JP2003270900A - Multicolor image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a multicolor image forming apparatus capable of efficiently forming a final image of improved color reproducibility by avoiding dependency on toner characteristics and increase of the toner consumption and also suppressing a toner reverse transfer phenomenon at several transfer processes. <P>SOLUTION: On the assumption that the apparatus is a tandem type multicolor image forming apparatus, it is provided with 1st to 4th image forming stations each of which is equipped with a photoreceptor, and a transfer carrying belt for carrying a recording medium, and wherein an image is developed by applying a developing bias voltage obtained by superposing an AC voltage on a DC voltage on the developing roller 52 in each image forming station. The duty ratio of the components of the developing bias voltage in a developing accelerating direction is reduced stepwise from the upstream side in accordance with the ranking of a toner transfer order. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sequential transfer of a developer image developed on a photoconductor of an electrophotographic color copying machine, a color printer or the like while a transfer material is conveyed by a semiconductive transfer belt. And a multicolor image forming apparatus for forming an image.

[0002]

2. Description of the Related Art Generally, in a multicolor image forming apparatus,
Various proposals are made up of a plurality of developing devices, each of which forms a visible image (toner image) of a different color, and these toner images are finally transferred onto the same transfer sheet in an overlapping manner. Has been done. In that case, the multicolor image forming apparatus is roughly classified into a single drum type and a multi-stage drum type (tandem type).

FIG. 7 is a diagram schematically showing the structure of a main part of a single-drum type multicolor image forming apparatus. As shown in the figure, the multicolor image forming apparatus a includes an initializing charger c along a peripheral surface of a single photosensitive drum b, a recording head d for irradiating a laser beam, and a photosensitive drum b. 4 developing devices e1
e4 is provided, and a paper winding semi-conductive drum f that forms a transfer portion is provided opposite to the photosensitive drum b. Inside the cylinder of the sheet winding semi-conductive drum f, a corona discharger g is disposed in the vicinity of the transfer portion to discharge a transfer current.

The above-mentioned sheet winding semi-conductive drum f winds the transfer sheet carried in from the lower left direction as shown by an arrow m in FIG. 7 in the counterclockwise direction as shown by an arrow h in FIG. And then go around.

The photosensitive drum b is rotated in the clockwise direction as shown by an arrow i in FIG. 7 at the same peripheral speed as the peripheral speed of the paper winding semi-conductive drum f, and first, M (magenta: red). A toner image of (dye) is formed, and this toner image is transferred onto a transfer sheet by the discharge current of the corona discharger g. Next, the paper winding semi-conductive drum f makes one revolution again, and in response thereto, the photoconductor drum b forms a C (cyan: blue with greenish tint) toner image.
Are transferred onto the transfer paper and stacked. Furthermore, the paper winding semi-conductive drum f rotates once again, and in response thereto, the photosensitive drum b forms a Y (yellow: yellow) toner image, which is transferred onto the transfer paper by the corona discharger g. Pile up. Finally, the paper winding semi-conductive drum f makes one revolution again, and in response thereto, the photosensitive drum b forms a Bk (black: black) toner image, which is transferred onto the transfer paper by the corona discharger g. Pile up. Then, when the transfer (overcoating) of the toner images of the four colors is completed, the winding of the transfer sheet is released, and the transfer sheet is conveyed by the conveyor belt j to the fixing unit k arranged on the left side in FIG. Then, the above-mentioned transferred toner images of four colors are heat-fixed on the paper surface.

As described above, the single-drum type multicolor image forming apparatus a is configured such that, for one sheet (one page) of transfer paper, M (magenta) toner and C (cyan) toner, which are the three primary colors of subtractive color mixture. Toner, Y (yellow) toner, and Bk (black) toner, which are exclusively used for printing the black portion, are superimposed on each other, so that each toner is individually printed (exposure recording, developing, and transferring). In this case, the printing process is repeated four times for one page of the transfer paper, and thus the printing process takes a long time.

On the other hand, in the tandem type multicolor image forming apparatus, four kinds of toners are sequentially superposed and transferred onto the paper in one step, so that the processing speed is almost four times that of the single drum type. Have For this reason, in recent years, the tandem-type multicolor image forming apparatus is becoming the mainstream together with the fact that the internal apparatus has been downsized and has been assembled (unitized) to be relatively inexpensive.

