JP6728636B2 - Image forming device - Google Patents

Description

The present invention relates to an image forming apparatus that obtains a final image by transferring a toner image carried on a photoconductor or an intermediate transfer body onto plain paper, an OHP sheet, or another recording material.

In an electrophotographic image forming apparatus such as a copying machine, a printer, a facsimile, or a composite machine having at least two functions thereof, it is necessary to form optical image information on an image carrier that is uniformly charged in advance. The charged latent image obtained by the above is visualized by the toner from the developing device, and the visible image is transferred and fixed on the recording material to form an image. In recent years, a wide variety of papers have been used as recording materials used in image forming apparatuses, and those with the image of a leather pattern with a high-quality feeling and those of Japanese paper are commercially available to form printed matter with various expressions. It is possible. These papers have unevenness on the surface due to embossing or the like in order to give a high-grade feeling. The toner is less likely to be transferred to the concave portion than to the convex portion, and there is a problem that when the toner is transferred to a paper having large irregularities, the toner is not sufficiently transferred to the concave portion and an image is missing. The following techniques have been proposed as techniques for improving the transfer rate to the recesses.

In Patent Document 1, immediately before the transfer of the toner image onto the paper, the paper is heated and charged to a polarity opposite to that of the toner, so that the transfer electric field at the time of transfer is strengthened, so that the toner is transferred to the concave portion. It is disclosed to do. However, even if such a method is used, it is not possible to obtain sufficient transferability in a large uneven portion of the uneven paper.

In addition, there is a proposal aiming at improvement of transfer rate and improvement of abnormal images such as voids by superimposing AC voltage on DC voltage (Patent Documents 2, 3, 4, 5). A transfer method in which a voltage in which an AC component and a DC component are superimposed is applied to the transfer nip is called "transfer by superimposed voltage".

In Patent Document 2, a transfer bias in which an AC voltage is superimposed on a DC voltage is used, and the toner is transferred to the concave portion by charging the surface of the paper to a polarity opposite to the polarity of the toner according to the unevenness before the transfer. It is disclosed to perform such control.

Patent Document 3 proposes a technique 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 equal to or less than twice the DC voltage.

In Patent Document 4, a fluororesin is used on the surface of the intermediate transfer member, and a transfer bias in which an AC voltage is superimposed on a DC voltage is used so that the peak-to-peak voltage of the AC voltage is 2.05 times or more the DC voltage. It is proposed to superimpose an AC voltage on.

Patent Document 5 proposes that a transfer bias in which an AC voltage is superimposed is used as a transfer bias, and the AC voltage is superimposed so that the number of cycles in the nip is 20 times or more.
However, as confirmed by the present inventors, in any of these methods, the superimposed AC voltage is small, and in the case of paper with large unevenness, toner is not so much transferred to the recesses even when voltage is applied as in the embodiment. , There was no effect.

Therefore, as a result of intensive investigations by the present inventors, a transfer bias in which an AC voltage is superimposed on a DC voltage is used, and at this time, an AC bias having a high amplitude that causes the toner to reciprocate is used to form the uneven surface. It has been found that the transferability of can be improved (Patent Document 6).

However, even with the configuration disclosed in Patent Document 6, if the conveyance speed of the paper is increased in order to improve the productivity, the number of reciprocations of the toner in the transfer nip is small and the transferability is deteriorated. Since the transit time is short, the DC component is not sufficiently charged, and there is a problem that the necessary DC component cannot be secured. Further, even if an attempt is made to increase the frequency in order to cope with this, there is a problem that the toner cannot follow the voltage change of the AC voltage and the reciprocating motion does not occur. Further, if the DC component is increased in order to obtain a sufficient DC component, there is a problem in that discharge occurs at the peak voltage and an image defect occurs. If the paper transport speed is high, good transferability cannot be obtained.

In order to realize good transfer efficiency with low transfer power consumption, in Patent Document 7, in the secondary transfer portion that transfers the toner image on the intermediate transfer member to the recording material, toner is present based on image information from the transfer head. It has been proposed to apply a transfer bias while applying electric field vibration only to the region. Further, it is shown that the transfer head is formed by a piezoelectric transformer having a planar shape, and a plurality of piezoelectric transformers are arranged in a staggered manner with respect to the nip surface so as to cover the entire image area.

It is effective to increase the nip width to increase the number of reciprocation of the toner without lowering the recording material conveyance speed.However, if the pressure of the secondary transfer part is increased to increase the nip width, the central part of the thin line There is a problem that worm-eating images and the like are generated. Further, in the configuration in which the transfer nip is formed by the plurality of piezoelectric transformers arranged in a staggered manner as in the reference document 7, since there is a gap between the plurality of piezoelectric transformers, uniform pressure cannot be applied at the transfer nip.

The present invention has been made in view of the above background, and an object thereof is to obtain sufficient transferability in a surface concave portion of a recording material having irregularities, even when the recording material conveying speed is high, and , To transfer the image uniformly.

In order to achieve the above object, in the present invention, a transfer nip is formed between an image carrier that carries a toner image on the front surface and the image carrier that contacts the front surface of the image carrier. A nip forming member that forms a sheet, a facing member that faces the nip forming member, and is provided on the back surface side of the image carrier, and a toner on the image carrier with respect to the recording material sandwiched in the transfer nip. In order to transfer an image, a power source that forms a potential difference between the nip forming member and the facing member is provided, and an electric field formed by the potential difference causes toner charged to a regular polarity from the image carrier side. In an image forming apparatus comprising an electric field in the direction of moving to the recording material side and an electric field in the direction of returning from the recording material side to the image carrier side, the facing member is a flat plate formed of a conductive metal. And has a flat surface that uniformly contacts the back surface of the image carrier at the transfer nip and extends downstream from the transfer nip in the recording material conveyance direction, and the flat surface of the facing member is the back surface of the image carrier. The length in the recording material conveyance direction in contact with the recording material is longer than the contact width in the recording material conveyance direction when the nip forming member is pressed.

