JP5659699B2 - Image forming apparatus - Google Patents

Description

The present invention relates to an image forming apparatus, and more particularly, relates superimposed bias control using a DC Ingredient and an AC component.

As is well known, in an image forming apparatus such as a copying machine, a printer, or a printing machine, an electrostatic latent image formed on a photosensitive member, which is a latent image carrier, is visualized by a developer supplied from a developing device. It is processed.
The visible image-processed toner image is transferred to a transfer material, and is then fixed by heating and pressurizing with a fixing device so that the toner melts and permeates to form a copy image.

  Some image forming apparatuses are configured to form not only a single color but also a plurality of colors such as full colors. In this case, images of different colors formed in a plurality of image forming units are belted. In some cases, a superimposed image is transferred onto a recording medium such as a recording sheet (secondary transfer) after being sequentially transferred (primary transfer) to an intermediate transfer body such as the above.

By the way, plain paper is often used as a recording medium for image transfer, but in recent years, a wide variety of paper has been used, and among them, the image of a leather pattern with a high-class feel or Japanese paper tone. Are commercially available, and it is possible to form printed materials having various expressions.
These recording papers used as printed materials having various expressions have irregularities on the surface due to embossing or the like in order to give a high-class feeling.

  The concave portion is less liable to transfer the toner than the convex portion, and particularly when the toner is transferred onto a recording paper having large irregularities, there is a problem that the toner is not sufficiently transferred to the concave portion and an image is lost.

Thus, the following is known as a method for improving transfer rate for concave portions and preventing transfer failure such as image omission (Patent Document 1).
In Patent Document 1, immediately before the transfer of the toner image onto the recording paper, the recording paper is heated and charged to a polarity opposite to that of the toner to increase the transfer electric field at the time of transfer, so that There is disclosed a method in which the is transferred.
However, in this method, it has been confirmed through experiments and the like that a sufficient transferability cannot be obtained in the uneven portion with large unevenness.

As a method for solving such a problem, a method has been proposed in which an alternating voltage is superimposed on a direct current voltage so as to improve the transfer rate in the concave portion and prevent a void from occurring by applying an electrostatic force to the toner and a vibrating action. (For example, Patent Documents 2 to 5).
Patent Document 2 uses a transfer voltage in which an AC voltage is superimposed on a DC voltage, and the toner is transferred to the concave portion by charging the surface of the recording paper to a polarity opposite to the polarity of the toner according to the unevenness before transfer. A method of performing control so as to be performed is disclosed.
Patent Document 3 uses a method in which an AC voltage is superimposed on a DC voltage as a transfer bias, and the AC voltage is superimposed so that the peak-to-peak voltage of the AC voltage is twice or less the DC voltage. Is disclosed.
In Patent Document 4, a fluororesin is used on the surface of an intermediate transfer member, and a DC voltage is superimposed on an AC voltage as a transfer bias. The peak-to-peak voltage of the AC voltage is 2.05 times or more of the DC voltage. Thus, a method characterized by superimposing an alternating voltage is disclosed.
Patent Document 5 uses a transfer voltage in which an AC voltage is superimposed on a DC voltage, and the AC voltage is superimposed so that the frequency of the AC voltage is 4 kHz or less and the number of cycles in the nip is 20 times or more. A featured method is disclosed.

  However, in the methods disclosed in these patent documents, the superimposed AC voltage is small, and on the paper with large unevenness, even if the voltage is applied as in the embodiment, the toner is not transferred much to the concave portion, and the effect is effective. The low was confirmed by the experimenter.

  Therefore, according to the experiments of the present inventors, a transfer bias in which an AC voltage is superimposed on a DC voltage is used, and the peak-to-peak value of the AC voltage is set to a value larger than four times the absolute value of the DC voltage. It has been found that the transferability of the uneven paper can be improved.

  However, it has been found that this method has a new problem that transfer dust tends to occur when a line or character is transferred in the concave portion of the uneven paper.

  The object of the present invention is to improve the image quality even when the surface irregularities are large in view of the above-described conventional image forming apparatus, particularly the problem of using a transfer bias in which an AC voltage component is superimposed on a DC voltage component. An object of the present invention is to provide an image forming apparatus having a transferable structure.

In order to achieve this object, the present invention has the following configuration.
(1) In an image forming apparatus equipped with a transfer device that transfers an image of electrostatic toner from an image carrier to a recording material at a transfer nip , when the recording material is an uneven recording material having unevenness formed on the surface , direct current A superimposing bias in which an alternating current component is superimposed on a component and its polarity is switched alternately, and a time-averaged polarity of the background color toner charged to a predetermined polarity is transferred from the recording material to the image carrier in the transfer nip. After the image of the base color toner is transferred to the surface of the recording material by a superimposing bias that forms an electric field in the direction of electrostatic movement, the toner whose time average polarity is charged to the predetermined polarity the transfer bias is polar to form the direction of the electric field electrostatically moving to the recording material from said image bearing member at the transfer nip, the image by the toner to the surface of the recording medium Image forming apparatus characterized by transferring.

(2) The image forming apparatus according to (1) , wherein the image carrier is an intermediate transfer member that temporarily holds the base color toner and the toner, respectively .

(3) the absolute value of the charge quantity of the background color toner, the image forming apparatus according to and smaller than the absolute value of the charge quantity of the toner (1) or (2).

( 4 ) The image forming apparatus according to any one of (1) to ( 3 ) , wherein when the image is transferred with the toner, the transfer bias includes only a DC bias .

( 5 ) The image forming apparatus according to any one of (1) to ( 4 ), wherein a toner of a color dedicated to a base is used as the base color toner .

( 6 ) The image forming apparatus according to any one of (1) to ( 5 ), wherein a clear toner (colorless transparent toner) is used as the base color toner .

( 7 ) The superimposed bias is a duty ratio of one cycle of the AC component (the voltage value of the superimposed bias is less than the time average value of the background color toner from the recording material to the image carrier in the transfer nip. And the time to become the voltage value on the side to be electrostatically moved) / (time of one cycle) × 100) is 50 (%), and the peak-to-peak voltage of the AC component is the absolute value of the voltage of the DC component. The image forming apparatus according to any one of (1) to ( 6 ), wherein the image forming apparatus is larger than four times .

  According to the present invention, the toner is transferred by the superimposed bias to the concavo-convex surface of the recording material before the image transfer, thereby smoothing the concavo-convex surface by the background transfer, and the surface of the smooth surface. Since an arbitrary image can be transferred, it is possible to prevent a transfer defect to the recess by filling the groove to be the recess.