FIG. 8 is a cross-sectional view of main parts showing a schematic configuration of a multicolor image forming apparatus to which the tandem system is applied. In FIG. 8, reference numeral 3'denotes a photosensitive drum.
′ Rotates in the direction of the arrow in FIG. 8, and a charging device 4 ′, a developing device 5 ′, and a cleaning device 7 ′ are arranged around it at least in the order of rotation. The charger 4'is the photosensitive drum 3
′ A contact charging member that constitutes a charging device for uniformly charging the surface, and is a conductive elastic member (a rubber material such as EPDM or urethane is used as a base material around a conductive core metal to adjust resistance. It is a cylindrical roller made of a material in which a conductive substance such as carbon black or metal powder is dispersed is integrally molded into a roller shape by die molding or the like, and its resistance is 1
It is about 0 ³ to 10 ⁷ Ω · cm. This charger 4 '
Is in contact with the surface of the photosensitive drum 3'and the photosensitive drum 3 '
It is rotating in the counter direction against. This has the effect of increasing the probability of contact with the photosensitive drum 3 '. The surface of the photoconductor drum 3'between the charger 4'and the developing device 5'is irradiated with laser light from an exposure unit 8'as a writing optical system, and an electrostatic latent image is formed on the photoconductor drum 3 '. To be formed. Then, four image forming stations S1 'to
S4 'is a transfer / transport belt 13' which is a transfer material transport means.
Are juxtaposed along. The transfer / transport belt 13 'is in contact with the photoconductor drum 3'between the developing device 5'and the cleaning device 7'of each of the image forming stations S1'-S4', and the photoconductor drum 3'of the transfer transport belt 13 '. A transfer roller 6'for applying a transfer bias is disposed on the surface (rear surface) which is the back side of the 'side. The image forming stations S1 'to S4' have the same configuration except for the toner color inside the developing device 5 '.

In the multicolor image forming apparatus X'having the structure shown in FIG. 8, the image forming operation is performed as follows. First, in each of the image forming stations S1 'to S4',
The photoconductor drum 3'is charged by the charger 4'rotating in the counter direction, and then an electrostatic latent image corresponding to the image of each color to be formed is formed by the laser beam in the exposure section 8 '. Next, the latent image is developed by the developing device 5'and a toner image is formed. The developing device 5'develops with Bk (black), C (cyan), M (magenta), and Y (yellow) toners, respectively, and each color formed on the four photoconductor drums 3 '. Toner images are superposed on the transfer paper. The transfer paper is sent out from the tray 19 'by a paper feed roller (not shown), and a pair of registration rollers 10',
It is temporarily stopped at 10 ', and is fed to the transfer / conveying belt 13' at the same timing as the image formation on the photosensitive drum 3 '. The transfer paper held on the transfer / transport belt 13 'is transported, and the toner images of the respective colors are transferred at the contact position (transfer portion) with the photosensitive drums 3'. The toner image on the photosensitive drum 3'is transferred onto the transfer paper by the electric field formed by the potential difference between the transfer bias applied to the transfer roller 6'and the photosensitive drum 3 '. Then, the recording paper on which the four color toner images are superposed through the four transfer portions (transfer rollers 6 ') is conveyed to the fixing device 2'and the toner is fixed, and the recording paper is ejected to a paper ejection portion (not shown). To be done. The residual toner remaining on each photoconductor drum 3 ′ without being transferred by the transfer unit is collected by the cleaning device 7 ′. In the example shown in FIG. 8, the image forming stations S1 'to S4' are arranged from the upstream side to the downstream side in the transfer paper conveyance direction from Bk (black), C (cyan), M (magenta), Y (yellow).
Although the colors are arranged in the order of, the color order is not limited to this order, and the color order is arbitrarily set.

As described above, the image forming stations S1 to S4 'have the same structure except for the toner color inside the developing device 5'. Therefore, the rotation speeds of the developing devices 5'of the image forming stations S1 to S4 'are also set to the same rotation speed.

However, in the tandem type multicolor image forming apparatus X ', the unfixed toner temporarily transferred onto the transfer material P'in the image forming station on the upstream side of the developing and forming stations is the image immediately downstream. A reverse transfer phenomenon, which is robbed by the forming station, occurs and a good color image cannot be obtained.

Here, in order to explain the reverse transfer phenomenon,
In the tandem type multicolor image forming apparatus X ′, the first image formed in the developing process of the most upstream image forming station S1 ′.
The toner image of the color is transferred onto the transfer material P ', but the second toner formed in the developing process of the next image forming station S2'.
When the color toner image is transferred onto the transfer material P ′ in an overlapping manner, a phenomenon occurs in which a part of the first color toner image transferred first is reversely transferred to the photosensitive drum 3 ′ side. This also occurs when the toner images of the third and fourth colors are transferred onto the transfer material P '. Therefore, after the transfer of the toner image of the fourth color is completed in the fourth image forming station S4 ′ at the most downstream side, the toner adhesion amount of the toner image of the first color on the transfer material P ′ is the same as the initial adhesion amount. It will decrease by several tens of percent compared to the amount. Such a phenomenon also occurs in the second color toner image and the third color toner image.

Therefore, for example, in the case where the steps of forming and transferring a toner image are performed four times, the final amount of each toner image adhered to the transfer material P'is that the toner image of the first color is the smallest. Therefore, “first color adhesion amount” <“second color adhesion amount” <“third color adhesion amount” <“fourth color adhesion amount”
The color scheme is out of balance. (See Figure 9).