According to the present invention, the facing member that forms the transfer nip together with the nip forming member has a flat surface that contacts the back surface of the image carrier at the transfer nip, and the flat surface of the facing member contacts the back surface of the image carrier. The length in the recording material conveyance direction is longer than the contact width in the recording material conveyance direction when the nip forming member is pressed . This makes it possible to obtain a wide transfer nip width even with a low pressure. Compared to the case where the transfer nip is composed of rollers and pressed against each other, a long nip time can be secured while preventing worm-eating images. Even when the recording material conveying speed is high, it is possible to sufficiently secure the transferability of the recording material having irregularities to the concave portions.

Further, since the flat surface of the facing member uniformly contacts the image carrier at the transfer nip, uniform pressure can be applied at the transfer nip, and the image can be transferred uniformly.

1 is a schematic configuration diagram showing a printer according to a first embodiment of the present invention. FIG. 2 is an enlarged configuration diagram showing a black image forming unit in the printer of FIG. 1. FIG. 6 is a schematic configuration diagram showing a printer according to a comparative example in which a back surface member forming a secondary transfer nip is a roller. 4 is a graph showing a relationship between a pressing force and a nip width in comparison between the printer of FIG. 1 and the printer of FIG. 3. FIG. 6 is a waveform diagram showing a waveform of a secondary transfer bias used in the printer of the first embodiment. It is a schematic block diagram which shows the printer which concerns on 2nd embodiment of this invention. FIG. 7 is a diagram showing images in which the evaluation results of the recessed portion transferability are ranks 1 to 5, respectively. It is a figure which respectively shows the image which the evaluation result of white spot appearance is rank 1-5. It is a figure which shows the image which becomes the evaluation result of convex part density reproducibility rank 1-5, respectively. FIG. 6 is a waveform diagram showing another secondary transfer bias waveform that can be used in the present invention. It is a wave form diagram which shows the waveform of further another secondary transfer bias which can be used in the present invention. It is a wave form diagram which shows the waveform of another secondary transfer bias which can be used in this invention. It is a wave form diagram which shows the waveform of another secondary transfer bias which can be used in the present invention. It is a wave form diagram which shows the waveform of the further different secondary transfer bias which can be used in the present invention.

An electrophotographic color printer (hereinafter referred to as a printer) will be described below as an image forming apparatus according to the first embodiment of the present invention. The resistance values of the members described below are values under the environment of a temperature of 22° C. and a relative humidity of 50% unless otherwise specified.

First, the basic configuration of the printer will be described. In FIG. 1, the printer includes four image forming units 1Y, 1M, 1C, and 1K for transferring yellow (Y), magenta (M), cyan (C), and black (K) toner images, and a transfer unit. A unit 30, an optical writing unit 80, a fixing device 90, a sheet feeding cassette 100, and a registration roller pair 101 are provided.

The four image forming units 1Y, 1M, 1C, and 1K use toners (Y, M, C, and K) of different colors, but otherwise have the same configuration and are replaced when the unit life is reached. ing. Taking the image forming unit 1K for forming a K toner image as an example, as shown in FIG. 2, the unit includes a charging device 6K, a developing device 8K, and a drum cleaning device 3K around a drum-shaped photoreceptor 2K. , A static eliminator and the like. Since these devices are held by a common holder and can be integrally attached to and detached from the printer main body, they can be simultaneously replaced as a process cartridge.

The photoconductor 2K has, for example, an outer diameter of about 60 [mm] in which an organic photosensitive layer is formed on the surface of a drum base, and is rotationally driven in the clockwise direction in the figure. The charging device 6K generates a discharge between the charging roller 7K and the photoconductor 2K while bringing the charging roller 7K, to which a charging bias is applied, into contact with or in the vicinity of the photoconductor 2K, so that the surface of the photoconductor 2K is made uniform. Charge it. In this embodiment, the toner is uniformly charged to the same negative polarity as the normal charging polarity. As the charging bias, a DC voltage on which an AC voltage is superimposed is adopted. The charging roller 7K is a metal cored bar having a surface covered with a conductive elastic layer made of a conductive elastic material. Instead of the method of bringing a charging member such as a charging roller into contact with or close to the photoconductor 2K, a method using a charging charger may be adopted.

The uniformly charged surface of the photoconductor 2K is optically scanned by a laser beam emitted from an optical writing unit, which will be described later, and carries an electrostatic latent image for K. The electrostatic latent image for K is developed by the developing device 8K to become a K toner image. Then, the image is primarily transferred onto an intermediate transfer belt 31 described later.

The drum cleaning device 3K removes transfer residual toner adhering to the surface of the photoconductor 2K after the primary transfer process (primary transfer nip described later). The cleaning brush roller 4K is driven to rotate, and the cleaning blade 5K has its free end brought into contact with the photoconductor 2K while being supported in a cantilever manner. The rotating cleaning brush roller 4K scrapes the transfer residual toner from the surface of the photoconductor 2K, and the cleaning blade 5K scrapes the transfer residual toner from the surface of the photoconductor 2K. The cleaning blade 5K is in contact with the photoconductor 2K in a counter direction in which the cantilevered support end side is more downstream than the free end side in the drum rotation direction.

The static eliminator removes residual charges on the photoconductor 2K after being cleaned by the drum cleaning device 3K. By this charge removal, the surface of the photoconductor 2K is initialized and prepared for the next image formation.

The developing device 8K will be described in more detail. The developing device has a developing portion 12K including a developing roll 9K and a developer transporting portion 13K that stirs and transports the K developer in the casing. The developer transport unit 13K has a first transport chamber that houses the first screw member 10K and a second transport chamber that houses the second screw member 11K. Each of the screw members includes a rotary shaft member whose axial ends are rotatably supported by bearings, and a spiral blade spirally projecting from the peripheral surface of the rotary shaft member.

The first transport chamber that houses the first screw member 10K and the second transport chamber that houses the second screw member 11K are partitioned by partition walls, but at both ends of the partition wall in the screw axis direction, A communication port is formed to connect both transfer chambers. The first screw member 10K conveys the K developer, which is held in the spiral blade, from the back side to the front side in the direction orthogonal to the paper surface in the drawing, while stirring the K developer in the rotation direction along with the rotation drive. Since the first screw member 10K and the developing roll 9K are arranged in parallel facing each other, the conveying direction of the K developer at this time is also the direction along the rotation axis direction of the developing roll 9K. Then, the first screw member 10K supplies the K developer to the surface of the developing roll 9K along the axial direction thereof.

