JP2014238445A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus that can provide sufficient image density at recesses on the surface of a recording sheet during printing at normal image density, and reduce the bleeding of character images and line images during printing at low image density.SOLUTION: An image forming apparatus transfers images in conditions in which the time average value of potentials of a transfer member and an opposing member is the same as an offset voltage that is the central value of the maximum value and the minimum value of the potentials of the transfer member and the opposing member, or on the transfer direction side with respect to the offset voltage, and controls the frequency of an AC current applied between the transfer member and the opposing member according to the set toner adhesion amount during development so that the frequency of the AC current is reduced as the set toner adhesion amount is smaller.

Description

  The present invention relates to an image forming apparatus for transferring a toner image on a surface of an intermediate transfer member to a recording material sandwiched in the nip at a transfer nip formed by contact between the intermediate transfer member and a nip forming member.

  An image forming apparatus such as an electrophotographic copying machine or a printer has been proposed that uses an AC voltage as a secondary transfer bias (Patent Document 1). Also, an apparatus that changes the output target value of the DC component or AC component of the secondary transfer bias based on image data has been proposed (Patent Document 2). According to these documents, sufficient image density is ensured on the concave portion of the recording paper surface rich in surface irregularities by using a high amplitude AC voltage that causes the toner to reciprocate between the recording material and the intermediate transfer member. it can. Furthermore, by controlling the voltage and current of the alternating current component and direct current component based on the image data, it is possible to realize a more uniform depression image density and suppression of white spots caused by discharge.

  However, when the reciprocating motion of the toner is generated by an AC electric field, a good image can be obtained in the case of a solid image, but the edge is disturbed in the case of a character image or a line image, and the image is blurred. Further, it has been found that the effect of improving the image density of the recording paper recess by the AC electric field varies depending on the toner adhesion amount, and when the toner adhesion amount is small, the image density of the recess is difficult to increase.

Next, observation experiments conducted by the present inventors will be described.
The present inventors manufactured a special observation experimental apparatus for observing the behavior of the toner in the secondary transfer nip in order to clarify the reason why the effect of the AC electric field becomes small when the toner adhesion amount is small. FIG. 11 is a schematic configuration diagram showing an observation experimental apparatus for observing the movement of toner by an alternating electric field.

  This observation experimental apparatus includes a transparent substrate 210, a developing device 231, a Z stage 220, an illumination 241, a microscope 242, a high-speed camera 243, a personal computer 244, and the like. The transparent substrate 210 includes a glass plate 211, a transparent electrode 212 made of ITO (Indium Tin Oxide) formed on the lower surface thereof, and a transparent insulating layer 213 made of a transparent material coated on the transparent electrode 212. doing. The transparent substrate 210 is supported at a predetermined height by substrate support means (not shown). This substrate support means can be moved in the vertical and horizontal directions in the drawing by a moving mechanism (not shown). In the illustrated example, the transparent substrate 210 is positioned on the Z stage 220 on which the metal plate 215 is placed. However, the movement of the substrate support means causes the developing device 231 disposed on the side of the Z stage 220 to move. It is also possible to move directly above. The transparent electrode 212 of the transparent substrate 212 is connected to an electrode fixed to the substrate support means, and this electrode is grounded.

  The developing device 231 has the same configuration as the developing device of the printer according to the first embodiment, and includes a screw member 232, a developing roll 233, a doctor blade 234, and the like. The developing roll 233 is rotationally driven in a state where a developing bias is applied by the power source 235.

  When the transparent substrate 210 is moved at a predetermined speed to a position directly above the developing device 231 and facing the developing roll 233 via a predetermined gap by the movement of the substrate support means, the toner on the developing roll 233 Is transferred onto the transparent electrode 212 of the transparent substrate 210. As a result, a toner layer 216 having a predetermined thickness is formed on the transparent electrode 212 of the transparent substrate 210. The toner adhesion amount per unit area with respect to the toner layer 216 includes the developer toner density, the toner charge amount, the developing bias value, the gap between the substrate 210 and the developing roll 233, the moving speed of the transparent substrate 210, and the rotation of the developing roll 233. The speed can be adjusted.