On the other hand, as can be seen from the relationship between the retransfer amount and the image density shown in FIG. 10, the image density decreases as the retransfer amount increases. For this reason, there has been a problem that the color density becomes higher toward the downstream color and a multicolor image is separated from the correct hue.

This is because when the toner adheres to the photoconductor from the developing roller according to the image potential of the photoconductor (drum or belt) in the image forming process, the toner is not always charged with a uniform negative charge. Rather, there is a mixture of those with a weak negative polarity and those with a positive polarity charge on the contrary, and there are those that become positive polarity after discharge occurs in the peeling process from the photoconductor after transfer, It is said that these phenomena are caused by the fact that each time a transfer charge having a positive polarity is received in the subsequent transfer process, the charge gradually returns to the photoconductor.

In order to solve such a problem, in a tandem type multicolor image forming apparatus having a plurality of developing devices, the charging potential of the photoconductor is reduced toward the downstream side, and the developing agent with a low charging amount is used to reduce the charging potential. It has been proposed to suppress the occurrence of an abnormal image such as a reverse transfer phenomenon by transferring with a voltage (see Japanese Patent Application Laid-Open No. 8-106197).

In other tandem type multicolor image forming apparatuses, the contact pressure of the doctor blade in the developing device of each image forming station is set to be gradually strengthened as the contact pressure shifts from the upstream side to the downstream side. In addition to the above, we have proposed a method that controls the amount of toner that adheres to the photoconductor on the upstream side more than it does on the downstream side, and handles the characteristics such as the particle size and fluidity of the toner used in each stage. (Japanese Patent Laid-Open No. 9-319179)
(See Japanese Patent Publication).

[0018]

However, according to the former proposal (Japanese Patent Laid-Open No. 8-106197), the contents are dealt with mainly depending on the characteristics of the toner, and it is troublesome to manage the characteristics of the toner. Not only is it costly, but the target charging potential and toner charge amount are simply changed for each image forming stage such as each image forming station. Therefore, from the perspective of the entire system, there is a high tendency to generate abnormal images related to the transfer process. Becomes

On the other hand, the latter proposal described above (Japanese Patent Laid-Open No. 9-319)
No. 179), the toner adhesion amount control is performed in advance so that the toner adhesion amount to the photoconductor is increased toward the upstream image forming stage in anticipation of a reverse transfer phenomenon. There is a drawback that it is likely to lead to an increase in

The present invention has been made in view of the above points, and an object of the present invention is to avoid the dependence on toner characteristics and the increase in toner consumption, and at the same time, the toner reverse transfer phenomenon in a plurality of transfer steps. It is an object of the present invention to provide a multi-color image forming apparatus capable of suppressing the above-mentioned problem and efficiently forming a final image having excellent color reproducibility.

[0021]

In order to achieve the above object, in the present invention, an image forming stage centered on a plurality of photoconductors arranged in parallel along the transfer material conveying direction and a transfer material on the outer peripheral surface. And a transfer material conveying belt that circulates so as to bring the transfer material into contact with at least one of the plurality of photoconductors, and a transfer unit corresponding to the plurality of photoconductors inside the conveyance belt. A plurality of transfer means for transferring a toner image to the transfer material, which is arranged to be configured and is in contact with the photoconductor, is provided, and the toner color images formed at each image forming stage are sequentially transferred in multiplex. Assuming a multicolor image forming apparatus for forming a multicolor image, the developing means of each image forming stage is
Development is performed by applying a developing bias voltage in which an AC voltage is superimposed on a DC voltage. Then, the duty ratio of the developing bias voltage is gradually changed according to the order of the toner transfer order.

According to this specific matter, the optimum toner adhesion amount is adjusted by changing the duty ratio of the developing bias voltage applied to the developing device of each image forming stage. That is, in order to change the amount of toner finally transferred to the paper in consideration of the density decrease due to the reverse transfer phenomenon of the toner color in the upstream image forming stage that occurs in the image forming station on the downstream side, each image forming By adjusting the amount of the developer supplied to the developing unit for each stage, the toner adhesion amount at each image forming stage is optimized.

In that case, if the voltage of the DC component of the developing bias is changed for density adjustment, the adjustment range becomes narrower,
The balance of Vd (dark part potential), Vl (bright part potential), the negative side peak of Vp-p, and the positive side peak of Vp-p is easily lost, and the image quality becomes unstable, but the duty ratio of the AC component changes. By doing so, the adjustment range is wide, and it is possible to stably obtain an image with an appropriate density.

Further, although the image forming stage on the downstream side is less susceptible to the reverse transfer phenomenon, a good color balance can be obtained by sequentially changing the amount of developer adhered to the photoconductor from the image forming stage arranged on the upstream side. It is possible to obtain a multicolor image of.