1 is a schematic diagram for explaining a configuration of an image forming apparatus according to a first embodiment of the present invention. FIG. 2 is a schematic diagram for explaining a configuration of an image forming unit used in the image forming apparatus shown in FIG. 1. It is a timing chart which shows an example of the transfer bias used for the Example of this invention. FIG. 2 is a diagram illustrating an image example according to an example of a transfer bias used in the image forming apparatus illustrated in FIG. 1. FIG. 5 is a diagram illustrating an example of an imager with a transfer bias different from the example of the image illustrated in FIG. 4. FIG. 6 is a diagram illustrating an image example with a transfer bias different from the image example illustrated in FIG. 5. It is a figure which shows the example of an image by the transfer bias different from the example of an image shown in FIG. It is a figure which shows the example of an image by the transfer bias different from the example of an image shown in FIG. FIG. 5 is a cross-sectional view illustrating a toner transfer state on an uneven surface by applying a transfer bias illustrated in FIG. 4. FIG. 6 is a cross-sectional view illustrating a toner transfer state on an uneven surface by applying a transfer bias illustrated in FIG. 5. FIG. 7 is a cross-sectional view illustrating a toner transfer state on an uneven surface by applying a transfer bias illustrated in FIG. 6. FIG. 8 is a cross-sectional view illustrating a toner transfer state on an uneven surface by applying a transfer bias illustrated in FIG. 7. FIG. 9 is a cross-sectional view illustrating a toner transfer state on an uneven surface by applying a transfer bias illustrated in FIG. 8. It is a schematic diagram for demonstrating the structure of the image forming apparatus by 2nd Example of this invention. It is a schematic diagram for demonstrating the structure of the image forming apparatus by 3rd Example of this invention. It is a schematic diagram for demonstrating the structure of the image forming apparatus by 4th Example of this invention. It is a table | surface figure which shows the experimental condition (transfer bias value) regarding the transfer state by the 1st Example of this invention, and the generation | occurrence | production state of a fine line dust, and an experimental result.

Embodiments of the present invention will be described below with reference to examples shown in the drawings.
FIG. 1 is a schematic diagram for explaining the configuration of the image forming apparatus according to the first embodiment of the present invention. The feature of the present invention is that the recording material is an uneven recording material having unevenness formed on the surface. Before transferring the image, the toner is transferred by the superimposed bias to the uneven surface of the recording material, and an arbitrary image is transferred to the surface.
The following is a description of the configuration of the embodiment including this feature together with the operation.
In FIG. 1, the printer according to the first embodiment uses each color of colorless and transparent clear (CL) as a fifth color in addition to four colors of yellow (Y), magenta (M), cyan (C), and black (K). It has a unit for output. In the following description, toner used for visible image processing of a black image is expressed as K toner.

In the present invention, the base color toner is transferred onto the recording material before any image output. However, the fifth color is not necessarily required, and the base may be formed using four basic colors.
However, by arranging an appropriate color as the background color such as white or clear for the fifth color, an effect of making the background inconspicuous can be obtained.

The image forming order includes five image forming units 1CL, Y, M, C, and K, a transfer unit 30 as a transfer device, an optical writing unit 80, a fixing device 90, a paper feed cassette 100, and a pair of registration rollers. 101.
The five image forming units 1CL, Y, M, C, and K use toners of different colors as image forming materials, but the other configurations are the same, and are replaced when the lifetime is reached.

Taking the image forming unit 1K for forming a K toner image as an example, the configuration is shown in FIG.
In FIG. 2, the image forming unit 1K includes a drum-shaped photosensitive member 2K as a latent image carrier, a drum cleaning device 3K, a charge eliminating device (not shown), a charging device 6K, a developing device 8K, and the like. These devices are held by a common holding body and integrally attached to and detached from the printer main body, so that they can be exchanged at the same time.

  The photoreceptor 2K has a drum shape having an outer diameter of about 60 (mm) in which an organic photosensitive layer is formed on the surface of a drum base, and is rotated in a clockwise direction in the drawing by a driving unit (not shown).

The charging device 6K generates a discharge between the charging roller 7K and the photosensitive member 2K while bringing the charging roller 7K to which a charging bias is applied into contact with or in proximity to the photosensitive member 2K, thereby making the surface of the photosensitive member 2K uniform. It is charged like this.
In this embodiment, the toner is uniformly charged to the same negative polarity as the normal charging polarity of the toner. As the charging bias, one in which an AC voltage is superimposed on a DC voltage is employed.

  The charging roller 7K is formed by coating a metal cored bar with a conductive elastic layer made of a conductive elastic material. Instead of a method in which a charging member such as a charging roller is brought into contact with or close to the photoreceptor 2K, a method using a charging charger may be adopted.

The surface of the uniformly charged photoreceptor 2K is optically scanned by a laser beam emitted from an optical writing unit described later and carries a K electrostatic latent image.
The electrostatic latent image for K is developed by a developing device 8K using K toner (not shown) to become a K toner image. The toner image processed with the K toner is subjected to a secondary transfer process using a secondary transfer nip forming roller 33 used in a primary transfer process using an intermediate transfer belt 31 and a secondary transfer process described later. Transferred onto the recording material P.

The drum cleaning device 3K removes the transfer residual toner attached to the surface of the photoreceptor 2K after the transfer process.
The drum cleaning device 3K is provided with a cleaning brush roller 4K that is rotationally driven, a cleaning blade 5K that abuts the free end of the drum 2K in a cantilevered state, and the like.
In this configuration, the transfer residual toner is scraped off from the surface of the photoreceptor 2K by the rotating cleaning brush roller 4K, and the transfer residual toner is scraped off from the surface of the photoreceptor 2K by the cleaning blade. The cleaning blade is in contact with the photosensitive member 2K in the counter direction in which the cantilevered support end side is directed downstream of the free end side in the drum rotation direction.

  The static eliminator (not shown) neutralizes the residual charge on the photoreceptor 2K after being cleaned by the drum cleaning device 3K. By this charge removal, the surface of the photoreceptor 2K is initialized and prepared for the next image formation.

In FIG. 2, the developing device 8K includes a developing unit 12K including a developing roll 9K, and a developer transport unit 13K that stirs and transports a K developer (not shown). The developer transport unit 13K includes a first transport chamber that houses the first screw member 10K and a second transport chamber that houses the second screw member 11K.
Each screw member includes a rotating shaft member whose both ends in the axial direction are rotatably supported by bearings, and a spiral blade projecting spirally on the peripheral surface thereof.

In the developing device 8K, the first transfer chamber that stores the first screw member 10K and the second transfer chamber that stores the second screw member 11K are partitioned by a partition wall. At both end portions in the axial direction, communication ports for communicating both the transfer chambers are formed.
The first screw member 10K is directed from the back side to the front side in the direction orthogonal to the paper surface in the drawing while stirring the K developer (not shown) held in the spiral blade in the rotation direction along with the rotation drive. Transport.
Since the first screw member 10K and a later-described developing roll 9K are arranged in parallel so as to face each other, the transport direction of the K developer at this time is also a direction along the rotation axis direction of the developing roll 9K. . The first screw member 10K can supply K developer along the axial direction to the surface of the developing roll 9K.

