but recently, a corona discharge product such as ozone is generated as compared with a corona charger. Since it has advantages such as a small number and a low applied voltage, a contact charging means for charging the image bearing member by bringing a charging member as a charger to which a voltage is applied into contact with the image bearing member is put into practical use. ing.

The voltage applied to the charging member is a DC voltage alone or an oscillating voltage in which a DC voltage and an AC voltage are superimposed.

The above-mentioned contact charging in which only a DC voltage is applied has a merit that an applied voltage is low and an inexpensive power source can be used, but on the other hand, spot-like charging unevenness is likely to occur and the charging member is dirty. It has a small latitude to obtain uniform charging even if it is weak.

On the other hand, the latter case of contact charging applying an oscillating voltage can significantly improve spot-like charging unevenness, and the power source is a little complicated and costly as compared with contact charging applying only a DC voltage. It has a large latitude to obtain a uniform charging property. As in USP4851960, the oscillating voltage is effectively a voltage in which an AC voltage having a peak-to-peak voltage that is at least twice the charging start voltage of the image carrier is superimposed on the DC voltage.

[0006]

However, in an image forming apparatus that charges an image carrier by applying an oscillating voltage to a charger and forms a latent image on the image carrier along a scanning line, the oscillating voltage is used. There is a problem that the charging frequency of the image carrier due to the frequency of 1 and the spatial frequency of the latent image formed on the image carrier by exposure interfere with each other to cause image unevenness. That is,
Since there is a slight change in the potential depending on the frequency of the oscillating voltage applied to the charger, when an image having a spatial frequency close to this frequency is formed, they interfere with each other to cause pitch unevenness and moire.

In a high resolution image forming apparatus such as a laser printer, an image such as a halftone may be output. Halftone is often formed by an image having spatial periodicity such as horizontal lines, vertical lines, and dots. When the exposure corresponding to the image information having such spatial periodicity is performed, the periodicity due to the charging on the image carrier and the periodicity of the exposure corresponding to the image information interfere with each other, and the lateral and vertical directions are Image irregularities such as fine pitch irregularities and moiré with a fine cycle are likely to occur.

Further, when a color image is formed by repeating an image forming process including a step of charging an image carrier by applying an oscillating voltage to a charger, a plurality of colors (for example, yellow, magenta and cyan) are successively superimposed. , Black (four colors), pitch unevenness, moire, and color unevenness occur. A periodical image disturbance that is not so noticeable in a single-color image is also emphasized when images of a plurality of colors are overlapped with each other, which causes a significant deterioration in image quality.

[0009]

SUMMARY OF THE INVENTION It is an object of the present invention to provide an image forming apparatus in which uneven charging of an image carrier due to a charging member is prevented.

Another object of the present invention is to provide an image forming apparatus which prevents uneven pitch and moire of an image.

Another object of the present invention is to provide an image forming apparatus which prevents color unevenness of images of a plurality of colors.

Further objects and features of the present invention will become more apparent by reading the following detailed description with reference to the accompanying drawings.

[0013]

According to the present invention, an image carrying means capable of carrying an image of a first color and an image of a second color different from the first color, and transferring the images of the first and second colors. A transfer means for superposing and transferring onto a material, a charging member for sequentially charging the surface of the image carrying means to form an image on the image carrying means, and a voltage applying means for applying an oscillating voltage to the charging member. In the image forming apparatus, the frequency of the vibration voltage applied to the charging member is different when forming the image of the first color and when forming the image of the second color.

[0014] Alternatively, the present invention includes a first color image, the second color and the image of which is different from the first color, Ri supportable der a single transfer
Position a single image bearing member and a transfer means for superimposing transferred to a transfer material the first and second colors image of the image carrier to form an image on the image carrier <br/> In an image forming apparatus having a charging member that charges the surface of the body and a voltage applying unit that applies an oscillating voltage to the charging member, when an image of the first color is formed, The phase of the oscillating voltage applied to the charging member is different from that when a two-color image is formed.