Here, when the duty ratio of the developing bias voltage applied to the developing device of each image forming stage in the developing promotion direction is changed to gradually decrease in accordance with the toner transfer order, the upstream side image is The amount of toner adhered to the photoconductor at the forming stage becomes larger than the amount of toner adhered at the downstream side image forming stage, and it is possible to suppress the image density reduction related to the toner color of the upstream side image forming stage due to the reverse transfer phenomenon. Become.

At this time, the lower the image forming stage on the downstream side, the less likely it is to be affected by the reverse transfer phenomenon. However, by decreasing the amount of developer adhered to the photoconductor in order from the image forming stage on the upstream side, good color can be obtained. It is possible to obtain a balanced multicolor image.

The duty ratio of the developing bias voltage is fixed to the frequency f or the amplitude Vp-p of the AC component of the developing bias voltage, and the pulse widths of the developing bias voltage component in the developing acceleration direction and the developing bias voltage component. If the frequency f is changed by changing the frequency f independently, the positive and negative pulse widths are changed independently or the peak voltage Vp-p and the positive and negative peak voltages are changed. By keeping the above constant and independently changing the positive and negative pulse widths, the control range of the developing bias voltage can be easily widened.

Further, when a pulse width control circuit for variably setting the duty ratio of the developing bias voltage is provided,
Although it is necessary to set the developing bias voltage individually in each image forming stage in order to provide a difference in the amount of toner adhered to the photoconductor in each image forming stage, by providing a pulse width control circuit It is possible to smoothly variably control the duty ratio of the bias.

[0029]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows a main part of a multicolor image forming apparatus according to an embodiment of the present invention. Inside the multicolor image forming apparatus X, a transfer / conveying belt mechanism 1 is provided. The transfer / conveying belt mechanism 1 includes a drive roller 11 rotatably supported on one side (left side in FIG. 1) and a driven roller 12 rotatably supported on the other side (right side in FIG. 1). , An endless transfer / transport belt 13 as a transfer material transport belt which is stretched between both rollers 11 and 12 and is driven in the direction of arrow Z shown in FIG. 1, and is transferred onto the surface of the transfer / transport belt 13. By electrostatically adsorbing the recording medium P as a material, the recording medium P supplied from the registration rollers 10, 10 is conveyed from the upstream side (right side in FIG. 1) to the downstream side (left side in FIG. 1). ing. A fixing device 2 is provided on the downstream side (left side in FIG. 1) of the transfer paper carrying belt mechanism 1 in the conveyance direction of the recording medium P, and the toner image transferred and formed on the recording medium P is formed on the recording medium P by the fixing device 2. It is fixed in. The fixing device 2 includes a thermal heat roller 21 as a fixing member and a pressure roller 22 on the upper and lower sides, and heats the recording medium P transported on the transfer paper transport belt mechanism 1 (transfer transport belt 13) to the thermal heat roller 21. The pressure roller 22 and the pressure roller 22 are passed through the nip.

Above the transfer / transport belt mechanism 1,
A first image forming station S1 as an image forming stage,
The second image forming station S2, the third image forming station S3, and the fourth image forming station S4 are arranged close to the transfer / conveying belt 13 at predetermined intervals from the upstream side (right side in FIG. 1) of the recording medium conveying path. Are installed side by side. In this case, the recording medium P on the transfer / transport belt 13 is the first image forming station S1, the second image forming station S2, and the third image forming station S.
It is sequentially conveyed to the third and fourth image forming stations S4.

Each of the image forming stations S1 to S4 has substantially the same structure, and is provided with a photoconductor drum 3 as a photoconductor that rotates in the arrow F direction shown in FIG. The periphery of each of the photosensitive drums 3 is
A charger 4 that charges each photoconductor drum 3 and forms an electrostatic latent image on the outer peripheral surface of each photoconductor drum 3, and a powder of the electrostatic latent image formed on the outer peripheral surface of the photoconductor drum 3. A developing device 5 as a developing unit that develops a visible image with toner T as a body (shown in FIG. 2), and a toner image (visible image) developed on the outer peripheral surface of the photosensitive drum 3 as a recording medium P. A transfer discharger 6 as a transfer unit that transfers the toner to the surface of the photosensitive drum 3 and a cleaning device 7 for removing the toner T remaining on the outer peripheral surface of the photosensitive drum 3 are arranged along the rotation direction of the photosensitive drum 3 (direction of arrow F). They are provided in sequence.

An exposure means 8 is provided above each photosensitive drum 3. The exposure means 8 is a writing means, and writes an image on the surface of the charged photosensitive drum 3 based on image information, for example, with light of a laser or the like. As a result, an electrostatic latent image is formed on the photosensitive drum 3.