In the first screw member 10K, the K developer transported to the vicinity of the front side end portion in FIG. 2 passes through the communication opening provided in the vicinity of the front end portion of the partition wall in the drawing and enters the second transport chamber. After entering, it is held in the spiral blade of the second screw member 11K. As the second screw member 11K is driven to rotate, the second screw member 11K is conveyed from the front side to the back side while being stirred in the rotation direction.
In the second transfer chamber, a toner concentration sensor (not shown) is provided on the lower wall of the casing, and detects the K toner concentration of the K developer in the second transfer chamber. As the K toner density sensor, a sensor composed of a magnetic permeability sensor is used.
Since the magnetic permeability of the K developer containing K toner and magnetic carrier has a correlation with the K toner concentration, the magnetic permeability sensor detects the K toner concentration.

  In the printer according to this embodiment, CL, Y (not shown) for individually supplying CL, Y, M, C, and K toners to the second storage chamber of the developing device for CL, Y, M, C, and K, respectively. , M, C, K toner replenishing means are provided. Then, the control unit of the printer stores Vtref for CL, Y, M, C, and K, which are target values of output voltage values from the CL, Y, M, C, and K toner density detection sensors, in a storage unit RAM (not shown). Is remembered.

  When the difference between the output voltage value from the CL, Y, M, C, K toner density detection sensor and the Vtref for CL, Y, M, C, K exceeds a predetermined value, the difference is determined. The CL, Y, M, C, and K toner replenishing means are driven for the time. As a result, CL, Y, M, C, and K toners are replenished into the second transport chamber in the CL, Y, M, C, and K developing devices.

  The developing roll 9K accommodated in the developing unit 12K faces the first screw member 10K and also faces the photoreceptor 2K through an opening provided in the casing. The developing roll 9K includes a cylindrical developing sleeve made of a nonmagnetic pipe that is rotationally driven, and a magnet roller that is fixed inside the developing roll 9K so as not to rotate with the sleeve. Then, the K developer supplied from the first screw member 10K is carried on the sleeve surface by the magnetic force generated by the magnet roller, and is conveyed to the developing region facing the photoreceptor 2K as the sleeve rotates.

  A developing bias having the same polarity as the toner and larger than the electrostatic latent image of the photosensitive member 2K and smaller than the uniform charging potential of the photosensitive member 2K is applied to the developing sleeve. As a result, a developing potential for electrostatically moving the K toner on the developing sleeve toward the electrostatic latent image acts between the developing sleeve and the electrostatic latent image on the photoreceptor 2K.

  Further, a non-developing potential that moves K toner on the developing sleeve toward the sleeve surface acts between the developing sleeve and the background portion of the photoreceptor 2K. By the action of the developing potential and the non-developing potential, the K toner on the developing sleeve is selectively transferred to the electrostatic latent image on the photoreceptor 2K, and the electrostatic latent image is developed into the K toner image.

  In the above-described FIG. 1, the image forming units 1CL, Y, M, and C for CL, Y, M, and C are similar to the image forming unit 1K for K on the photoreceptors 2Y, M, and C. Y, M, and C toner images are formed.

Above the image forming units 1CL, Y, M, C, and K, an optical writing unit 80 serving as a latent image writing unit is disposed.
The optical writing unit 80 optically scans the photoreceptors 2CL, Y, M, C, and K with laser light emitted from a laser diode based on image information transmitted from an external device such as a personal computer.
By such optical scanning, electrostatic latent images for CL, Y, M, C, and K are formed on the photoreceptors 2CL, Y, M, C, and K. Specifically, of the entire surface of the uniformly charged surface of the photoconductor, the portion irradiated with the laser light attenuates the potential. Thereby, an electrostatic latent image is obtained in which the potential of the laser irradiation portion is smaller than the potential of the other portion (background portion). The optical writing unit 80 irradiates the photosensitive member through a plurality of optical lenses and mirrors while polarizing the laser light L emitted from the light source in the main scanning direction by a polygon mirror rotated by a polygon motor (not shown). To do. You may employ | adopt what performs optical writing by the LED light emitted from several LED of the LED array.

  In the present embodiment, as described above, when the recording material is an uneven recording material having an uneven surface, the superimposed bias is applied to the uneven surface of the recording material prior to image transfer. It is assumed that the toner is transferred and an arbitrary image is transferred to the surface. For this purpose, prior to transferring the color image, the image using the CL unit is transferred to smooth the uneven surface. Then, an arbitrary color image is transferred. As a result, the concave portion is filled in advance, and transfer defects to the concave portion can be eliminated.

  Since the background color CL unit used to exhibit this feature only deals with a specific pattern such as a solid image, the CL unit may have a simple configuration different from other colors, such as limiting only the CL unit to full exposure. .

Below the image forming units 1CL, Y, M, C, and K, as a primary transfer device provided with an intermediate transfer belt 31 used for sequentially transferring toner images carried on the photoreceptor of the image forming unit. The transfer unit 30 is disposed.
The transfer unit 30 includes a driving roller 32, a secondary transfer back roller 33, rollers 34, 5 in addition to the intermediate transfer belt 31 as an image carrier to which the toner images carried on the photoreceptor of the image forming unit are sequentially transferred. Two primary transfer rollers 35CL, Y, M, C, K, a nip forming roller 36 disposed at the secondary transfer position, a belt cleaning device 37, a potential sensor 38, and the like are provided.

  The intermediate transfer belt 31 is used as a member for primary transfer of a toner image carried on each photoconductor, and includes a drive roller 32, a secondary transfer back roller 33, and a cleaning backup roller 34 disposed inside the loop. , And five primary transfer rollers 35CL, Y, M, C, and K. Then, it is moved endlessly in the same direction by the rotational force of the drive roller 32 that is driven to rotate counterclockwise in the figure by a drive means (not shown).

As the intermediate transfer belt 31, a belt having the following characteristics is used.
That is, the thickness is set to 20 (μm) to 200 (μm), preferably about 60 (μm). Further, the volume resistivity is selected from 1e6 (Ωcm) to 1e12 (Ωcm), preferably about 1e9 (Ωcm) (measured with a Hiresta UP MCP HT45 manufactured by Mitsubishi Chemical under the condition of an applied voltage of 100 V). The material is made of carbon-dispersed polyimide resin.

  The five primary transfer rollers 35CL, Y, M, C, and K sandwich the intermediate transfer belt 31 that is moved endlessly between the photoreceptors 2CL, Y, M, C, and K. As a result, primary transfer nips for CL, Y, M, C, and K where the surface of the intermediate transfer belt 31 and the photoreceptors 2CL, Y, M, C, and K abut are formed.

  A primary transfer bias is applied to the primary transfer rollers 35CL, Y, M, C, and K by a transfer bias power source (not shown). Thereby, a transfer electric field is formed between the CL, Y, M, C, and K toner images on the photoreceptors 2CL, Y, M, C, and K and the primary transfer rollers 35CL, Y, M, C, and K. Thus, each color image is sequentially transferred to the intermediate transfer belt 31.