  The transparent substrate 210 on which the toner layer 216 is formed is translated to a position facing the recording paper 214 attached to the planar metal plate 215 with a conductive adhesive. The metal plate 215 is installed on a substrate 221 provided with a weight sensor, and the substrate 221 is installed on the Z stage 220. The metal plate 215 is connected to the voltage amplifier 217. A transfer bias composed of a DC voltage and an alternating voltage is input to the voltage amplifier 217 by the waveform generator 218, and a transfer bias amplified by the voltage amplifier 217 is applied to the metal plate 215. The Z stage 220 is driven and controlled to raise the metal plate 215, and the metal plate 215 is brought close to the recording paper 214 and the toner layer 216 until a certain gap is provided.

  With the gap width set to a predetermined value, a transfer bias is applied to the metal plate 215 to observe the behavior of the toner. After the observation, the Z stage 220 is driven and controlled, the metal plate 215 is lowered, and the recording paper 214 is separated from the transparent substrate 210. Then, a part of the toner layer 216 is transferred onto the recording paper 214.

  The behavior of the toner was observed using a microscope 242 and a high-speed camera 243 disposed above the substrate 210. Since the substrate 210 is composed of a transparent material for each layer of the glass plate 211 and the transparent electrode 212, the behavior of the toner below the transparent substrate 210 is observed from above the transparent electrode 210 through the transparent substrate 210. Can do. As the microscope 242, a zoom lens made of KEYENCE zoom lens VH-Z75 was used. As the high-speed camera 243, FASTCAM-MAX 120KC manufactured by Photoron Co. was used. Photolon FASTCAM-MAX 120KC is driven and controlled by a personal computer 244. The microscope 242 and the high-speed camera 243 are supported by camera support means (not shown). This camera support means is configured so that the focus of the microscope 242 can be adjusted.

  The behavior of the toner on the transparent substrate 210 was photographed as follows. That is, first, the illumination light irradiates the observation position of the toner behavior with the illumination 241 to adjust the focus of the microscope 242. Next, a transfer bias is applied to the metal plate 215 to move the toner of the toner layer 216 attached to the lower surface of the transparent substrate 210 toward the recording paper 214. The behavior of the toner at this time was photographed with a high-speed camera 243.

  Since it is difficult to measure the initial adhesion amount after development and the weight of the reciprocating toner, in order to examine the ratio of the reciprocating toner in this observation experiment, the coverage area of the toner on the transparent electrode 212 in the observation region is determined. used. In other words, the area of the observation area is Ao, the initial coverage area of the developing toner in the observation area is Ai, the coverage area of the toner remaining on the transparent electrode 212 in each voltage period is Ar, and the initial coverage ratio of the developed toner is θi. [%], The ratio of the reciprocating toner is represented by Rm [%]. At this time, θi and Rm are calculated by equations (1) and (2).

θi = (Ai / Ao) × 100 (1)
Rm = [(Ai−Ar) / Ai] × 100 (2)

As the applied voltage, the peak-to-peak voltage Vpp of the AC component was set to 1.2 [kV], the DC component Voff was set to 0 [V], and the frequency was set to 500 [Hz].
Table 1 shows the results of examining the relationship between the reciprocating toner ratio Rm and the number of reciprocating motions at various initial coverages θi.

  As a result, the smaller the initial toner adhesion amount, the smaller the ratio Rm of the reciprocating toner becomes smaller even if the toner reciprocates, so that the ratio Rm of the reciprocating toner decreases. Not much improved. In addition, since toner that does not reciprocate has high adhesion and is difficult to be transferred, the transfer rate at a certain number of reciprocations (= frequency of constant AC voltage) decreases as the amount of adhesion decreases.

  Therefore, the present invention provides an image formation that can obtain a sufficient image density at the concave portion of the recording paper surface when printing at a normal image density, and can reduce bleeding of a character image or a line image at the time of printing at a low image density. It is an object to provide an apparatus.