[0015]

FIG. 1 is a schematic configuration diagram of an image forming apparatus as a reference example. The apparatus of this example is a laser beam printer for a transfer electrophotographic process.

Reference numeral 1 denotes a rotary drum type electrophotographic photosensitive member (hereinafter referred to as a photosensitive drum) as an image bearing member, and an organic photosensitive member (OPC) is provided on the outer peripheral surface of a grounded aluminum cylinder.
Or A-Si (amorphous silicon), Cds, Se
A photoconductive layer of an inorganic photoconductor is formed and is rotationally driven in the clockwise direction indicated by an arrow at a predetermined peripheral speed (process speed) by a driving unit (not shown).

Numeral 2 is a contact charging roller as a charging device. The charging roller 2 is rotated by being driven by the photosensitive drum 1, and a predetermined voltage is applied from high voltage power sources 24 and 25 as will be described later, thereby rotating. The peripheral surface of the photosensitive drum 1 is uniformly primary-charged to a predetermined polarity and potential.

Next, the primary charging surface of the rotary photosensitive drum 1 is exposed by a laser scanner 3 as an exposure device along a scanning line in accordance with target image information, and the surface of the rotary photosensitive drum 1 is exposed with the image information. Electrostatic latent image of is formed.

The latent image is developed as a toner image by the developing device 4. In this example, the charging polarity of the toner is the charging device 2
The reversal development method, which is the same as the charging polarity of, is used. The toner image is transferred to the transfer nip portion 6a at a predetermined timing from a sheet feeding portion (not shown) at a transfer nip portion 6a which is a pressure contact portion between the transfer roller 6 as a transfer device and the photosensitive drum 1. It is transferred to the surface of the material P. Transfer roller 6
Is applied with a transfer bias voltage having a polarity opposite to the charging polarity of the toner.

The transfer material 6a on which the toner image has been transferred at the transfer nip portion 6a is fixed by the fixing device 8 by a conveying means (not shown).
The toner image is fixed to the toner image as a permanent image and is output as an image-formed product (print / copy).

After the transfer of the toner image onto the transfer material P, the surface of the photosensitive drum 1 is cleaned by removing the transfer residual toner and the like by a cleaning device 7, and is repeatedly used for image formation.

The charging roller 2 in this example has EPDM (a terpolymer of ethylene propylene diene), NBR (nitrile butadiene rubber), and silicone rubber on a core metal 21 as shown in the schematic cross-sectional view of FIG. A conductive elastic layer 22 made of, for example, is further provided, and a urethane rubber layer 23 having a volume resistance of 10 ⁵ Ωcm by dispersing carbon is provided on the outer peripheral surface of the conductive elastic layer 22.

The core metal 21 of the charging roller 2 is connected in series with an AC voltage power supply 25 for generating a sine wave to a DC high voltage power supply 24. The charging roller 2 has an oscillating voltage whose voltage value periodically changes with time. Is applied.

The AC voltage power supply 25 is an AC voltage amplifier 25.
1 has a sine wave generating circuit 252 capable of frequency modulation connected thereto, a signal generated by the sine wave generating circuit 252 is amplified by the AC high-voltage amplifier 251, and a predetermined bias is applied to the charging roller 2. .

FIG. 3 shows a voltage output waveform of an AC power source which changes with time. During charging for one image formation, the highest frequency f1 is modulated through the lowest frequency f2 to the highest frequency f1, and modulation (composition) is performed. ) The frequency is indicated by f0. The modulation frequency f0 is composed of a plurality of different frequencies, and has the same voltage value every cycle 1 / f0 seconds. Here, if the highest frequency f1 is too high, power consumption increases and charging noise increases, and if the lowest frequency f2 is too low, uneven charging occurs. Therefore, the process speed Vps (mm) of the photosensitive drum of the apparatus to be used. / Sec) but each is 200
It is preferably 0 Hz or less and 200 Hz or more.