  The primary transfer rollers 35CL, Y, M, C, and K can be separated, and when only CL and K are transferred, the units of other colors are separated. First, the CL toner formed on the surface of the CL photoreceptor 2CL enters the CL primary transfer nip as the photoreceptor 2CL rotates. Then, the image is primarily transferred from the photoreceptor 2CL onto the intermediate transfer belt 31 by the action of the transfer electric field or nip pressure.

The primary transfer rollers 35CL, Y, M, C, and K are formed of an elastic roller having a metal core and a conductive sponge layer fixed on the surface thereof. It has characteristics.
That is, the outer shape is 16 (mm). The diameter of the mandrel is 10 (mm). Further, it flows when a voltage of 1000 (V) is applied to the primary transfer roller mandrel while a grounded metal roller having an outer diameter of 30 (mm) is pressed against the sponge layer with a force of 10 (N). The resistance R of the sponge layer calculated from the current I based on Ohm's law (R = V / I) is about 3E7Ω.
A primary transfer bias is applied to such primary transfer rollers 35CL, Y, M, C, and K by constant current control. Instead of the primary transfer rollers 35CL, Y, M, C, and K, a transfer charger, a transfer brush, or the like may be employed.

  The nip forming roller 36 of the transfer unit 30 corresponds to a secondary transfer position where the color image other than the CL image carried on the intermediate transfer belt 31 is transferred to the recording material P in a state where the image is superimposed. The intermediate transfer belt 31 is sandwiched between the secondary transfer back roller 33 disposed inside the loop of the transfer belt 31 and the secondary transfer back roller 33 inside the loop. As a result, a secondary transfer nip where the surface of the intermediate transfer belt 31 and the nip forming roller 36 abut is formed.

  While the nip forming roller 36 is grounded, a secondary transfer bias is applied to the secondary transfer back roller 33 by a secondary transfer bias power source 39. As a result, a secondary transfer electric field is formed between the secondary transfer back roller 33 and the nip forming roller 36 to electrostatically move the negative polarity toner from the secondary transfer back roller 33 side toward the nip forming roller 36 side. Is done.

Below the transfer unit 31, a paper feed cassette 100 that stores a plurality of recording materials P in a bundle of paper is disposed.
In the paper feeding cassette 100, a paper feeding roller 100a is brought into contact with the top recording material P of the paper bundle, and the recording material P is directed to the paper feeding path by being driven to rotate at a predetermined timing. And send it out.
A registration roller pair 101 is disposed near the end of the paper feed path.
The registration roller pair 101 stops the rotation of both rollers as soon as the recording material P fed from the paper feed cassette 100 is sandwiched between the rollers. Then, rotation driving is resumed at a timing at which the sandwiched recording material P can be synchronized with the CL toner image on the intermediate transfer belt 31 in the secondary transfer nip, and the recording material P is sent out toward the secondary transfer nip.

The CL toner image on the intermediate transfer belt 31 brought into close contact with the recording material P at the secondary transfer nip is secondarily transferred onto the recording material P by the action of the secondary transfer electric field or nip pressure, and transferred onto the recording material P. The
The recording material P to which the CL toner image is transferred in this way is separated from the nip forming roller 36 and the intermediate transfer belt 31 by the curvature when passing through the secondary transfer nip. Thus, when the recording material P having an uneven surface is used by transferring the CL image to the recording material P, the surface thereof is smoothed.

The secondary transfer back roller 33 has the following characteristics.
That is, the outer diameter is about 24 (mm). The diameter of the cored bar is about 16 (mm). The surface of the metal core is covered with a conductive NBR rubber layer, and its resistance R is 1e6 (Ω) to 1e12 (Ω), preferably about 4E7 (Ω). The resistance R is a value measured by the same method as that for the primary transfer roller.

The nip forming roller 36 has the following characteristics.
That is, the outer diameter is about 24 (mm). The diameter of the cored bar is about 14 (mm). The surface of the metal core is covered with a conductive NBR rubber layer, and its resistance R is 1E6Ω or less. The resistance R is a value measured by the same method as that for the primary transfer roller.

The secondary transfer bias power supply 39 has a DC power supply and an AC power supply, and can output a DC voltage superposed with an AC voltage as a secondary transfer bias.
The output terminal of the secondary transfer bias power source 39 is connected to the core of the secondary transfer back roller 33, and the potential of the core of the secondary transfer back roller 33 is the output voltage value from the secondary transfer bias power source 39. And almost the same value. Further, the core metal of the nip forming roller 36 is grounded (ground connection).

  Instead of grounding the core metal of the nip forming roller 36 while applying the superimposed bias to the core metal of the nip forming roller 36, the secondary transfer back roller while applying the superimposed bias to the core metal of the nip forming roller 36. 33 core bars may be grounded. In this case, the polarity of the DC voltage is varied. Specifically, when applying a superimposed bias to the secondary transfer back roller 33 under the condition that a negative polarity toner is used and the nip forming roller 36 is grounded, a DC voltage having the same negative polarity as the toner is used. Thus, the time average potential of the superimposed bias is set to the same negative polarity as that of the toner.

  On the other hand, when the secondary transfer back surface roller 33 is grounded and the superimposed bias is applied to the nip forming roller 36, a DC voltage having a positive polarity opposite to that of the toner is used, and the time average of the superimposed bias is used. Is set to a positive polarity opposite to that of the toner. Instead of applying the superimposed bias to the secondary transfer back roller 33 or the nip forming roller 36, a DC voltage may be applied to one of the rollers and an AC voltage may be applied to the other roller. As the AC voltage, a sinusoidal waveform is used, but a rectangular waveform may be used.

  In addition, when using a recording material P having a small surface irregularity such as plain paper without using a large surface irregularity such as rough paper, a shading pattern that follows the irregular pattern does not appear. A transfer bias composed only of a DC voltage may be applied. However, when using a paper with large surface irregularities such as rough paper, it is necessary to switch the transfer bias from a DC voltage only to a superimposed bias.

Untransferred toner that has not been transferred to the recording material P adheres to the intermediate transfer belt 31 that has passed through the secondary transfer nip. This is cleaned from the belt surface by a belt cleaning device 37 in contact with the surface of the intermediate transfer belt 31.
A cleaning backup roller 34 disposed inside the loop of the intermediate transfer belt 31 backs up the cleaning of the belt by the belt cleaning device 37 from the inside of the loop.

  The potential sensor 38 is disposed outside the loop of the intermediate transfer belt 31. In the entire area of the intermediate transfer belt 31 in the circumferential direction, the intermediate transfer belt 31 is opposed to the grounded driving roller 32 with a gap of about 4 mm. When the toner image primarily transferred onto the intermediate transfer belt 31 enters a position facing the intermediate transfer belt 31, the surface potential of the toner image is measured. As the potential sensor 38, EFS-22D manufactured by TDK Corporation is used.

A fixing device 90 is disposed on the left side of the secondary transfer nip in the drawing.
The fixing device 90 forms a fixing nip with a fixing roller 91 containing a heat source such as a halogen lamp and a pressure roller 92 that rotates while contacting with the fixing roller 91 with a predetermined pressure. The recording material P fed into the fixing device 90 is sandwiched between the fixing nips in such a posture that the unfixed toner image carrying surface is in close contact with the fixing roller 91. Then, the toner in the toner image is softened by the influence of heating and pressurization, and the CL toner image is fixed.