  To solve this problem, an intermediate transfer body on which a toner image on an image carrier developed by a developing device is primarily transferred, and a toner image carried on the intermediate transfer body is secondarily transferred to a transfer material. A transfer member to which a ground or a voltage is applied, a transfer member that is opposed to the transfer member through the intermediate transfer body, and that is applied with a voltage or grounded, and a transfer bias output unit. In the image forming apparatus in which a potential difference due to an alternating current is formed between the transfer member and the opposing member by the transfer bias output means, the toner formed by the potential difference is charged to a normal polarity. An electric field in a transfer direction to be transferred from the body side to the transfer material side, and an electric field in a return direction to return the toner from the transfer material side to the intermediate transfer body side, and the electric potential of the transfer member and the opposing member Time average value is The secondary transfer is performed under a condition that is equal to or equal to the offset voltage, which is the center value of the maximum value and the minimum value of the potential of the transfer member and the opposing member, and located on the transfer direction side from the offset voltage. The present invention proposes an image forming apparatus characterized in that the frequency of alternating current applied between members is controlled in accordance with the set toner adhesion amount at the time of development, and the alternating frequency is lowered as the set toner adhesion amount decreases. .

  For images with a small amount of toner set during development, the blurring of character images and line images during printing can be reduced by lowering the frequency of the AC component to reduce the number of reciprocating movements of the toner. Even when printing is performed at a normal image density, a sufficient image density can be obtained at the concave portion of the recording paper surface.

It is a schematic block diagram which shows the printer which concerns on 1st thru | or 3rd embodiment. FIG. 2 is an enlarged configuration diagram illustrating an enlarged image forming unit for K in the printer. FIG. 4 is a waveform diagram showing a waveform of a secondary transfer bias composed of a superimposed voltage output from a secondary transfer bias power source of the printer. It is an example of the alternating voltage waveform which made transfer time longer than return time. It is an example of the alternating voltage waveform which made transfer time longer than return time. It is an example of the alternating voltage waveform which made transfer time longer than return time. It is an example of the alternating voltage waveform which made transfer time longer than return time. It is an example of the alternating voltage waveform which made transfer time longer than return time. It is an example of the alternating voltage waveform which made transfer time longer than return time. It is an example of the alternating voltage waveform which made transfer time longer than return time. It is a schematic block diagram which shows the observation experiment apparatus for observing the motion of the toner by an alternating electric field.

Hereinafter, a first embodiment of an electrophotographic printer (hereinafter simply referred to as a printer) will be described as an image forming apparatus to which the present invention is applied. In addition, the resistance value of the member described below shows the value in an environment with a temperature of 22 ° C. and a relative humidity of 50% unless otherwise limited.
First, a basic configuration of the printer according to the embodiment will be described. FIG. 1 is a schematic configuration diagram illustrating the printer according to the first embodiment. In the figure, the printer according to the first embodiment includes five image forming units for forming transparent (T), yellow (Y), magenta (M), cyan (C), and black (K) toner images. 1T, 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 registration roller pair 101.

  In FIG. 1, the five image forming units 1T, Y, M, C, and K use Y, M, C, and K toners of different colors from the transparent T toner as the image forming material. It is exchanged when the life is reached. As an example, an image forming unit 1K for forming a K toner image includes a drum-shaped photosensitive member 2K as an image carrier, a drum cleaning device 3K, a charge eliminating device (not shown), as shown in FIG. A charging device 6K, a developing device 8K, and the like are provided. 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.

FIG. 2 is an enlarged configuration diagram of an image forming unit 1K for K in the printer.
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. Charge like this. In the first 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 uniformly charged surface of the 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. Then, primary transfer is performed on an intermediate transfer belt 31 described later.

  The drum cleaning device 3K removes transfer residual toner adhering to the surface of the photoreceptor 2K after the primary transfer step (primary transfer nip described later). It includes a cleaning brush roller 4K that is driven to rotate, a cleaning blade 5K that abuts the free end of the cleaning brush roller 4K in a cantilevered state, and the like. 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 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.

  The developing device 8K includes a developing unit 12K that includes the 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 of the screw members includes a rotary shaft member whose both ends in the axial direction are rotatably supported by bearings, and a spiral blade projecting in a spiral manner on the peripheral surface thereof.

  The first transfer chamber containing the first screw member 10K and the second transfer chamber containing the second screw member 11K are partitioned by a partition wall, but both ends of the partition wall in the screw axial direction. A communication port for communicating the two transfer chambers is formed at each location. 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. . Then, the first screw member 10K supplies the K developer along the axial direction to the surface of the developing roll 9K.