Further, the difference between the highest frequency f1 and the lowest frequency f2 was effective at 50 Hz or more, preferably 100 Hz or more. If the modulation frequency f0 has a waveform of 3 waves or more, preferably 5 waves or more within the period, good results are obtained, and 600 Hz or less is preferable. It should be noted that one wave includes one peak and one trough. Further, if the modulation frequency f0 is too low, the change of the frequency becomes slow and the effect of the modulation is deteriorated. Therefore, the image on the transfer material is 10 mm or less, preferably 5 mm or less.
It is better to modulate at mm or less. That is, 10 mm / Vps
Within (second), preferably within 5 mm / Vps (second).

In the printer of this reference example, regarding the charging bias for the charging roller 2, the maximum frequency f1 is set to 6
00Hz, minimum frequency f2 is 400Hz, modulation frequency f
0 was set to 122 Hz, and the occurrence of moire was examined in a horizontal line image formed by repeating a line having a width of 1 dot and a gap having a width of 1 to 4 dots. As a comparative example, the charging frequency is 5
The generation of moire when the frequency was kept constant at 00 Hz was examined in the same manner. In any case, the process speed is 100 mm /
sec and a resolution of 600 dpi. The results are shown in Table 1.

The one-dot two-space means that a horizontal line image is created by repeating the line scanning for one dot and the portion not scanning for two dots by the laser.

[0029]

[Table 1] ◯ indicates that moire did not occur, Δ indicates that slight moire occurred, and x indicates that moire occurred remarkably.

As can be seen from the above results, in the comparative example, a slight moire was generated in the image of 1 dot 3 spaces and a remarkable moire was generated in the image of 1 dot 4 spaces, but in this embodiment, No moire was found in any of the images.

Further, when the charging frequency of the comparative example was kept constant during charging and variously changed from 500 Hz, moire occurred in some images.

As described above, by modulating the frequency of the oscillating voltage applied to the charger 2 during charging, a high quality image without moire can be obtained. The charging device 2 may be a charging blade, a charging brush or the like instead of the charging roller of this reference example.

<First Embodiment> FIG. 4 is a schematic configuration diagram of an image forming apparatus as a first embodiment. The apparatus of this example is a color image forming apparatus.

The rotary photosensitive drum 1 as an image carrier is uniformly charged to a predetermined potential by a charging roller 2 as a charger. Although not shown in the figure, the charging roller 2 has a high-voltage power supply 24/25 (FIG. 1) similar to that of the printer of the reference example.
Are connected.

A signal according to a yellow component image pattern of a target color image is input to the exposure device (laser scanner) 3, the photosensitive drum 1 is subjected to scanning exposure L of the image pattern, and the image pattern is formed on the photosensitive drum 1. Latent image is formed.

In the turret type rotary developing device 40, the support 5 is rotated so that the developing device 4Y containing yellow toner faces the photosensitive drum 1, and the developing device 4Y is rotated.
Thus, the latent image on the photosensitive drum 1 is visualized as a yellow toner image.

The yellow toner image is transferred onto the transfer paper P wound and held on the outer peripheral surface of the transfer drum 60.

The toner image is transferred onto the transfer material P as follows. That is, the transfer sheet P is fed from the transfer sheet cassette 10 to the transfer drum 60 by the pickup roller 9 in synchronization with the toner image on the photosensitive drum 1. The transfer drum 60 is configured by providing an elastic layer 62 on a conductive support 61, and further stretching a dielectric sheet 63 on the elastic layer 62, and rotating in the counterclockwise direction of the arrow at substantially the same speed as the photosensitive drum 1. Driven. When the transfer paper P is supplied to the transfer drum 6, the front end of the transfer paper is held by the gripper 66 provided on a part of the support, and the transfer paper is electrostatically adsorbed by the adsorbing device 67 including an adsorbing roller. The transfer paper is held around the transfer drum 60 in a wound state. The toner image on the photosensitive drum 1 is transferred to the transfer drum 6 by applying a bias voltage having a polarity opposite to the charging polarity of the toner to the support 61.
The image is transferred onto the transfer paper P that is wound around 0 and held.