  The recording material P discharged from the fixing device 90 is smoothed when the surface has irregularities due to the transfer of the CL image, and again passes through the lower conveyance path B in the drawing, and is again in the secondary transfer nip. It is conveyed toward the part. A registration roller pair 102 is disposed near the end of the conveyance path.

  The registration roller pair 102 stops the rotation of both rollers as soon as the recording material P conveyed after fixing is sandwiched between the rollers. On the other hand, the Y toner image is primarily transferred to the intermediate transfer belt 31 in the same manner as when the CL toner image is transferred to the intermediate transfer belt 31, and then the primary transfer nips for M, C, and K are sequentially transferred. pass. Then, the M, C, and K toner images on the photoreceptors 2M, C, and K are sequentially superimposed and superimposed on the Y toner image.

  A four-color superimposed toner image is formed on the intermediate transfer belt 31 by the superimposing primary transfer described above. Then, rotation driving is resumed at a timing that can be synchronized with the four-color superimposed toner image on the intermediate transfer belt 31 in the secondary transfer nip, and the recording material P on which the CL toner image is fixed is directed to the secondary transfer nip. And send it out.

  The four-color superimposed toner image on the intermediate transfer belt 31 that is in close contact with the recording material P at the secondary transfer nip is secondarily transferred onto the recording material P by the action of the secondary transfer electric field and nip pressure, and is a full-color toner image. It becomes. At this time, only a DC voltage may be applied to the secondary transfer, or a voltage in which an AC voltage and a DC voltage are superimposed may be applied.

  When the CL toner image sufficiently fills the concave portion, it is possible to transfer the image with energy saving when a DC voltage is applied to the secondary transfer. The recording material P to which the full-color toner image has been transferred is fixed by the fixing device 90 and then discharged outside the apparatus. Further, when an image is formed on the surface of the recording material P opposite to the surface on which the toner image is transferred, the full-color toner image is fixed and then conveyed to the transfer material reversing mechanism 70 to be transferred to the transfer material reversing mechanism 70. To reverse the recording material. Thereafter, an image is also formed on the opposite surface in the same manner as in the transfer step.

FIG. 3 is a waveform diagram showing the waveform of the secondary transfer bias composed of the superimposed bias output from the secondary transfer bias power source 39.
In the figure, the secondary transfer bias is applied to the core metal of the secondary transfer back roller as described above. The secondary transfer bias power source 39 as voltage output means functions as a transfer bias applying means for applying a transfer bias. In addition, as described above, when the secondary transfer vice is applied to the core of the secondary transfer back roller, the core of the secondary transfer back roller as the first member and the core of the nip forming roller as the second member A potential difference occurs between the two. Thus, the secondary transfer bias power supply 39 also functions as a potential difference generating unit.

The potential difference is generally handled as an absolute value, but in this embodiment, it is handled as a value with polarity. More specifically, a value obtained by subtracting the potential of the core metal of the nip forming roller from the potential of the core metal of the secondary transfer back roller is treated as a potential difference.
Normally, the time average value of the potential difference, as shown in the embodiment, in the configuration using a negative polarity toner, when the polarity becomes negative, the potential of the nip forming roller is changed to the secondary transfer back surface roller. Therefore, the potential of the toner is larger on the opposite polarity side (plus side in this example) to the charged polarity of the toner. Therefore, the toner is electrostatically moved from the secondary transfer back roller side to the nip forming roller side.

However, as a result of the study by the present inventor, even when the polarity of the time average value of the potential difference is in the direction in which the opposite toner is electrostatically moved from the nip forming roller side to the secondary transfer back roller side, the recording is performed. It has been found that there is a condition for transferring the toner only to the concave portion of the material P.
In the present embodiment, when the toner is transferred only to the concave portion of the recording material P, the direct current component of the voltage applied to the secondary transfer back surface roller 33 is set to a positive polarity, and when the concave portion convex portion is transferred, it is applied to the secondary transfer back surface roller. The DC component of the voltage is negative polarity.
In FIG. 3, the offset voltage Voff is the value of the DC component of the secondary transfer bias. The peak-to-peak voltage Vpp is the peak-to-peak voltage of the AC component of the secondary transfer bias.

In the printer according to the first embodiment, as described above, the secondary transfer bias is obtained by superimposing the offset voltage Voff and the peak-to-peak voltage Vpp, and the time average value thereof is the same value as the offset voltage Voff. become. In the printer according to the first embodiment, as described above, the secondary transfer bias is applied to the core metal of the secondary transfer back roller, and the core metal of the nip forming roller is grounded (0 V). . Therefore, the potential of the core metal of the secondary transfer back roller becomes the potential difference between both core bars as it is.
The potential difference between both the cores is composed of a DC component (Eoff) having the same value as the offset voltage Voff and an AC component (Epp) having the same value as the peak-to-peak voltage Vpp.
In FIG. 3, a negative polarity is applied as the offset voltage Voff.

By making the polarity of the offset voltage Voff of the secondary transfer bias applied to the secondary transfer back roller 33 negative, the negative polarity toner is transferred from the secondary transfer back roller 33 side to the nip forming roller in the secondary transfer nip. It becomes possible to extrude to the 36 side relatively. As a result, the toner on the intermediate transfer belt 31 is transferred onto the recording paper P.
On the other hand, when the polarity of the secondary transfer bias is a positive polarity opposite to that of the toner, the negative polarity toner is directed from the nip forming roller 36 side to the secondary transfer back surface roller 33 side in the secondary transfer nip. Pulls electrostatically. As a result, the toner transferred to the recording paper P is attracted again to the intermediate transfer belt 31 side. However, when the time average value of the secondary transfer bias (in this example, the same value as the offset voltage Voff) is negative, the toner is relatively electrostatic from the secondary transfer back roller 33 side to the nip forming roller 36 side. Pushed out.

  On the other hand, in the case of plus, relatively, the toner is electrostatically pulled back from the nip forming roller 36 side to the secondary transfer back surface roller 33 side. Although it is pulled back, the toner remains in the recess. In the figure, the return potential peak value Vr indicates a positive peak value having a polarity opposite to that of the toner.

Next, the transfer portion of the base color for the uneven recording material will be described in detail.
The secondary transfer bias power supply 39 can apply a voltage obtained by superimposing a DC voltage on an AC voltage. By applying this superimposed voltage to the secondary transfer back roller 33, the CL toner can be satisfactorily transferred even to a recording material having an uneven surface.
First, a CL toner solid image is transferred to a recording material as a base of an arbitrary image to be formed. At this time, there are a case where the unevenness on the surface of the recording material is transferred to the concave portion and the convex portion, and a case where the recording material is transferred only to the concave portion. When transferring to both the concave and convex portions, the secondary transfer bias power source 39 generates the following as a potential difference between the core metal of the secondary transfer back roller 33 and the core metal of the secondary transfer roller 36. It is preferable to output a transfer bias.