  After the K developer transported to the vicinity of the front side end of the first screw member 10K enters the second transport chamber through the communication opening provided near the front end of the partition wall in the figure. The second screw member 11K is held in the spiral blade. 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 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 this printer, T, Y, M, C, and K toners (not shown) for individually supplying T, Y, M, C, and K toners to the second storage chamber of the developing device for T, Y, M, C, and K, respectively. C and K toner replenishing means are provided. Then, the printer control unit stores Vtref for T, Y, M, C, and K, which are target values of output voltage values from the T, Y, M, C, and K toner density detection sensors, in the RAM. Yes. When the difference between the output voltage value from the T, Y, M, C, K toner density detection sensor and the Vtref for T, Y, M, C, K exceeds a predetermined value, the difference is determined. The T, Y, M, C, and K toner replenishing means are driven for the time. As a result, T, Y, M, C, and K toners are replenished into the second transfer chamber of the developing device for T, Y, M, C, and K.

  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 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 FIG. 1 described above, the image forming units 1T, Y, M, and C for T, Y, M, and C also have the photosensitive members 2T, Y, M, and C in the same manner as the image forming unit 1K for K. T, Y, M, and C toner images are formed on C.

  Above the image forming units 1T, 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 2T, Y, M, C, and K with laser light emitted from a laser diode based on image information sent from an external device such as a personal computer. By this optical scanning, electrostatic latent images for T, Y, M, C, and K are formed on the photoreceptors 2T, Y, M, C, and K. Specifically, in the entire area of the uniformly charged surface of the photoconductor 2T, the portion irradiated with the 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 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 with 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.

  Below the image forming units 1T, Y, M, C, and K, there is disposed a transfer unit 30 as a transfer device that endlessly moves the endless intermediate transfer belt 31 in a counterclockwise direction while stretching. ing. In addition to the intermediate transfer belt 31 that is an intermediate transfer member, the transfer unit 30 is a driving roller 32, a driven roller 38, a nip forming roller 36 that is a transfer member, and a counter member that faces the nip forming roller 36 via the intermediate transfer belt 31. Secondary transfer back roller 33, secondary transfer bias power source 39 as a transfer bias output means, cleaning backup roller 34, five primary transfer rollers 35T, Y, M, C, K, belt cleaning device 37, etc. Yes.

  The intermediate transfer belt 31 includes a driving roller 32, a driven roller 38, a secondary transfer back roller 33, a cleaning backup roller 34, and five primary transfer rollers 35T, Y, M, C, and K disposed inside the loop. It is stretched by. Then, it is moved endlessly in the same direction by the rotational force of the driving roller 32 that is driven to rotate counterclockwise in the figure by a driving means (not shown). 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 a Hiresta UP MCP HT450 manufactured by Mitsubishi Chemical Analytech under the condition of an applied voltage of 100 V). The material is made of carbon-dispersed polyimide resin.

  The five primary transfer rollers 35T, Y, M, C, and K sandwich the intermediate transfer belt 31 that is moved endlessly between the photoreceptors 2T, Y, M, C, and K. Thus, primary transfer nips for T, Y, M, C, and K where the front surface of the intermediate transfer belt 31 and the photoreceptors 2T, Y, M, C, and K abut are formed. A primary transfer bias is applied to the primary transfer rollers 35T, Y, M, C, and K by a transfer bias power source (not shown). As a result, a transfer electric field is formed between the T, Y, M, C, and K toner images on the photoreceptors 2T, Y, M, C, and K and the primary transfer rollers 35T, Y, M, C, and K. Is done. The T toner formed on the surface of the T photoconductor 2T enters the T primary transfer nip as the photoconductor 2T rotates. Then, the image is primarily transferred from the photoreceptor 2T onto the intermediate transfer belt 31 by the action of the transfer electric field or nip pressure. The intermediate transfer belt 31 on which the T toner image has been primarily transferred in this way then passes sequentially through the primary transfer nips for Y, M, C, and K. Then, Y, M, C, and K toner images on the photoreceptors 2Y, 2M, 2C, and 2K are sequentially superimposed on the T toner image and primarily transferred. By this superimposing primary transfer, a five-color superimposed toner image is formed on the intermediate transfer belt 31.