Next, formation of a latent image for a magenta component image pattern of the target color image → development by a developing device 4M containing magenta toner → transfer of the magenta toner image to a transfer paper, formation of a latent image for a cyan component image pattern → Development by developing device 4C containing cyan toner → Transfer of the cyan toner image to transfer paper, latent image formation for black image pattern → Development by developing device 4B containing black toner → Transfer paper of the black toner image Transfer is sequentially performed, and four toner images of a yellow toner image, a magenta toner image, a cyan toner image, and a black toner image are sequentially arranged on the surface of the same transfer material P that is wound around and held on the transfer drum 60. The transferred color toner image is formed on the transfer material P by being superimposed and transferred.

The transfer material P which has undergone the above-mentioned four toner image superposition transfer is peeled from the transfer drum 60 by the separation charger 64 and the separation claw 65, melted and mixed by the heat fixing device 8 and fixed as a permanent image. It is discharged to the outside.

The untransferred toner on the photosensitive drum 1 is cleaned by the cleaning device 7. Also, the transfer drum 6
It is also preferable to clean the toner on No. 0 by a transfer drum cleaning device 68 such as a fur brush web if necessary.

In the color image forming apparatus described above, toner images of four colors of yellow, magenta, cyan, and black are superposed on the transfer paper to form a color image. Therefore, in addition to the pitch unevenness in each single color image described above. As a result, color unevenness may appear due to color overlapping.

That is, when the primary charging by the charging roller of each color is made to have the same frequency and phase, the charging unevenness and the density unevenness due to this overlap, and even if the density unevenness is not a problem for each single color. However, in the case of a color image, unevenness in the color tone occurs. Specifically, when a color image was formed under the same charging conditions as those of the comparative example in Table 1 described in the reference example, color unevenness was generated even in an image of 1 dot 1 space where moire did not occur in a monochrome image. did. On the other hand, the problem of color unevenness can be solved by making the charging frequency of each color the same and changing the phase of the frequency for each color, or by making the charging frequency of each color different. Specifically, the high voltage power source of the charging roller may be configured to change the phase or frequency, and the image writing timing of each color image may be switched for at least one of the phase and frequency for each detection signal. Further, by modulating the frequency of the primary bias voltage during charging of one color as in the case of the reference example, it was possible to prevent moire and color unevenness even when forming a monochromatic image. That is, the primary charging bias voltage for the charging roller 2 in each of the yellow, magenta, cyan, and black toner image forming processes was set as follows.

The above-described embodiment and a comparative example in which a bias of 500 Hz without changing the charging frequency was applied in the toner image forming process of each color,
As comparative example 2, the output image in each case was compared with the case where the bias shown in the reference example was applied under the same condition in the toner image forming process of each color. In addition, the compared images are horizontal line images similar to those in the reference example, and a single color image and a full color image are output. The results are shown in Table 2.
It was shown to.

[0045]

[Table 2]

As can be seen from the above results, in the present embodiment, no moire was observed in either the monochromatic image or the color image.

In Comparative Example 1, both the single color image and the color image are small in the image of 1 dot 3 space.
Moire was noticeably seen in the image of dot 4 space. In particular, in a color image, moire appeared as color unevenness, resulting in a marked deterioration in image quality.

In Comparative Example 2, moire did not occur in the monochromatic image, but color unevenness occurred in the color image, although not as severe as in Comparative Example 1.

As described above, in the color image forming apparatus for forming the color image by repeating the steps of charging, exposing, developing, and transferring the photosensitive drum 1 a plurality of times, the frequency of the bias applied to the charging roller 2 is set. By varying the frequency and changing the frequency for each color, it is possible to prevent moire and color unevenness even in a color image.