  That is, in the waveform representing the time change of the potential difference between the metal core of the secondary transfer back roller 33 and the metal core of the secondary transfer roller 36, the absolute value of the peak value Vr of the polarity opposite to the polarity at which the toner is transferred is the potential difference. The time average Voff is greater than the absolute value of “| Vr |> | Voff |”, and the offset voltage Voff is the polarity in the direction in which the toner is normally transferred. In addition, when a waveform having a duty ratio of one cycle of AC component (= time required for voltage in the positive direction / time of one cycle × 100) as 50% is applied as the AC voltage, the peak-to-peak of the AC component is applied. The peak voltage Vpp (V) and the offset voltage Voff which is the time average value of the potential difference have a relationship of “¼ × Vpp> | Voff |”, and the offset voltage Voff is a direction in which the normal toner is transferred. The polarity.

In order to transfer the toner image on the intermediate transfer body to the concave and convex portions of the transfer material having irregularities on the surface, transfer is performed with a voltage Vr having a reverse polarity that is relatively higher than the voltage | Voff | This is because it is necessary to push the toner back and reciprocate the toner.
When transferring only to the concave portion, the absolute value of the peak value Vr of the polarity opposite to the polarity to which the toner is transferred is larger than the absolute value of the time average Voff of the potential difference, which is referred to as “| Vr |> | Voff |” It is preferable that the offset voltage Voff has a relationship opposite to that of the normal toner transfer polarity.

  When a waveform having a duty ratio of 50 (%) in one cycle of the AC component is applied as the AC voltage, the peak-to-peak voltage Vpp (V) of the AC component and the offset voltage Voff that is the time average value of the potential difference are applied. It is preferable to satisfy the relationship of “¼ × Vpp> | Voff |” and to set the offset voltage Voff to a polarity opposite to the polarity at which the toner is transferred.

4 to 8 show a blue (blue) solid image in which cyan and magenta are overlapped by the printer according to the first embodiment, with a voltage obtained by superimposing an AC voltage and a DC voltage of a sine wave duty 50% in the secondary transfer. An example of a transferred image is shown.
Originally, it is preferable to form the background with a toner of an inconspicuous color such as a clear toner, but here a blue image was transferred in order to confirm transferability.

In the upper part of the image, the peak-to-peak voltage Vpp (kV) and the offset voltage Voff (kV) of the AC component of the voltage obtained by superimposing the AC voltage and the DC voltage applied by the secondary transfer bias power source 39 are shown.
Since the image in FIG. 4 was transferred with Vpp = 4.6 (kV) and Voff = −2.3 (kV), Vr = 0 (kV). At this time, the toner is sufficiently transferred to the convex portion, but the concave portion is hardly transferred.

Since the image in FIG. 5 was transferred with Vpp = 5.6 (kV) and Voff = −1.8 (kV), Vr = 1.0 (kV). At this time, the toner is sufficiently transferred to the convex portion, but the transfer amount of the concave portion is small.
Since the image in FIG. 6 was transferred with Vpp = 6.2 (kV) and Voff = −1.5 (kV), Vr = 1.7 (kV). At this time, the toner is sufficiently transferred to the convex portion, and the concave portion is also sufficiently transferred.
Since the image in FIG. 7 was transferred with Vpp = 7.0 (kV) and Voff = −1.1 (kV), Vr = 2.4 (kV). At this time, the toner is sufficiently transferred to the convex portion, and the concave portion is also sufficiently transferred.
Since the image of FIG. 8 was transferred with Vpp = 9.6 (kV) and Voff = 0.2 (kV), Vr = 5.0 (kV). At this time, the toner is hardly transferred to the convex portion and is sufficiently transferred to the concave portion.

As shown in FIGS. 4 and 5, when a transfer material with large irregularities is transferred only with a DC voltage, or when transfer is performed with a voltage in which an AC voltage and a DC voltage are superimposed without satisfying both the conditions, as shown in FIGS. The toner is transferred only to the convex portion, and the transferability of the concave portion is not sufficient.
FIG. 9 schematically shows a state of a transfer material cross section relating to transferability including the above-described recess.

On the other hand, when the transfer is performed by applying a voltage in which an AC voltage and a DC voltage are superimposed so as to satisfy the condition for transferring the toner to the concave and convex portions, the convex portion is also provided on the concave portion as shown in FIGS. In addition, the toner is sufficiently transferred.
FIG. 10 schematically shows a cross-sectional state of the transfer material relating to transferability including the above-described recesses.
FIG. 11 shows a cross-sectional state of the transfer material when the transfer material having the toner sufficiently transferred to the concave and convex portions as shown in FIG. 10 is fixed.

When the transfer material having the toner sufficiently transferred to the concave portion and the convex portion is fixed, the toner melts and flows slightly into the concave portion. For this reason, as shown in FIG. 11, the effect of making the unevenness smaller than before fixing is obtained, and when an arbitrary image is transferred to the toner surface after fixing, image omission is less likely to occur. In addition, when the transfer is performed by applying a voltage in which an AC voltage and a DC voltage are superimposed so as to satisfy the condition for transferring the toner only to the concave portion, as shown in FIG. Transcribed.
FIG. 12 schematically shows a cross-sectional state of the transfer material relating to transferability in such a case.
FIG. 13 shows a cross-sectional state of the transfer material when the transfer material with the toner sufficiently transferred only to the concave portion is fixed as shown in FIG.

When the transfer material having the toner sufficiently transferred only to the concave portion is fixed, the effect of making the unevenness smaller than before fixing is obtained as shown in FIG. 13, and when transferring an arbitrary image to the toner surface after fixing. , Image omission is less likely to occur. In this way, by applying a voltage in which an AC voltage and a DC voltage are superimposed and transferring the background color toner so as to fill the unevenness of the recording material, the unevenness of the transfer material is reduced, and the upper surface of the background color toner is reduced. When transferring an arbitrary image, image omission can be reduced.
In addition, in an image forming apparatus that performs transfer by applying a voltage in which an alternating voltage and a direct current voltage are superimposed without transferring the background color toner, dust is generated when fine lines or characters are transferred to the concave portion of the transfer material. In the image forming apparatus of the present invention, even when a similar image is formed, the generation of dust is less than that of the former.

Regarding the results shown in FIGS. 4 to 8, the present inventor conducted experiments based on conditions different from the conditions for obtaining the results shown in FIGS. 4 to 8, and as shown in FIG. It was confirmed that the same tendency as the result shown in FIG. 8 was obtained.
Here, an imagio NeoC600 modified machine (manufactured by Ricoh Co., Ltd.) was used as an experimental machine, the linear speed of the intermediate transfer belt 31 was set to 283 mm / sec, and the recording material was Rezac 66 175 Kg, thickness 228 μm, dent (unevenness) The thickness was 104 μm.
The following conditions were set as experimental contents.
Experiment 1: DC transfer of image after filling only concave portion with background toner 2: DC transfer experiment for image after filling entire surface with background toner 3: Image transfer for AC transfer after filling entire surface with background toner 4: Image of background AC transfer of image without toner transfer Experiment 5: DC transfer of image without transfer of underlying toner And, as a concave density rank, it is graded from 1 (bad) to 5 (good) as a fine line dust rank Stage 1 (bad) to 5 (good).
The above conditions are based on the premise that the image that has been subjected to the primary transfer to the intermediate transfer belt 31 is transferred to the recording material. And the applied voltage is Voff or Vpp.