  The primary transfer rollers 35T, Y, M, C, and K are made 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, when a grounded metal roller having an outer diameter of 30 [mm] is pressed against the sponge layer with a force of 10 [N], it flows when a voltage of 1000 [V] is applied to the primary transfer roller mandrel. The resistance R of the sponge layer calculated from the current I based on Ohm's law (R = V / I) is about 3E7 [Ω]. A primary transfer bias is applied to such primary transfer rollers 35T, Y, M, C, and K by constant current control. Instead of the primary transfer rollers 35T, 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 is disposed outside the loop of the intermediate transfer belt 31, and the intermediate transfer belt 31 is sandwiched between the secondary transfer back roller 33 inside the loop. As a result, a secondary transfer nip where the front 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 surface roller 33 and the nip forming roller 36 to move the negative polarity toner from the secondary transfer back surface roller 33 side toward the nip forming roller 36 side. .

  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. A setting value such as an AC component frequency of the voltage applied from the secondary transfer bias power source 39 to the secondary transfer back roller 33 is controlled according to the image area ratio on the intermediate transfer belt 31. A specific method for setting the frequency and the like will be described later.

  Below the transfer unit 30, a paper feed cassette 100 is provided that stores a plurality of recording papers P as a material to be transferred in a stack of paper sheets. In this paper feed cassette 100, a paper feed roller 100a is brought into contact with the top recording paper P of the paper bundle, and this recording paper P is fed into the paper feed path by being driven to rotate at a predetermined timing. 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 paper P delivered from the paper feed cassette 100 is sandwiched between the rollers. Then, rotation driving is restarted at a timing at which the sandwiched recording paper P can be synchronized with the five-color superimposed toner image on the intermediate transfer belt 31 in the secondary transfer nip, and the recording paper P is directed to the secondary transfer nip. Send it out. The five-color superimposed toner image on the intermediate transfer belt 31 brought into intimate contact with the recording paper P at the secondary transfer nip is secondarily transferred onto the recording paper P by the action of the secondary transfer electric field or nip pressure. Combined with the white color of P, a full color toner image is obtained. The recording paper P having the full-color toner image formed on the surface in this manner is separated from the nip forming roller 36 and the intermediate transfer belt 31 by the curvature when passing through the secondary transfer nip.

  The secondary transfer back roller 33 includes a cored bar and a conductive NBR rubber layer coated on the surface thereof, 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 includes a cored bar and a conductive NBR rubber layer coated on the surface thereof, and its resistance R is 1E6 [Ω] or less. The resistance R is a value measured by the same method as that for the primary transfer roller.

  The output terminal of the secondary transfer bias power source 39 is connected to the core metal of the nip forming roller 36. The potential of the core metal of the nip forming roller 36 is almost the same as the output voltage value from the secondary transfer bias power source 39. Further, the core metal of the secondary transfer back roller 33 is grounded (ground connection). Instead of grounding the core metal of the nip forming roller 36 while applying the superimposed voltage to the core metal of the secondary transfer back roller 33, the secondary transfer is performed while applying the superimposed voltage to the core metal of the nip forming roller 36. The core metal of the back roller 33 may be grounded. In this case, the polarity of the DC voltage is varied. Specifically, as shown in the figure, when a superimposed voltage is applied to the secondary transfer back surface roller 33 under the condition that a negative polarity toner is used and the nip forming roller 36 is grounded, the DC voltage is the same as that of the toner. Using the polarity, the time average potential of the superimposed voltage 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 a superimposed voltage 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 voltage is used. Is set to a positive polarity opposite to that of the toner. Instead of applying the superimposed voltage to the secondary transfer back surface 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.

  In the first embodiment, a sine wave is used as the waveform of the AC voltage superimposed on the DC voltage. However, as will be described later, the waveform of the AC voltage superimposed on the DC voltage may be a sine wave, a rectangular wave, a triangular wave, or a trapezoidal wave. It is also possible to use a waveform whose duty cycle is not 50%.

  Untransferred toner that has not been transferred to the recording paper P adheres to the intermediate transfer belt 31 after passing through the secondary transfer nip. This is cleaned from the belt surface by a belt cleaning device 37 in contact with the front 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.