A pixel signal corresponding to the black component image of the color original image is input to the exposing means 8 of the first image forming station S1 located on the most upstream side of the transfer / conveying belt 13 in the conveying direction, and the next second The pixel signal corresponding to the yellow color component image of the color original image is input to the exposing means 8 of the image forming station S2, and the magenta color of the color original image is input to the exposing means 8 of the next third image forming station S3. The pixel signal corresponding to the component image is input, and the pixel signal corresponding to the cyan component image of the color original image is input to the exposure unit 8 of the fourth image forming station S4 located on the most downstream side. ing. This allows
An electrostatic latent image corresponding to the color-converted document image information is formed on the outer peripheral surface of each photoconductor drum 3.

The developing device 5 of the first image forming station S1 contains the black toner T, the developing device 5 of the second image forming station S2 contains the cyan toner T, and the third image. The developing device 5 of the forming station S3 stores magenta toner T, and further,
The yellow toner T is stored in the developing device 5 of the image forming station S4. The electrostatic latent image on the outer peripheral surface of each photoconductor drum 3 is developed into a visible image by the toner T of each color, whereby the document image information is reproduced as a toner image by the toner T of each color. ing.

A recording medium suction charger 15 is provided between the first image forming station S1 and the transfer / transport belt 13. The charging device 15 for attracting the recording medium charges the surface of the transfer / conveyance belt 13, and is supplied from a paper feeding cassette 19 provided in the lower portion of the image forming apparatus X by a paper feeding mechanism (not shown). By reliably adsorbing the recording medium P on the transfer / transport belt 13,
The recording medium P is conveyed between the first image forming station S1 and the fourth image forming station S4 without being displaced.

On the other hand, between the fourth image forming station S4 and the fixing device 2, a discharger 16 for static elimination is provided almost directly above the drive roller 11. An AC current for separating the recording medium P electrostatically adsorbed on the transfer / transport belt 13 from the transfer / transport belt 13 is applied to the discharging / discharger 16.

Then, when the recording medium P is sent out from a sheet feeding cassette (not shown) and supplied into the guide of the recording medium conveying path of the sheet feeding mechanism, the leading end portion of the recording medium P is a sensor (not shown). Is detected by the pair of registration rollers 1 based on a detection signal output from the sensor.
It is temporarily stopped by 0, 10. Then, the recording medium P
Is transferred onto the transfer / conveying belt 13 rotating in the arrow Z direction in FIG. 1 at a timing with each of the image forming stations S1 to S4. At this time, the transfer conveyance belt 1
The recording medium P is stably conveyed and supplied while passing through the image forming stations S <b> 1 to S <b> 4 because the recording medium adsorption charger 15 has given a predetermined charge to the recording medium 3.

In each of the image forming stations S1 to S4, a toner image of each color is formed, and the image forming stations S1 to S4 are formed on the supporting surface of the recording medium P that is electrostatically attracted and conveyed by the transfer conveying belt 13. Toner images are superimposed and the images are transferred. Then, when the transfer of the image by the fourth image forming station S4 is completed, the recording medium P is sequentially peeled from the transfer conveyor belt 13 by the negative discharger 16 from the leading end portion thereof, and is guided to the fixing device 2. . The recording medium P on which the toner image is fixed by the fixing device 2 is discharged to the outside of the image forming apparatus X by a discharge roller (not shown).

The developing device 5 of each of the image forming stations S1 to S4 is of a one-component developing type and, as shown in FIG. 2, a supply roller for supplying the toner T from the toner containing portion 51 to the developing roller 52. 53, and a blade 54 that presses against the developing roller 52 and uniformizes the thickness of the toner layer on the developing roller 52 that is conveyed by the rotation of the developing roller 52 to the developing area V that is a portion facing the photosensitive drum 3. I have it.

The developing roller 52 faces the surface of the photosensitive drum 3 with a predetermined gap therebetween and performs non-contact development, and is rotationally driven in the arrow direction (counterclockwise direction) shown in FIG. Further, an appropriate developing bias voltage such as direct current, alternating current, alternating current of direct current superposition, pulse voltage, etc. is applied to the developing roller 52. In this case, in order to stabilize the flying of the toner in the non-contact development, a voltage having an alternating component (AC, AC with superimposed DC, or pulse voltage) is applied.