The K developer that has been transported to the vicinity of the front end of the first screw member 10K in the drawing passes through the communication opening provided near the front end of the partition wall in the drawing, and then enters the second transport chamber. , Held in the spiral blade of the second screw member 11K. Then, 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 in the drawing while being stirred in the rotation direction.

A well-known toner concentration sensor is provided on the lower wall of the casing in the second transport chamber to detect the K toner concentration of the K developer in the second transport chamber. A magnetic permeability sensor is used as the K toner concentration sensor. Since the magnetic permeability of the K developer containing the K toner and the magnetic carrier has a correlation with the K toner concentration, it means that the magnetic permeability sensor detects the K toner concentration.

In this printer, Y, M, C, and K toner replenishment for individually replenishing Y, M, C, and K color toners into the second accommodating chamber of the Y, M, C, and K developing devices, respectively. Means are provided. Then, the control unit of the printer stores Vtref for Y, M, C, and K, which are target values of the output voltage values from the Y, M, C, and K toner density detection sensors, in the RAM. When the difference between the output voltage value from each of the Y, M, C, and K toner density detection sensors and the Vtref for Y, M, C, and K exceeds a predetermined value, Y is set for a time corresponding to the difference. , M, C, K toner replenishing means are driven. As a result, Y, M, C, and K color toners are replenished into the second transport chamber in the Y, M, C, and K developing devices.

The developing roller 9K housed in the developing section 12K faces the first screw member 10K and also faces the photoconductor 2K through an opening provided in the casing. Further, the developing roller 9K includes a cylindrical developing sleeve made of a non-magnetic pipe that is rotationally driven, and a magnet roller fixed inside the developing sleeve 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 area facing the photoconductor 2K as the sleeve rotates.

To the developing sleeve, a developing bias having the same polarity as that of the toner, which is larger than the electrostatic latent image of the photoconductor 2K and smaller than the uniform charging potential of the photoconductor 2K is applied. 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 photoconductor 2K. Further, a non-development potential that directs the K toner on the developing sleeve to the sleeve surface acts between the developing sleeve and the background portion of the photoconductor 2K. Due to the effects 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 photoconductor 2K to develop the electrostatic latent image into the K toner image. In the image forming units 1Y, 1M, and 1C for Y, M, and C, toner images of Y, M, and C are formed on the photoconductors 2Y, 2M, and 2C as in the image forming unit 1K for K. (Hereinafter, Y, M, C, and K are omitted as appropriate if the color difference of the toner does not matter).

Returning to FIG. 1, the optical writing unit 80 disposed above the image forming units 1Y, 1M, 1C, and 1K is operated by a laser diode based on image information sent from an external device such as a personal computer. The photoconductors 2Y, 2M, 2C, and 2K are optically scanned by the emitted laser light. By this optical scanning, Y, M, C, and K electrostatic latent images are formed on the photoconductors 2Y, 2M, 2C, and 2K. Specifically, in the entire area of the uniformly charged surface of the photoconductor 2Y, the portion irradiated with laser light attenuates the potential. As a result, an electrostatic latent image in which the potential of the laser irradiation portion is smaller than the potential of the other portion (background portion) is obtained. The optical writing unit 80 irradiates each photoconductor through a plurality of optical lenses and mirrors while polarizing the laser light emitted from the light source in the main scanning direction with a polygon mirror. A device that performs optical writing by LED light emitted from a plurality of LEDs of the LED array may be adopted.

Below the image forming units 1Y, 1M, 1C, and 1K, there is arranged an intermediate transfer unit which circulates counterclockwise in the drawing while stretching the endless intermediate transfer belt 31. The transfer unit 30 includes a driving roller 32, two assist rollers 33A and 33B, a cleaning backup roller 34, four primary transfer rollers 35, a nip forming roller 36, a belt cleaning device, in addition to an intermediate transfer belt 31 which is an image carrier. 37, a secondary transfer back surface member 40, a density sensor and the like.

The intermediate transfer belt 31 is stretched around a driving roller 32, assist rollers 33A and 33B, a cleaning backup roller 34, a secondary transfer back surface member 40, and a primary transfer roller 35, which are belt stretching members arranged inside the loop. ing. Then, by the rotational force of the drive roller 32 that is rotationally driven in the counterclockwise direction in the figure, it is moved endlessly in the same direction. As the intermediate transfer belt 31, a belt having the following characteristics is used. That is, the thickness is about 20 [μm] to 200 [μm], preferably about 60 [μm]. The volume resistivity is about 1e6 [Ωcm] to 1e12 [Ωcm], preferably about 1e9 [Ωcm] (measured with Mitsubishi Chemical Hirestor UP MCP HT45 under an applied voltage of 100 V). The material is carbon-dispersed polyimide resin.

The four primary transfer rollers 35Y, 35M, 35C, and 35K sandwich the intermediate transfer belt 31 that can be moved endlessly between the photoconductors 2Y, 2M, 2C, and 2K. As a result, the respective primary transfer nips for Y, M, C, and K in which the front surface of the intermediate transfer belt 31 and the photoconductors 2Y, 2M, 2C, and 2K contact each other are formed. A primary transfer bias is applied to each primary transfer roller 35, which is a toner image forming unit. As a result, a transfer electric field is formed between each toner image of Y, M, C, and K on each photoconductor 2 and each primary transfer roller 35. The Y toner formed on the surface of the Y photoconductor 2Y enters the Y primary transfer nip as the photoconductor 2Y rotates. Then, by the action of the transfer electric field and the nip pressure, primary transfer is performed from the photoconductor 2Y onto the intermediate transfer belt 31. The portion of the intermediate transfer belt 31 to which the Y toner image is primarily transferred in this manner then sequentially passes through the respective primary transfer nips for M, C, and K. Then, the M, C, and K toner images on the photoconductors 2M, 2C, and 2K are sequentially superposed on the Y toner image and primarily transferred. By this primary transfer of superposition, a four-color superposition toner image is formed on the intermediate transfer belt 31.