Next, a printer according to a second embodiment of the present invention will be described. Unless otherwise specified, the configuration of the printer according to the second embodiment is the same as that of the first embodiment and each modification.
FIG. 14 is a schematic configuration diagram showing a printer according to the second embodiment.
This printer is different from the printer according to the first embodiment in that only the background color image forming unit is provided separately from the other color image forming units.

  The background color image forming unit performs direct transfer onto paper, but may also employ a method of transferring to paper via an intermediate transfer member. The recording material P delivered from the paper feed cassette 100 via the registration roller pair 101 is transferred with the CL toner image on the photoreceptor 2CL in the transfer nip of the background color image forming unit 50.

  The recording material P to which the CL toner image has been transferred has the four-color superimposed toner image on the intermediate transfer belt 31 continuously transferred to the secondary transfer nip continuously at the next secondary transfer nip, or the secondary transfer nip. After the transfer roller 36 is separated and the CL toner image on the recording material P is once fixed, it passes through the conveyance path B positioned below the drawing and is transferred by the secondary transfer roller 36 as in the first embodiment. At the secondary transfer nip, the four-color superimposed toner image is collectively transferred to the secondary transfer nip.

  When secondary-transferring a four-color superimposed toner image, the applied voltage may be only a DC voltage or a voltage obtained by superimposing an AC voltage and a DC voltage. The recording material P to which the full-color toner image has been transferred is fixed by the fixing device 90 and then discharged outside the apparatus.

When an arbitrary image is transferred onto the upper surface of the recording material P after filling the concave portion on the surface of the recording material P with the base color, the transfer to the paper can be continuously performed, so that the printing speed is improved.
When the next transfer is performed without fixing after transferring the background color, the charge of the background color toner may affect the transfer of the other color toner. For example, the transfer rate of the recesses may decrease due to the CL toner charge concentrated on the recesses, resulting in image unevenness.
In order to prevent such a decrease in image quality, it is effective to make the absolute value of the charge amount of the background color toner smaller than the absolute value of the charge amount of the toner for forming an arbitrary image.

  In this embodiment, the average charge amount of Y, M, C, and K toners is about −30 (μC / g), whereas the average charge amount of CL toners is about −10 (μC / g). The charge amount of the base toner may be opposite to the charge amount of other toners.

  On the other hand, once the background color is fixed, the charge of the background color toner disappears and the image is likely to be stable. The user or service person can set whether to perform the transfer once or not through the operation panel or the like.

  The transfer bias power supply 40 that applies a voltage to the transfer roller 40 can apply a voltage obtained by superimposing a DC voltage on an AC voltage. Here, a CL toner solid image is transferred to a recording material as a base of an arbitrary image to be formed. At this time, there are a case where the unevenness on the surface of the recording material is transferred to the concave portion and the convex portion, and a case where the recording material is transferred only to the concave portion.

  When transferring to both the concave and convex portions, it is preferable to output a transfer bias that generates the following as a potential difference between the electrostatic latent image of the photoreceptor 2CL and the core metal of the transfer roller 40. That is, in the waveform representing the time change of the potential difference between the electrostatic latent image on the photoreceptor 2CL and the cored bar of the transfer roller 40, the absolute value of the peak value Vr of the polarity opposite to the polarity at which the toner is transferred is the potential difference. The time average Voff is greater than the absolute value of “| Vr |> | Voff |”, and the offset voltage Voff is the polarity in the direction in which the toner is normally transferred. In addition, when a waveform having a duty ratio of one cycle of AC component (= time required for voltage in the positive direction / time of one cycle × 100) as 50% is applied as the AC voltage, the peak-to-peak of the AC component is applied. The peak voltage Vpp (V) and the offset voltage Voff which is the time average value of the potential difference have a relationship of “¼ × Vpp> | Voff |”, and the offset voltage Voff is a direction in which the normal toner is transferred. The polarity.

  Further, when transferring only to the concave portion, the absolute value of the peak value Vr of the polarity opposite to the polarity at which the toner is transferred is larger than the absolute value of the time average Voff of the potential difference “| Vr |> | Voff | It is preferable that the offset voltage Voff has a polarity opposite to that of the normal toner. When a waveform having a duty ratio of 50 (%) in one cycle of the AC component is applied as the AC voltage, the peak-to-peak voltage Vpp (V) of the AC component and the offset voltage Voff that is the time average value of the potential difference are applied. It is preferable to satisfy the relationship of “¼ × Vpp> | Voff |” and to set the offset voltage Voff to a polarity opposite to the polarity at which the toner is transferred.

Next, a printer according to a third embodiment of the invention will be described. Unless otherwise specified, the configuration of the printer according to the third embodiment is the same as that of the first embodiment and each modification.
FIG. 15 is a schematic configuration diagram showing a printer according to the third embodiment.
This printer is different from the printer according to the first embodiment in that only the background color image forming unit is provided separately from the other color image forming units and the background color fixing fixing device is provided separately. .

  The background color image forming unit 51 performs direct transfer onto a recording material, but may employ a method of transferring to paper via an intermediate transfer member. The recording material P delivered from the paper feed cassette 100 via the registration roller pair 101 is transferred with the CL toner image on the photoreceptor 2CL in the transfer nip of the background color image forming unit 50. A voltage obtained by superimposing a DC voltage on an AC voltage is applied to the transfer roller 40.

  The recording material P to which the CL toner image has been transferred is fixed by the fixing device 110, and the four-color superimposed toner image on the intermediate transfer belt 31 is continuously transferred to the secondary transfer nip at the same time. At this time, only a DC voltage may be applied to the secondary transfer, or a voltage in which an AC voltage and a DC voltage are superimposed may be applied. The recording material P to which the full-color toner image has been transferred is fixed by the fixing device 90 and then discharged outside the apparatus.

In the third embodiment as described above, since the filling of the concave portion with the base color, the fixing of the base color toner, the arbitrary image transfer to the upper surface of the base color toner, the fixing of the arbitrary image can be performed continuously, the stable Printing speed can be improved while forming an image.
The background color image forming unit 51 can be attached to and detached from the image forming apparatus main body, and the image forming unit 51 can be added to the image forming apparatus main body as an option later. The image forming unit 51 can also be used independently as a recording material pre-processing device.