<Second Embodiment> (FIG. 6) In this embodiment, in the color image forming apparatus of the first embodiment, a bias having a variable frequency is used as the bias applied to the charging roller 2, and toner of each color is used. By changing the phase of the waveform for each image forming process, it is possible to prevent moire and color unevenness in a color image, as in the first embodiment.

FIG. 6A shows a bias waveform applied to the charging roller 2 in the first color yellow toner image forming process. The bias waveform is from the lowest frequency f1 = 300 Hz to the highest frequency f2 = 600 Hz and then again to the lowest frequency f1. Is a waveform returning to. Here, modulation is performed with 6 waves as one cycle.

(2) in the figure is a bias waveform in the case of the magenta toner image forming process of the second color, and (1)
A waveform for advancing the bias indicated by 1 by one wave is applied.

In the following, during the third color magenta toner image forming process and the fourth color black toner image forming process, the biases that proceed by one wave (3) and (4) are applied to the charging roller 2.

The bias applied to the charging roller 2 is frequency-modulated for each color, and the phase is changed for each color toner image forming process, so that images having the same frequency do not overlap in the same region even when the images are overlapped by four colors. Therefore, it is possible to prevent moire and color unevenness in the color image.

The transfer drum 60 or the photosensitive drum 1
It is preferable to control the image to be printed, the writing start timing, and the phase of the charging bias in synchronization with the rotation of the.

The color image forming apparatus may be of a type in which toner images of a plurality of required colors are successively formed on the image carrier 1 and the superimposed toner images are collectively transferred to a transfer material. Alternatively, the toner images sequentially formed on the intermediate image carrier may be sequentially superimposed and transferred to the intermediate image carrier, and the toner images superimposed on the intermediate image carrier may be collectively transferred to the transfer material.

As shown in FIG. 7, a yellow toner image,
It has first to fourth process mechanism sections 4 to 4 for respectively forming a magenta toner image, a cyan toner image and a black toner image, and transfers the transfer material P to the transfer sections 71 to 74 of the four image forming process mechanism sections. It is also possible to adopt a system in which a color image is obtained by sequentially conveying the four toner images by the conveying means 75 and sequentially transferring the four toner images.

By applying the present invention to any color image forming apparatus, it is possible to output a color image without moire or color unevenness.

The charging member should not be brought into contact with the image bearing member, and should be disposed in a non-contact manner so as to have a slight air gap (air gap) between the charging member and the surface of the image bearing member which may cause a discharge phenomenon. You can also When the charging member is brought close to the image carrier, it is desirable that the distance between the charging member and the image carrier is 1000 μm or less.

Further, the charging member may have a blade type, a rod type, a brush type or the like other than the roller shape.

The waveform of the oscillating voltage applied to the charging member is not only a sine wave, but also a rectangular wave, a sawtooth wave, a triangular wave, a pulse wave, etc., and is formed by periodically turning on and off a DC power supply. It may be a rectangular wave.

In the image forming apparatus that charges the image carrier by applying an alternating voltage to the charger, the charging frequency is modulated during charging, so that the periodicity due to the charging on the image carrier and the exposure corresponding to the image information are performed. It is possible to reduce the interference of the periodicity of. Further, in the color image forming apparatus, by making the frequency different in each of the repeated image forming processes or changing the phase thereof, images having the same frequency do not overlap in the same region even in the case of overlapping images of plural colors. It was possible to prevent color unevenness in a color image.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of an image forming apparatus of a reference example.

FIG. 2 is a sectional view of a layer structure of a charging roller.

FIG. 3 is a waveform diagram of a bias applied to a charging roller.

FIG. 4 is a schematic configuration diagram of a color image forming apparatus according to a first embodiment.

FIG. 5 is a waveform diagram of a bias applied to a charging roller in a toner image forming process of each color.

FIG. 6 is a waveform diagram of a bias applied to a charging roller in a toner image forming process of each color in the second embodiment.