The primary transfer roller 35 is an elastic roller including a metal cored bar and a conductive sponge layer fixed on the surface thereof, and has an outer diameter of 16 [mm] (diameter of the cored bar). Is 10 [mm]). A current I flowing when a voltage of 1000 [V] is applied to the core of the primary transfer roller in a state where a grounded metal roller having an outer diameter of 30 [mm] is pressed against the sponge layer with a force of 10 [N]. Therefore, the resistance R of the sponge layer calculated based on Ohm's law (R=V/I) is about 3E7Ω. A primary transfer bias is applied to such a primary transfer roller 35 by constant current control. Instead of the primary transfer roller 35, a transfer charger, a transfer brush or the like may be adopted.

The nip forming roller 36 is arranged outside the loop of the intermediate transfer belt 31, and is sandwiched between the intermediate transfer belt 31 and the secondary transfer back surface member 40 inside the loop, so that the nip forming roller 36 is provided on the front surface of the intermediate transfer belt 31. A secondary transfer nip is formed in contact with the nip forming roller 36. The secondary transfer back surface member 40 has a plate shape, and a wide nip is formed by pressing the nip forming roller 36. Further, while the nip forming roller 36 is grounded, the secondary transfer bias power supply 39 applies a secondary transfer bias to the secondary transfer back surface member 40. As a result, a secondary transfer electric field is formed between the secondary transfer back surface member 40 and the nip forming roller 36 to electrostatically move the negative polarity toner from the secondary transfer back surface member 40 side toward the nip forming roller 36 side. To be done.

The secondary transfer bias power source 39 has a DC power source 39A and an AC power source 39B, and can output a DC voltage with an AC voltage superimposed thereon as the secondary transfer bias. The set value such as the frequency of the AC component of the transfer bias applied from the secondary transfer bias power source 39 to the secondary transfer back surface member 40 is controlled according to the image area ratio on the intermediate transfer belt 31. A specific method of setting the frequency and the like will be described later, but by changing the frequency of the AC component of the transfer bias according to the transfer nip width and the sheet transport speed. By optimizing the number of reciprocations of the toner with respect to the nip time (nip passage time), it is possible to obtain the optimum concave portion transferability. If this frequency is too high, the toner group in the concave portion of the paper cannot sufficiently reciprocate, so the frequency is set to a frequency at which the toner can reciprocate in the transfer nip to prevent the transferability of the concave portion from decreasing. When the paper P having a small surface irregularity such as rough paper is used as the paper P and a paper having a small surface irregularity such as plain paper is used, a light and shade pattern following the concavo-convex pattern does not appear. A bias composed of only a DC voltage may be applied to suppress the appearance of dust that is likely to occur in a convex thin line or the like when a superimposed voltage is applied. However, when using a paper having a large surface roughness such as rough paper, it is necessary to switch the transfer bias from a DC voltage only to a superposition bias. Further, although the constant voltage is used in the present embodiment to form the electric field, the electric field may be formed by another method. For example, it is possible to obtain a predetermined effect by performing a constant current control on a DC component and applying a voltage to the AC component.

The secondary transfer back surface member 40 is a flat plate having a length of about 30 [mm] and a thickness of about 15 [mm] in the paper transport direction (recording material transport direction), and is formed of a conductive metal such as copper or aluminum. It constitutes a single-layer electrode. The surface in contact with the intermediate transfer belt 31 is uniform and smooth, and a coating layer made of fluorine-based resin or the like is provided on the surface to reduce frictional force. To reduce the frictional force, a lubricant such as silicone oil or grease may be appropriately applied to the surface. The length of the plate-shaped secondary transfer back surface member 40 that slides on the inner peripheral surface of the intermediate transfer belt 31 in the paper transport direction is longer than the contact width in the paper transport direction when the nip forming roller 36 is pressed. To secure a sufficient nip width. Therefore, it is preferably longer than the outer diameter of the nip forming roller.

The secondary transfer back surface member 40 has a flat surface 40A that uniformly contacts the back surface of the intermediate transfer belt 31 in the transfer nip N shown in FIG. The flat surface 40A and the intermediate transfer belt uniformly contact each other at the transfer nip N without any gap. That is, the flat surface 40A and the intermediate transfer belt 31 uniformly contact each other in the running direction and the width direction of the belt. Thereby, uniform pressure can be applied in the transfer nip N, and the image can be transferred uniformly.

As described above, in Patent Document 7, a transfer member corresponding to the nip forming member in the present embodiment is formed by a plurality of flat plate-shaped piezoelectric transformers, and these piezoelectric transformers are nipped so that the entire image area can be covered. A configuration is disclosed in which the surfaces are staggered. A piezoelectric transformer is a transformer that uses a piezoelectric element such as piezoelectric ceramics, and the piezoelectric element is an element that vibrates when a voltage is applied.

When transferring with a transfer bias having an AC component, it is necessary to uniformly apply an alternating voltage to the toner at the transfer nip, and it is necessary to apply a uniform pressure and electric field to the recording material passing through the transfer nip. In the configuration disclosed in the cited document 7, since the voltage applying element vibrates, a gap is generated between the transfer belt, which is an image carrier, and the element to affect the electric field, and uniform pressure cannot be applied.

In the transfer process, by applying a potential difference between the nip forming member and the opposing member, the electric field strength between the surface of the recording material and the surface of the image carrier is increased, and the charged toner particles are transferred to the recording material. In the transfer nip, in the direction from the nip forming member to the opposing member, the electric field in the gap between the image carrier and the recording material gradually increases according to the resistance of the member. When the recording material is conveyed at a high speed, the transfer nip passage time is short, so that the recording material passes through the nip before the electric field strength is saturated. Therefore, when the recording material is conveyed at a high speed, it is necessary to increase the potential difference applied between the transfer member and the facing member, as compared with the case where the conveying speed is slow and the nip time is long. In the transfer by the superposed voltage, the time average voltage Vave corresponds to the transfer voltage of only direct current, and it is necessary to increase the absolute value of Vave as the recording material conveying speed is higher. However, in the transfer by the superimposed voltage, if |Vave| is made too large, discharge is performed at the peak voltage, and a defective image is generated. Therefore, |Vave| cannot be made too large. Further, as will be described later, in the transfer by the superimposed voltage, the larger the number of reciprocation of the toner, the better the concave portion transferability to the uneven paper. Become. On the other hand, when the AC frequency is increased, the toner cannot follow the voltage change of the AC voltage, and the reciprocating motion does not occur. For these problems, it is effective to widen the nip width, but if the pressure at the secondary transfer portion is increased to widen the nip width, the problem of insect-eating images in which the central portion of the thin line comes out may occur. is there.