The supply roller 53 can hold the toner T inside the vicinity of the surface, and at least the material of the surface is
The one that can contact the developing roller 52 and give desired triboelectric charging to the toner T and the developing roller 52 is adopted. Further, the resistance value is such that the counter charge in triboelectrification is not accumulated, for example, 1 × 10 6 Ωc
Those having a resistance value of m or less are preferable. For example, an elastic roller such as a carbon kneading foamed polyurethane sponge roller having a volume resistance of about 1 × 10 6 Ωcm, or a fur brush roller having many fur brushes planted therein is used. Then, the supply roller 53 is rotated so that the surface thereof has a peripheral speed difference from the surface of the developing roller 52 at the contact portion W with the developing roller 52. The rotation direction of the supply roller 53 may be counterclockwise as in the example shown in FIG. 1 or may be clockwise. In addition, the same voltage as that applied to the developing roller 52 is applied to the core metal of the supply roller 53, or the potential difference which receives an electrostatic force such that the charged toner T on the supply roller 53 is transferred to the developing roller 52 side is the development. Roller 52
It is also possible to apply a voltage such that it is formed between In addition,
Linear velocity ratio between the supply roller 53 and the developing roller 52 and both rollers 5
The potential difference between 3, 52 affects the amount of toner supplied to the developing roller 52, and is set so that a desired toner adhesion amount can be obtained on the developing roller 52.

The blade 54 is arranged so as to contact the developing roller 52 with a predetermined linear pressure. It is also possible to apply a voltage to the blade 54, and this applied voltage also becomes a factor that influences the charge amount of the passing toner T. Also,
Since the adhesion force to the surface of the developing roller 52 varies depending on the charge amount of the toner T, it becomes a factor that influences the amount of adhered toner on the developing roller 52 passing through the blade 54.

In the above structure, the toner T in the toner containing portion 51 adheres to the surface of the supply roller 53, and the rotation of the supply roller 53 causes the supply roller 53 and the developing roller 52 to rotate.
It is conveyed toward the contact portion W with. Meanwhile, the supply roller 5
The surface portion of the developing roller 52 that has passed through the developing area due to the rotation of the developing roller 52 in the direction of the arrow (counterclockwise) shown in FIG.

At the contact portion W, the surface of the supply roller 53 and the surface of the developing roller 52 move with a relative speed difference, so that they do not adhere to the surface of the photosensitive drum 3 when passing through the developing area and remain on the surface of the developing roller 52. The residual toner T in the non-image portion is mechanically and electrically scraped by the supply roller 53 (for example, a sponge roller), and the charge on the developing roller 52 is also made constant by the frictional charging by the supply roller 53. As a result, the surface of the developing roller 52 is initialized. Then, due to the friction between the developing roller 52, the toner T, and the supply roller 53, the dielectric portion of the developing roller 52 is charged with a polarity opposite to the desired toner charging polarity (normal development (P
/ P) has the same polarity as the photoconductor charge, and reverse development (N / P) has the opposite polarity to the photoconductor charge) to give a microfield (closed electric field) on the developing roller 52. Are formed.

On the other hand, the toner T attached to the supply roller 53 is slid between the developing roller 52 and the supply roller 53,
Most of them are charged to a desired polarity (in normal development, the polarity is opposite to that of the photoreceptor charge, and in reversal development, it is the same polarity as that of the photoreceptor charge). Then, the charged toner T on the supply roller 53 is electrostatically attracted by the electric field of the microfield on the developing roller 52, and adheres to the surface of the developing roller 52 in multiple layers. As a result, the developing roller 52 holds the sufficiently charged toner T in a multi-layered state in the contact portion W.
To leave.

The toner layer on the developing roller 52 that has passed the contact portion W is in contact with the blade 5 on the developing roller 52.
In step 4, the thickness is evenly rubbed, and unevenness at the time of rubbing at the contact portion W with the supply roller 53 is eliminated. Then, the developing roller 52 rotates to convey the developing roller 52 to the developing area. In this developing area, development is performed while the surface of the developing roller 52 to which the optimum developing bias is applied by the contact or non-contact developing method and the surface of the photosensitive drum 3 move at a substantially constant speed. In this developing area, the conductor portion of the developing roller 52 exerts an electrode effect, so that the toner T on the developing roller 52 is transferred to the photosensitive drum 3.
An electric field is also formed that tends to adhere to the.

As a characteristic part of the present invention, the duty ratio D of the developing bias voltage applied to each developing roller 52 in the developing device 5 of each image forming station S1 to S4 will be described.

As shown in FIG. 3A, the duty ratio D is the voltage waveform of the developing bias voltage (rectangular voltage waveform) from when the voltage waveform starts to rise to the positive side until it falls to the negative side. Where A is the duration (pulse width) and B is the duration (pulse width) from when the voltage waveform starts to fall on the negative pole side to when it rises on the positive pole side, it is expressed by the following relational expression. is there.

Va component (positive electrode side) duty ratio D = A / (A + B) Vb component (negative electrode side) duty ratio D = B / (A + B) The frequency f or the amplitude Vp-p can be changed by changing the duty ratio. It is done on the precondition that it remains constant without change.

Further, as shown in FIG. 3B, when the voltage waveform of the developing bias voltage is a waveform other than the rectangular wave (non-rectangular voltage waveform), the above-mentioned duration A (pulse width) As for the duration B (pulse width), the starting point when the voltage switches from the negative polarity to the positive polarity, the so-called rising start point is a, the starting point when the voltage switches from the positive polarity to the negative polarity, the so-called falling start point. Let b be
The duration A is a time between a and b, and the duration B is a time between b and a.