FIG. 7 is a schematic configuration diagram of a color image forming apparatus having another configuration.

[Explanation of symbols]

1 photosensitive drum 2 charging roller 24 DC power supply 25 AC power supply 251 AC voltage amplifier 252 Sine wave generator

─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Hiroshi Sasame 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (56) Reference JP-A-4-373254 (JP, A) JP-A-3 -101765 (JP, A) JP-A-4-328778 (JP, A) JP-A-4-16867 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) G03G 15/02 G03G 15/00 303 G03G 15/01

Claims (16)

(57) [Claims] 1. An image carrying unit capable of carrying an image of a first color and an image of a second color different from the first color, and the images of the first color and the second color are superimposed and transferred onto a transfer material. And a voltage applying unit for applying an oscillating voltage to the charging member, in order to form an image on the image carrying unit, and a charging member for sequentially charging the surface of the image carrying unit. An image forming apparatus, wherein the frequency of the vibration voltage applied to the charging member is different between when the image of the first color is formed and when the image of the second color is formed. 2. The image forming apparatus according to claim 1, wherein the phase of the oscillating voltage is different when the image of the first color is formed and when the image of the second color is formed. 3. The image forming apparatus according to claim 1, wherein the oscillating voltage is a superimposed voltage of a DC voltage and an AC voltage. 4. The image forming apparatus according to claim 1, wherein the charging member is capable of contacting with the image carrier. 5. The charging member faces the image bearing means with a minute gap therebetween.
Image forming apparatus of any one of. 6. The charging member comprises the image bearing means 10 and
The image forming apparatus according to claim 5, wherein the image forming apparatuses face each other with a gap of 00 μm or less. 7. The image bearing means includes the first color and the second color.
The apparatus has a single image carrier capable of carrying a color image, and the apparatus has a transfer material carrier that carries and conveys the transfer material in order to transfer the image of the image carrier to the transfer material. The image forming apparatus according to any one of claims 1 to 6. 8. The image bearing means includes a first image bearing member capable of bearing the image of the first color and a second image bearing member capable of bearing the image of the second color. 7. The image according to claim 1, wherein the apparatus has a transfer material carrier that carries and conveys the transfer material for transferring the images of the first and second image carriers to the transfer material. Forming equipment. 9. An image of yellow, magenta, cyan, and black colors can be carried on the image carrying means, and a full-color image can be formed on the transfer material to form images of the respective colors. The image forming apparatus according to claim 1, wherein the frequency of the oscillating voltage is different for each. 10. A first-color image, the second color and the image of which is different from the first color, Ri supportable der a single with a single transfer position
As an image bearing member, a charging member for charging a transfer unit that superimposes transferred to a transfer material the first and second colors the image of the surface of the image carrier to form an image on said image bearing member And a voltage applying unit that applies an oscillating voltage to the charging member, the charging is performed when an image of the first color is formed and when an image of the second color is formed. An image forming apparatus, wherein the vibration voltages applied to the members have different phases. 11. The image forming apparatus according to claim 10, wherein the oscillating voltage is a superimposed voltage of a DC voltage and an AC voltage. 12. The charging member, image forming apparatus according to claim 10 or 11, characterized in that it is contactable to said image bearing member. 13. The charging member, image forming apparatus according to claim 10 or 11, characterized in that the opposing via the image bearing member with a small gap. 14. The charging member, the image carrier and 10
14. The image forming apparatus according to claim 13, which are opposed to each other with a gap of not more than 00 μm. 15. The apparatus according to claim 10 to 14 or of the image and having a transfer material carrying member for bearing and conveying a transfer material to transfer the transfer material the image of the image carrier Forming equipment. 16. The image carrier is capable of carrying images of yellow, magenta, cyan, and black colors, and is capable of forming a full-color image on a transfer material to form images of the respective colors. The image forming apparatus according to any one of claims 10 to 15 , wherein the phase of the oscillating voltage is different for each.