In the configuration of the present embodiment, the secondary transfer back surface member 40 is a plate-shaped member that rubs against the intermediate transfer belt 31. With such a configuration, it is possible to obtain a wide transfer nip width even at low pressure. Compared to the case where the transfer nip is composed of rollers and pressed against each other, the voltage peak is low and a long nip time can be secured with uniform pressure. Even when the recording material conveying speed is high, it is possible to sufficiently secure the transferability of the recording material having irregularities to the concave portions. In addition, since the secondary transfer back surface member 40 constitutes a conductive single-layer electrode and the surface contacting the intermediate transfer belt 31 is uniform and smooth, it does not vibrate like a piezoelectric element and does not move to the transfer nip. A stable and uniform AC electric field and pressure can be applied.

The outer diameter of the nip forming roller 36 is about 24 [mm], and the diameter of its core metal is about 14 [mm]. The surface of the cored bar is covered with a conductive NBR rubber layer, and its resistance R is 1E6Ω or less. The resistance R is a value measured by a method similar to that of the primary transfer roller.

In order to increase the nip width, there is a method of increasing the pressure of the nip forming roller 36, but when the rollers are in contact with each other (via the intermediate transfer belt) in the conventional configuration, the pressure in the central portion becomes extremely high. However, an image defect due to an impact when the sheet plunges into the nip, or a worm-eating image in which the central portion of a fine line such as a character is missing is likely to occur. However, if it is a flat plate, the nip width can be widened with a pressure that does not cause image defects.

In order to confirm that the nip width becomes wide, as a comparison, a comparison is shown in FIG. 3 in which a roller-shaped secondary transfer back surface member 40′ is arranged instead of the flat plate secondary transfer back surface member 40 and the assist rollers 33A and 33B. The printer of the form was used. The secondary transfer back surface member 40' is a roller member having an outer diameter of about 24 [mm] and a core metal diameter of about 16 [mm], and a surface of the core metal covered with a conductive NBR rubber layer. .. A pressure distribution measuring instrument I-Scan manufactured by Nitta Co., Ltd. was used for measuring the nip width and the pressure distribution. At this time, when the image is transferred between the first embodiment in which the secondary transfer back surface member is a flat plate and the comparative embodiment in which the secondary transfer back surface member is a roller, and the nip forming roller is pressed into contact with the maximum force that does not cause a problem. Was normally applied. In the comparative form, the comparison was made also in the case where the pressing force was increased to increase the nip width. The result is shown in FIG. In addition, when an image is formed by the pressurization force UP of the comparative form, a streak-shaped abnormal image called a shock jitter due to an impact when a thin line is dropped or a sheet is rushed. As can be seen from FIG. 4, in the comparative embodiment, the nip width at the applied pressure that gives a good image was about 3.0 mm. On the other hand, in the printer of the first embodiment of the present invention, it was about 6.0 mm. Compared to the comparative mode, the nip width is wide even if the peak of the pressing force is about the same, so that the transfer electric field by the superimposed voltage can be applied for a long nip time. Further, when the pressing force is increased to increase the nip width in the comparative form (comparative example in the figure: pressing force UP), the nip width reaches about 6.0 mm, but the peak of the pressing force is high. .. This causes an abnormal image.

In the first embodiment, the pressure applied to the secondary transfer back surface member 40 is variable by a known configuration, and the nip width during sheet passing can be adjusted. The pressing force of the nip forming roller 36 may be variable. In the transfer by the superposed voltage, the sheet conveying speed becomes high and the transfer property of the concave portion deteriorates when the nip time is short. Therefore, it is necessary to widen the nip width under the condition that the nip time is short. However, if the nip width is too long, there is a problem that misalignment is likely to occur due to fine lines or the like. Therefore, when the unevenness is large and it is necessary to enhance the transferability of the recessed portion, the transferability of the recessed portion may be suppressed by increasing the pressing force of the secondary transfer back surface member or the nip forming roller to widen the nip and reducing the unevenness. In this case, the image state may be controlled by reducing the pressure applied to the secondary transfer back surface member or the nip forming roller to narrow the nip width.

In the first embodiment, the output terminal of the secondary transfer bias power source 39 is connected to the metal portion of the secondary transfer back surface member 40. The potential of the metal portion of the secondary transfer back surface member becomes almost the same value as the output voltage value from the secondary transfer bias power source 39. The core of the nip forming roller 36 is grounded (grounded). The secondary transfer is performed while applying the superimposing bias to the metal portion of the secondary transfer back surface member 40 and applying the superimposing bias to the core metal of the nip forming roller 36 instead of grounding the core metal of the nip forming roller 36. The metal part of the back member may be grounded. In this case, the polarity of the DC voltage is made different. Specifically, as shown in the figure, when a negative polarity toner is used and the nip forming roller 36 is grounded, when a superimposing bias is applied to the secondary transfer back surface member 40, the DC voltage is the same as that of the toner. By using a negative polarity, the time average potential of the superimposed bias is set to the same negative polarity as the toner. On the other hand, when the secondary transfer back surface member 40 is grounded and a superimposing 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 superimposing 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 surface member 40 or the nip forming roller 36, a DC voltage may be applied to either one and an AC voltage may be applied to the other. In the first embodiment, as the AC voltage, a sinusoidal waveform is used as shown in FIG. 5, but any AC voltage whose size and direction change periodically may be used. A triangular waveform or a trapezoidal waveform may be used. Furthermore, it is also possible to use a waveform whose duty is not 50%. When the paper P having a small surface unevenness such as rough paper is used instead of the paper having a large surface unevenness such as rough paper, a light and shade pattern that follows the uneven pattern appears. Therefore, a transfer bias composed of only a DC voltage may be applied. However, when using a paper having a large surface roughness such as rough paper, it is necessary to switch the transfer bias from a DC voltage only to a superposition bias.