Next, the relationship between the duty ratio and the image density will be described.

Regarding the relationship between the minus side (development acceleration direction) peak Vb of Vp-p and the bright portion potential Vl, the reproducibility of the image portion is higher as the electric field strength is higher. FIG. 4 shows the relationship between the duty ratio of the negative peak Vb and the image density.

The duty ratio of the negative peak Vb,
Image density means that as the duty ratio of the negative peak Vb increases, the image density increases and the duty ratio becomes 6
The image density is saturated at about 0%. Conversely, the positive side of Vp-p (development suppressing direction) peak voltage V
In the relationship of the duty ratio of a, the plus-side peak voltage Va
Negative duty peak V as the duty ratio of
Since the duty ratio of b decreases, the image density decreases.

As shown in FIG. 5, the control device 9 as a pulse width control circuit for controlling the peripheral speed of each developing roller 52 includes a rotary drive device 91 for rotationally driving the developing roller 52 and the developing roller 52. Rotation control device 9 for drive control
2, a bias power source 93 for the developing roller 52, and a bias control device 94 for controlling the developing bias voltage.

The rotation control device 92 is the rotation drive device 9
Developing roller 5 with respect to the peripheral speed of the photosensitive drum 3
Control the peripheral speed of 2.

Further, on the operation panel surface Xa of the main body of the multicolor image forming apparatus X, an image forming mode (full color image formation or monochrome image formation) is input through a push button or the like which can be operated by the user. Mode input device 95
And a main control unit 9 including a CPU and a memory
6 and the I / O unit 97 are provided at appropriate places.

In the above structure, when full-color image formation is executed, the user operates the mode input device 95 to select the full-color image formation mode. The signal of the setting data corresponding to this image forming mode is I / O.
It is input to the main control unit 96 via the unit 97. Then, the rotation control device 9 is transferred from the main control unit 96 via the I / O unit 97.
A control signal corresponding to the full-color image forming mode is sent to 2 to set the driving condition and the image forming condition of the rotation driving device 91.

Here, an example of setting the duty ratio will be described.

A bias power source 93 for the developing roller 52.
Is configured so that an AC voltage, a pulse voltage, etc. on which a DC voltage is superimposed can be freely applied to the developing roller 52. The bias control device 94 also functions as a developing bias voltage control unit that controls the type of bias voltage such as DC voltage and AC voltage applied to the developing roller 52. Further, the bias control device 94 applies the voltage Va for generating the electric field in the direction in which the toner T is pulled back to the developing roller 52 side in one cycle of the bias voltage, and the toner T moves to the photosensitive drum 3 side. The ratio of the voltage Vb for generating the electric field in the direction to the application time B can be independently controlled. In this case, in FIG. 6, the application time A of the voltage Va on the development suppressing side and the application time B of the voltage Vb on the development promoting side are represented by an integer ratio.
And the application time B of the voltage Vb on the development promoting side are not necessarily limited to the integer ratio.

As described above, each image forming station S
Since the application time A of the potential on the development suppressing side and the application time B of the potential on the development promoting side are independently controlled and applied to the developing rollers 52 of 1 to S4, the image forming stations S1 to S4. In S4, a difference in the time during which development is accelerated occurs (at the same time, a difference in the time during which development is suppressed also occurs), and due to this difference in time, the photoconductors of the image forming stations S1 to S4 are exposed. The amount of toner adhered to the drum 3 is controlled, and as shown by a chain double-dashed line in FIG. 9, it is possible to suppress a decrease in the image density related to the toner color of the upstream image forming station due to the occurrence of the reverse transfer phenomenon in the downstream image forming station. You can

Further, the change of the duty ratio of the developing bias voltage fixes both the frequency f and the amplitude Vp-p of the AC component of the developing bias voltage, and the application time A of the developing bias voltage Va on the development suppressing side. Since the ratio of the developing bias voltage to the application time B of the voltage Vb on the development promoting side is independently controlled, the control range of the developing bias voltage can be easily widened.

Further, since the controller 9 for variably setting the duty ratio of the developing bias voltage is provided, the duty ratio of the developing bias of each of the image forming stations S1 to S4 can be easily variably controlled, which is very practical. Will be advantageous.

[0064]

As described above, according to the present invention, the following effects are exhibited. First, by gradually changing the duty ratio of the developing bias voltage applied to the developing unit of each image forming stage according to the order of the toner transfer order,
By optimizing the toner adhesion amount at each image forming stage to stably obtain an image having an appropriate density, it is possible to obtain a multicolor image having a good color balance.