Next explained is a printer according to the fourth embodiment of the invention. Unless otherwise specified, the configuration of the printer according to the fourth embodiment is the same as that of the first embodiment and each modification.
FIG. 16 is a schematic configuration diagram showing a printer according to the fourth embodiment.
In this printer, as an nip forming member, an endless paper conveying belt 121 is brought into contact with each color photoconductor 2CL, Y, M, C, Bk instead of the intermediate transfer belt in the first embodiment. Is different from the printer according to.

First, the paper conveying belt 121 conveys the recording material P held on its surface to the CL transfer nip as it moves endlessly.
In this process, the CL toner image on the photoreceptor 2CL is transferred to the surface of the recording material P. Thereafter, the recording material P continuously passes through the transfer nips for Y, M, C, and Bk, and the Y, M, C, and K toner images on the photoreceptors 2Y, M, C, and K are transferred to the recording paper. The image is transferred onto the surface in a superimposed manner, or after the CL toner is transferred, the transfer rollers 25Y, M, C, and K for Y, M, C, and K are separated to transfer only the CL toner. After the toner is fixed once, it is conveyed in a direction to transfer the other color image onto the upper surface of the CL toner image after fixing, and sequentially passes through the transfer nips for Y, M, C, and Bk to obtain a full color toner image.

A voltage obtained by superimposing a DC voltage on an AC voltage is applied to the transfer roller 25CL by a transfer bias power supply 81CL. Only the DC voltage may be applied to the transfer rollers 25Y, M, C, and K, or a voltage obtained by superimposing the DC voltage on the AC voltage may be applied.
Instead of bringing the paper transport belt 121 into contact with the photoreceptors 2CL, Y, M, C, and K, the transfer rollers 25CL, Y, M, C, and K are directly applied to the photoreceptors 2CL, Y, M, C, and K. The primary transfer nips for CL, Y, M, C, and K may be formed by contact.
In this case, the transfer rollers 25CL, Y, M, C, and K function as nip forming members. The recording material P to which the full-color toner image has been transferred is fixed by the fixing device 90 and then discharged outside the apparatus. When an arbitrary image is transferred onto the upper surface of the recording material P after filling the concave portion on the surface of the recording material P with the base color, the transfer to the paper can be continuously performed, so that the printing speed is improved.

  When the next transfer is performed without fixing after transferring the background color, the absolute value of the charge amount of the background color toner is preferably smaller than the absolute value of the charge amount of the toner for forming an arbitrary image.

On the other hand, once the background color is fixed, the charge of the background color toner disappears and the image is likely to be stable. The user or service person can set whether to perform the transfer once or not through the operation panel or the like.
The transfer bias power supply 81CL that applies a voltage to the transfer roller 25CL can apply a voltage obtained by superimposing a DC voltage on an AC voltage. Here, a CL toner solid image is transferred to a recording material as a base of an arbitrary image to be formed.

At this time, there are a case where the unevenness on the surface of the recording material is transferred to the concave portion and the convex portion, and a case where the recording material is transferred only to the concave portion. When transferring to both the concave and convex portions, it is preferable to output a transfer bias that generates the following as a potential difference between the electrostatic latent image on the photoreceptor 2CL and the cored bar of the transfer roller 25CL.
That is, in the waveform representing the time change of the potential difference between the electrostatic latent image on the photoreceptor 2CL and the core of the core of the transfer roller 25CL, the absolute value of the peak value Vr of the polarity opposite to the polarity to which the toner is transferred is the potential difference. The time average Voff is greater than the absolute value of “| Vr |> | Voff |”, and the offset voltage Voff is the polarity in the direction in which the toner is normally transferred. In addition, when a waveform having a duty ratio of one cycle of AC component (= time required for voltage in the positive direction / time of one cycle × 100) as 50% is applied as the AC voltage, the peak-to-peak of the AC component is applied. The peak voltage Vpp (V) and the offset voltage Voff which is the time average value of the potential difference have a relationship of “¼ × Vpp> | Voff |”, and the offset voltage Voff is a direction in which the normal toner is transferred. The polarity.

Further, when transferring only to the concave portion, the absolute value of the peak value Vr of the polarity opposite to the polarity at which the toner is transferred is larger than the absolute value of the time average Voff of the potential difference “| Vr |> | Voff | It is preferable that the offset voltage Voff has a polarity opposite to that of the normal toner.
When a waveform in which the duty ratio of one cycle of the AC component is 50 (%) is applied as the AC voltage, the peak-to-peak voltage Vpp (V) of the AC component and the offset voltage Voff that is the time average value of the potential difference. It is preferable to satisfy the relationship of “¼ × Vpp> | Voff |” and to set the offset voltage Voff to a polarity opposite to the polarity at which the toner is transferred.

  The present invention has been described with reference to the examples. However, the present invention is not limited to these examples, and various modifications can be made without departing from the scope of the invention.

DESCRIPTION OF SYMBOLS 1 Image forming unit 2 Photoconductor 2CL Transparent toner photoconductor 30 Transfer unit 31 Intermediate transfer belt 33 Secondary transfer back roller 36 Nip forming roller 39 Secondary transfer bias power supply P Recording material

JP 2008-185890 A JP 2006-267486 A JP 2008-058585 A JP 09-146433 A Japanese Patent Laid-Open No. 04-0868878

Claims (7)

In an image forming apparatus including a transfer device that transfers an image of electrostatic toner from an image carrier to a recording material at a transfer nip .
When the recording material is an uneven recording material with unevenness formed on the surface ,
A superimposing bias in which an alternating current component is superimposed on a direct current component and the polarity of the alternating current component is alternately switched , and the background color toner whose time average value is charged to a predetermined polarity is transferred from the recording material to the image carrier in the transfer nip. After the image of the base color toner is transferred to the surface of the recording material by a superimposed bias that is a polarity that forms an electric field in the direction of electrostatic movement to
The surface of the recording material has a polarity whose time average value is a polarity that forms an electric field in a direction in which the toner charged to the predetermined polarity is electrostatically moved from the image carrier to the recording material in the transfer nip. An image forming apparatus for transferring an image using the toner . The image forming apparatus according to claim 1, wherein the image carrier is an intermediate transfer member that temporarily holds each of the base color toner and the toner . The absolute value of the charge quantity of the background color toner, the image forming apparatus according to claim 1 or 2, characterized in that the smaller than the absolute value of the charge amount of the toner. 4. The image forming apparatus according to claim 1 , wherein when the image is transferred with the toner, the transfer bias includes only a DC bias . 5. Wherein as the base-color toner image forming apparatus according to one of claims 1 乃 optimum 4, which comprises using a color toner of the base only. Wherein as the base-color toner image forming apparatus according to one of claims 1 to 5, characterized by using the clear toner (clear toner). The superimposing bias is a duty ratio of one cycle of the alternating current component (the voltage value of the superimposing bias is electrostatically transferred from the recording material to the image carrier in the transfer nip than the time average value thereof. (Time to be a voltage value on the moving side) / (time of one cycle) × 100) is 50 (%), and the peak-to-peak voltage of the AC component is more than four times the absolute value of the voltage of the DC component. The image forming apparatus according to claim 1 , wherein the image forming apparatus is also larger .

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