Transfer residual toner that has not been transferred to the paper P adheres to the intermediate transfer belt 31 that has passed through the secondary transfer nip. This is cleaned from the belt surface by the belt cleaning device 37 which is in contact with the front surface of the intermediate transfer belt 31. The cleaning backup roller 34 arranged 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.

In FIG. 1, below the transfer unit 30, a paper feed cassette 100 that accommodates a plurality of paper sheets P that are recording materials in a stacked state is arranged. In this paper feed cassette 100, a paper feed roller 100a is brought into contact with the uppermost paper P of the paper bundle, and the paper P is directed toward the paper feed path by rotating the paper feed roller 100a at a predetermined timing. Send out. A registration roller pair 101 is arranged near the end of the paper feed path. The registration roller pair 101 stops rotating as soon as the paper P sent from the paper feed cassette 100 is sandwiched between the rollers. Then, the sandwiched paper P is restarted at a timing in which it can be synchronized with the four-color superimposed toner image on the intermediate transfer belt 31 in the secondary transfer nip, and the paper P is sent toward the secondary transfer nip. .. The four-color superposed toner images on the intermediate transfer belt 31, which are brought into close contact with the paper P at the secondary transfer nip, are secondarily transferred onto the paper P all at once by the action of the secondary transfer electric field and the nip pressure. In combination with the white color, a full-color toner image is formed. When the sheet P having the full-color toner image formed on the surface thereof passes through the secondary transfer nip, it is curvature-separated from the nip forming roller 36 and the intermediate transfer belt 31.

A fixing device 90 is arranged on the downstream side of the secondary transfer nip in the sheet conveying direction. The fixing device 90 has a fixing roller 91 that contains a heat source such as a halogen lamp, and a pressure roller 92 that rotates while contacting the fixing roller 91 at a predetermined pressure, and forms a fixing nip. The sheet P fed into the fixing device 90 is sandwiched between the fixing nip in a posture in which the unfixed toner image bearing surface is brought into 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 full-color image is fixed. The paper P ejected from the inside of the fixing device 90 is ejected to the outside of the machine after passing through the post-fixing conveyance path.

Regarding the secondary transfer bias in the first embodiment, the secondary transfer bias, which is a superimposed bias, is applied to the metal portion of the secondary transfer back surface member 40. When a secondary transfer bias is applied to the metal portion of the secondary transfer back surface member 40, the metal portion of the secondary transfer back surface member 40 serving as the first member and the core metal of the nip forming roller 36 serving as the second member are provided. , A potential difference occurs. Therefore, the secondary transfer bias power supply 39 also functions as a potential difference generating means. Note that the potential difference is generally treated as an absolute value, but in this specification, it is treated as a value with polarity. More specifically, a value obtained by subtracting the potential of the core metal of the nip forming roller 36 from the potential of the metal portion of the secondary transfer back surface member 40 is treated as the potential difference. As for the time average value of the potential difference, in the configuration in which the toner having a negative polarity is used as in the first embodiment, when the polarity becomes negative, the potential of the nip forming roller 36 is changed to the secondary transfer back surface member 40. The potential is set to be larger than the potential on the side opposite to the charging polarity of the toner (the positive side in this embodiment). Therefore, the toner is electrostatically moved from the secondary transfer back surface member side to the nip forming roller side.

Further, in the printer of the present embodiment, as can be seen from FIG. 5, the offset voltage Voff, which is the center voltage between the maximum value and the minimum value of the voltage difference between the intermediate transfer belt and the paper, is the DC of the secondary transfer bias. The value of the component. The peak-to-peak voltage Vpp is the peak-to-peak voltage of the AC component of the secondary transfer bias, and is in the direction (transfer direction) in which the toner charged to the normal polarity is moved from the intermediate transfer belt 31 to the paper P. Let Vt be the potential difference and Vr be the potential difference in the direction of returning from the paper P to the intermediate transfer belt 31. As described above, the secondary transfer bias is a combination of the offset voltage Voff and the peak-to-peak voltage Vpp, and its time average becomes the same value as the offset voltage Voff.

Further, as already described, the secondary transfer bias is applied to the metal portion of the secondary transfer back surface member 40, and the core metal of the nip forming roller 36 is grounded (0 V). The potential of the metal part of the above becomes the potential difference of both parts as it is. The potential difference between the two parts is composed of a DC component (Vave) 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.

As shown in FIG. 5, the printer of the first embodiment employs a negative polarity offset voltage Voff. By making the polarity of the offset voltage Voff of the secondary transfer bias applied to the secondary transfer back surface member 40 negative, the toner of negative polarity is transferred from the secondary transfer back surface member 40 side to the nip forming roller in the secondary transfer nip. It becomes possible to push it relatively to the 36 side. When the polarity of the secondary transfer bias is the same negative polarity as the toner, the toner of the negative polarity is electrostatically pushed out from the secondary transfer back surface member 40 side to the nip forming roller 36 side in the secondary transfer nip. As a result, the toner on the intermediate transfer belt 31 is transferred onto the paper P. On the other hand, when the polarity of the secondary transfer bias is the positive polarity opposite to the toner, the toner of the negative polarity is directed from the nip forming roller 36 side toward the secondary transfer back surface member 40 side in the secondary transfer nip. It attracts electrostatically. As a result, the toner transferred to the paper P is attracted again to the intermediate transfer belt 31 side. However, since the value of the time average voltage of the secondary transfer bias (the same value as the offset voltage Voff in the present embodiment) has a negative polarity, the toner relatively moves from the secondary transfer back surface member 40 side to the nip forming roller 36. It is electrostatically pushed to the side. In the figure, the return potential peak value Vr indicates the peak value on the plus side, which has a polarity opposite to that of the toner.

By alternately forming the electric field while changing the polarity, the toner is reciprocated between the intermediate transfer belt as the image carrier and the paper as the recording material. The toner that comes into contact with the reciprocating toner has a weaker adhesive force with the intermediate transfer belt, and reciprocates. By repeating this operation, a sufficient amount of toner is transferred to the concave portion of the paper.