Here, by changing the duty ratio of the developing bias voltage for the developing device of each image forming stage in the developing promotion direction to the stepwise decreasing direction according to the toner transfer order, the upstream side image due to the reverse transfer phenomenon is changed. It is possible to suppress a decrease in image density relating to the toner color in the formation stage, and it is possible to obtain a multicolor image having good color balance.

Further, the duty ratio of the developing bias voltage is changed by fixing the frequency f or the amplitude Vp-p of the AC component of the developing bias voltage, and by changing the developing bias voltage component in the developing promotion direction and the developing suppression component. By independently controlling the pulse width, it is possible to easily widen the control range of the developing bias voltage.

Further, by providing a pulse width control circuit for variably setting the duty ratio of the developing bias voltage, the duty ratio of the developing bias of each image forming stage can be smoothly variably controlled, which is very advantageous for implementation. Becomes

[Brief description of drawings]

FIG. 1 is a sectional view of a main part of a multicolor image forming apparatus according to an exemplary embodiment of the present invention as seen from the front.

FIG. 2 is a cross-sectional view showing configurations of a photosensitive drum and a developing device.

FIG. 3A is a characteristic diagram showing a characteristic of a rectangular voltage waveform of a developing bias voltage with respect to time. (B) is a characteristic diagram showing characteristics of a non-rectangular voltage waveform of the developing bias voltage with respect to time.

FIG. 4 is a characteristic diagram showing characteristics of image density with respect to a duty ratio of a negative peak.

FIG. 5 is a block diagram showing a configuration of a control device.

FIG. 6 is an explanatory diagram showing a setting example of a time ratio and a duty ratio for each toner color in each image forming station.

FIG. 7 is a cross-sectional view of a main portion of a single-drum type multicolor image forming apparatus according to a conventional example as viewed from the front.

FIG. 8 is a sectional view of a main part of a tandem-type multicolor image forming apparatus according to another conventional example, as viewed from the front.

FIG. 9 is a characteristic diagram showing a characteristic of image density with respect to a toner layer according to each image forming stage.

FIG. 10 is a characteristic diagram showing a characteristic of image density with respect to a retransfer amount.

[Explanation of symbols]

13 Transfer Conveying Belt (Transfer Material Conveying Belt) 3 Photoreceptor Drum (Photoreceptor) 5 Developing Device (Developing Means) 6 Transfer Discharger (Transfer Means) D Control Device (Pulse Width Control Circuit) S1 to S4 1st to 1st Fourth image forming station (image forming stage) P recording medium (transfer material) X image forming apparatus (multicolor image forming apparatus)

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Takayuki Yamanaka             22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka             Inside the company (72) Inventor Kaori Takeshita             22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka             Inside the company (72) Inventor Takashi Kitagawa             22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka             Inside the company F term (reference) 2H073 AA02 AA05 BA04 BA13 BA21                       CA02 CA14 CA22                 2H300 EA12 EB04 EB12 ED06 EF02                       EF08 EF13 EG03 EH16 EJ09                       EJ27 EJ31 EJ37 EJ39 EJ46                       EJ47 EJ58 FF05 GG01 GG02                       GG03 GG12 GG30 GG33 GG34                       GG41 GG48 HH23 KK07 MM25                       PP06 QQ09 QQ25 RR34 TT01                       TT02 TT03 TT05

Claims (4)

[Claims] 1. An image forming stage centered on a plurality of photoconductors arranged in parallel along a transfer material conveying direction, and at least one of the plurality of photoconductors by adsorbing a transfer material on an outer peripheral surface. A transfer material conveying belt that circulates so as to bring the transfer material into contact with the transfer material, and a transfer portion that is arranged inside the transfer belt so as to correspond to the plurality of photoconductors and that is in contact with the photoconductor. In a multicolor image forming apparatus, which comprises a plurality of transfer means for transferring a toner image to the transfer material, and sequentially multiplies the toner color images formed at the respective image forming stages to form a multicolor image, Development is performed by applying a developing bias voltage in which an AC voltage is superimposed on a DC voltage to the developing unit at the forming stage, and the duty ratio of the developing bias voltage is gradually changed according to the order of the toner transfer order. To Multi-color image forming apparatus, characterized in that by becoming. 2. The multicolor image forming apparatus according to claim 1, wherein the duty ratio of the component of the developing bias voltage in the development promoting direction is gradually reduced according to the toner transfer order. Color image forming apparatus. 3. The multicolor image forming apparatus according to claim 1, wherein the duty ratio of the developing bias voltage is such that the frequency f or the amplitude Vp-p of the AC component of the developing bias voltage is fixed and the developing bias voltage A multicolor image forming apparatus, which is configured to be changed by independently controlling pulse widths of a component in a development promoting direction and a component in a development suppressing direction. 4. The multicolor image forming apparatus according to claim 3, further comprising a pulse width control circuit for variably setting a duty ratio of a developing bias voltage.