When transferring with a transfer bias in which an AC voltage is superimposed on a DC voltage, it is necessary to take care not to cause periodical image unevenness due to the AC voltage. When the process linear velocity v of the printer is 282 [mm/s], the result of examining the frequency f capable of avoiding the occurrence of pitch unevenness between the first embodiment shown in FIG. 1 and the comparative embodiment shown in FIG. Is Table 1. The results are the results of visual evaluation of the presence/absence of pitch unevenness by outputting all solid black monochromatic images on ordinary plain paper, and ◯ indicates no unevenness, and x indicates unevenness. As shown in Table 1, the printer of the first embodiment did not cause pitch unevenness even at a low frequency.

On the other hand, basically, when the number of reciprocating movements of the toner is increased, the transferability of the toner is improved. Table 2 shows an image of a concave portion of a paper when a black solid full-color image is transferred to an embossed paper called LASERK 66 260 kg manufactured by Special Paper Manufacturing Co., Ltd. by using the printer of the first embodiment and the printer of the comparison embodiment. It is the result of evaluating the density in 10 ranks of 1.0 to 5.0 (the larger the value, the better the image quality). At the time of evaluation, an AC bias of Voff-1.2 kV and Vpp 7.2 kV was used. The results of DC (-3 kV) are also shown. The higher the frequency, the higher the density of the recesses, but the density decreases when the frequency exceeds a certain frequency. That is, since the toner reciprocates in the concave portion of the paper by the electric field due to the applied voltage, the adhesive force of the untransferred toner in the concave portion is suggested and the transfer property is improved, but if the period of the applied AC electric field is too short, the toner group Cannot move back and forth because the polarity changes before it moves sufficiently. Therefore, if the frequency of the alternating-current component is too high, the transferability of the concave portions will be deteriorated. In Table 2, the transferability of the concave portions is good up to 1000 Hz, but it decreases at 1500 Hz. From this, the frequency of the AC component of the transfer bias applied between the nip forming member and the opposing member is set to a frequency at which toner can reciprocate in the transfer nip according to the nip time determined by the nip width and the transport speed. By doing so, it is possible to secure good recessed portion transferability. In addition, the recess concentration can be increased at a lower frequency in the embodiment.

Basically, the larger the adhesion amount, the larger the required Vpp and Voff. Therefore, by controlling not only the frequency but also Vpp and Voff, a better transferred image can be realized.
In addition, the reciprocating motion of the toner needs to be sufficiently generated in order to sufficiently transfer the toner to the concave portion regardless of the image area ratio. Therefore, it is desirable to set the voltage so that the relationship between the absolute value of the peak value Vr of the return voltage and the absolute value of the time average voltage Vave satisfies the following expression.
|Vr|>|Voff|

Further, the configuration in which the nip width is widened with a low pressure is not limited to the printer of the first embodiment, and the same effect can be obtained with other configurations. For example, as shown in FIG. 6, in the second embodiment, a transfer/conveyance belt 41 is provided, which is stretched by the assist rollers 37A, 37B, 37C, 37D in addition to the nip forming roller 36 and is rotationally driven. The sheet is sandwiched and conveyed between the intermediate transfer belt 31 and the intermediate transfer belt 31. The core of the nip forming roller 36 is grounded or a voltage is applied. Further, the nip forming roller 36 and the assist rollers 37A and 37D bring the intermediate transfer belt 31 and the transfer conveyance belt 41 into contact with each other to form a secondary transfer nip. As a result, the transfer/transport belt 41 transfers the toner image at the nip portion with the intermediate transfer belt while transporting the sheet. With such a configuration, it is possible to secure a wider nip width and further improve the transferability of the concave portion by the superimposed voltage transfer. The structure is the same as that of the first embodiment except for the mechanism around the transfer/transport belt 41. In this configuration, when the nip width was measured as in the first embodiment, the nip width was about 9.0 mm.

Next, the experiments actually performed by the present inventors will be described.
The present inventors have prepared a print tester having the same configuration as the printer of the first embodiment (FIG. 1), the printer of the second embodiment (FIG. 6), and the printer of the comparison embodiment (FIG. 3). Various print tests were carried out using the print tester manufactured by. The transfer bias is applied with a constant voltage for both the DC component and the AC component.

In this experiment, a full-color image was continuously output on paper A and image evaluation was performed. The evaluation criteria are the transferability of concave portions, the density reproducibility of convex portions (smooth portions), and the presence/absence of white spots caused by discharge. As the paper A, 175 kg of Resac paper (paper having a thickness of about 220 μm and a maximum unevenness difference of about 100 μm) was used.

The transferability of the recesses was evaluated as follows. That is, a case where a sufficient amount of toner entered the concave portions of the surface irregularities and sufficient image density was obtained in the concave portions was evaluated as rank 5. In addition, rank 4 was evaluated when a very small area in the concave portion was a blank area or when the image density of the concave portion was slightly lower than that of the smooth portion. The case where the whiteout area is larger than that of rank 4 or the case where the density decrease is noticeable was evaluated as rank 3. Rank 2 was evaluated when the whiteout area was larger than that of Rank 3 or when the decrease in density was noticeable. Then, when the depressions were entirely white and the state of the grooves was clearly recognizable as a whole, or when they were even worse, it was evaluated as rank 1.

The appearance of white spots due to discharge was evaluated as follows. That is, a state in which white spots, which are considered to be caused by discharge, were not recognized was evaluated as rank 5. Further, although white spots were slightly recognized, the number of recognized white spots was small and the size was also small. Therefore, a level of no problem in image quality provided to the user was evaluated as rank 4. A larger number of white spots were recognized as compared with Rank 4, and the more prominent state was evaluated as Rank 3. The case where more white spots were recognized as compared with Rank 3 was evaluated as Rank 2. White spots were observed in the entire image, and a state worse than Rank 2 was evaluated as Rank 1. In addition, white spots caused by the discharge are generated in dots, whereas when the density of the recess is very low, the entire recess becomes white.

The density reproducibility of the convex portion (smooth portion) was evaluated as follows. That is, the case where sufficient image density was obtained in the smooth portion was ranked as rank 5. The case where the density was slightly thinner than Rank 5 but had no problem was evaluated as Rank 4. It was evaluated as rank 3 when it is thinner than rank 4 and poses a problem for the image quality provided to the user. The case where the smooth portion was thinner than that of rank 3 was evaluated as rank 2, and the case where the smooth portion was whitish as a whole or thinner than that was evaluated as rank 1.

For reference, FIGS. 7 to 9 show black solid images of each rank in terms of transferability of concave portions, appearance of white spots due to discharge, and density reproducibility of convex portions (smooth portions). The acceptable level of image quality that can be provided to the user is rank 4 or higher.

The linear velocity was 282 mm/s and 564 mm/s, and the reference applied voltage value when the alternating voltage and the direct current were superposed was as follows. As the waveform of the alternating voltage, a sine wave as shown in FIG. 5 was used.
DC voltage: -1.2 [kV], AC voltage amplitude Vpp: 7.2 [kV], AC frequency: 500 Hz

Table 3 shows the evaluation results. From Table 3, in Example 1 (first embodiment) and Example 2 (second embodiment), good images were obtained regardless of the image evaluation criteria and linear velocities, while Comparative Example 1 ( In the comparative form), there was no problem at a low linear velocity, but the transferability of the recesses deteriorated at a high linear velocity.

Therefore, by using a plate-like member as the secondary transfer back surface member as in the first embodiment, the nip width is widened without extremely increasing the nip pressure, so that a sheet having large irregularities even at a high linear velocity can be obtained. It was confirmed that a good image could be obtained.

The waveform of the voltage application portion of the AC bias (voltage waveform) is not limited to the waveform used in the first embodiment, and may be a waveform whose size and direction change periodically (for example, rectangular wave AC). It is not necessary to be limited to symmetrical waves (for example, triangular wave alternating current). In the case of a sine wave, Vave, which is the time average value of the potential difference between the voltage application times of the secondary transfer back surface member 40 and the nip forming roller 36, is equal to the offset voltage Voff which is the DC component of the secondary transfer bias (FIG. 5). However, with such an AC voltage, Vt inevitably becomes larger than Vr required for the reciprocating motion, and especially when the amount of adhered toner is large or the resistance of the paper is high, a discharge mark is generated. Since a problem that occurs easily occurs, it is necessary to adjust Voff by reducing it. On the other hand, it is preferable to use the ones shown in FIGS. In such a voltage waveform, the area of the waveform on the side where the toner is returned is smaller than the area of the waveform on the side where the toner is transferred across Voff of the AC component. Further, the time average voltage (Vave) of the voltage waveform is closer to the transfer direction than the center voltage (Voff) of the maximum value and the minimum value of the voltage waveform. The present inventors have confirmed that by using such a waveform, generation of discharge marks can be suppressed without applying an excessively high Vt.

1 Image Forming Unit 31 Intermediate Transfer Belts 33A, 33B Assist Roller 35 Primary Transfer Roller 36 Nip Forming Roller 40 Secondary Transfer Backside Member

JP, 2008-185890, A JP, 2006-267486, A JP, 2008-058585, A JP, 09-146381, A Japanese Patent Laid-Open No. 04-086878 JP 2012-063746A JP, 2007-057902, A

Claims (11)

An image carrier that carries a toner image on the front surface, a nip forming member that contacts the front surface of the image carrier to form a transfer nip between the image carrier, and the nip forming member And a nip forming member for transferring the toner image on the image carrier to the recording material sandwiched in the transfer nip, and a facing member provided on the back side of the image carrier. A power source that forms a potential difference between the opposing members, and an electric field formed by the potential difference is a direction in which toner charged to a regular polarity is moved from the image carrier side to the recording material side; In an image forming apparatus comprising an electric field in the direction of returning from the material side to the image carrier side,
The facing member is a flat plate formed of a conductive metal, and has a flat surface that uniformly contacts the back surface of the image carrier at the transfer nip and extends from the transfer nip to a downstream side in the recording material conveyance direction. Then
The length in the recording material conveyance direction in which the flat surface of the facing member is in contact with the back surface of the image carrier is longer than the contact width in the recording material conveyance direction when the nip forming member is pressed. Image forming apparatus. Transfer is performed under a voltage condition in which the time average voltage (Vave) is the same as the center voltage (Voff) of the maximum value and the minimum value of the voltage difference between the nip forming member and the facing member, or is on the transfer direction side. The image forming apparatus according to claim 1, wherein: The opposing member is a plate-shaped member that slides on the image carrier, constitutes a conductive single-layer electrode, and has a uniform and smooth surface in contact with the image carrier. The image forming apparatus according to Item 1 or 2. The image forming apparatus according to claim 1, wherein a coating layer is provided on a surface of the facing member that contacts the image carrier. The image forming apparatus according to any one of claims 1 to 3, wherein a lubricant is applied to a surface of the facing member that comes into contact with the image carrier. 6. The transfer conveyance belt, which is stretched between the nip forming member and a plurality of rollers and which nips and conveys the recording material between the image carrier and the image carrier. Image forming apparatus. 7. The frequency of the AC component of the transfer bias applied between the nip forming member and the facing member is changed according to the transfer nip width and the sheet conveying speed. The image forming apparatus described. 8. The frequency of the AC component of the transfer bias applied between the nip forming member and the facing member is set to a frequency at which toner can reciprocate in the transfer nip. The image forming apparatus according to item 1. The transfer bias applied between the nip forming member and the facing member can be switched between a voltage in which a direct current component and an alternating current component are superposed and a voltage composed of only the direct current component, depending on the recording material. The image forming apparatus according to any one of claims 1 to 8. The image forming apparatus according to claim 1, wherein the pressure applied to the nip forming member or the facing member can be changed. In the voltage waveform in which the AC component and the DC component are superimposed, the time average voltage (Vave) of the superimposed voltage is on the transfer direction side of the center voltage (Voff) of the maximum value and the minimum value of the superimposed voltage. The image forming apparatus according to claim 1.