  A fixing device 90 is disposed downstream of the secondary transfer nip. 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 paper 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 full color image is fixed.

  In the case of single-sided printing, the recording paper P discharged from the fixing device 90 is discharged to the outside of the apparatus according to the conveyance path 1 and is usually placed on the paper discharge tray in such a manner that the image surface faces downward by a finisher (not shown). Discharged. It is security and privacy considerations that the image surface is turned downward by the finisher. On the other hand, in the case of double-sided printing, the recording paper P is reversed by the transfer material reversing mechanism 14 according to the conveyance path 2 and is printed on the back surface by passing through the secondary transfer nip again, and is discharged after being fixed by the fixing device 90. It is discharged to the paper tray.

  FIG. 3 is a diagram showing a secondary transfer bias (in the case of a sine waveform) composed of a superimposed voltage output from the secondary transfer bias power supply 39. In the figure, the offset voltage Voff [V] represents the time average value of the secondary transfer bias. As shown in the figure, the secondary transfer bias composed of the superimposed voltage has a sinusoidal shape, and has a positive peak value and a negative peak value. Of these two peak values, the sign Vt is the peak value of the one that moves toner from the belt side to the recording paper side in the secondary transfer nip (minus side in this example) ( Hereinafter referred to as “feed peak value Vt”). Also, the symbol Vr is the peak value (hereinafter referred to as “return peak value Vr”) of the toner returning from the recording paper side to the belt side (in this example, the plus side). Even if an AC voltage consisting only of an AC component is applied instead of the superimposed voltage as shown, the toner can be reciprocated between the belt and the recording paper in the secondary transfer nip. However, with AC voltage, the toner cannot be transferred onto the recording paper simply by reciprocating. By applying a superimposed voltage containing a direct current component and setting the offset voltage Voff [V], which is a time average value thereof, to the same negative polarity as that of the toner, the toner is moved back and forth while relatively moving from the belt side to the recording paper side. It is possible to transfer to the recording paper.

  The secondary transfer bias is applied to the core metal of the secondary transfer back roller 33. Further, as described above, when a secondary transfer bias is applied to the core of the secondary transfer back roller 33, there is a potential difference between the core of the secondary transfer back roller 33 and the core of the nip forming roller 36. Occur. Therefore, the secondary transfer bias power supply 39 also functions as a potential difference generating unit. Although the potential difference is generally handled as an absolute value, it is assumed here that 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 36 from the potential of the core metal of the secondary transfer back surface roller 33 is handled as a potential difference. The time average value of the potential is such that when a negative polarity toner is used as in the first embodiment, the potential of the nip forming roller 36 is opposite to the potential of the secondary transfer back roller 33 (the polarity opposite to the toner charging polarity). In this example, it is increased to the plus side). Therefore, the toner is moved from the secondary transfer back roller 33 side to the nip forming roller 36 side.

  In the figure, the offset voltage Voff is the central value of the maximum value and the minimum value of the potential difference between the nip forming roller 36 and the secondary transfer back surface roller 33, and 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, the secondary transfer bias is applied to the core of the secondary transfer back roller 33, and the core of the nip forming roller 36 is grounded (0 V). Therefore, the potential of the core metal of the secondary transfer back roller 33 becomes the potential difference between the core bars as it is. The potential difference between the two metal cores 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 the figure, the printer according to the first embodiment employs a negative polarity 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 36 side. When the secondary transfer bias has the same negative polarity as the toner, the negative polarity toner is electrostatically pushed from the secondary transfer back roller 33 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 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, since the time average value of the secondary transfer bias (in this example, the same value as the offset voltage Voff) has a negative polarity, the toner is relatively static from the secondary transfer back roller 33 side to the nip forming roller 36 side. It is pushed out electrically.

  By the way, 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 periodic image unevenness due to the AC voltage. When the process linear velocity v of the printer of the first embodiment is 282 [mm / s], it is known that the lower limit value of the frequency f that can avoid the occurrence of pitch unevenness is 400 [Hz]. Table 2 shows the result of visual evaluation of the presence or absence of pitch unevenness by outputting all solid images of K on general plain paper. ○ indicates that no unevenness is confirmed, and × indicates that unevenness is confirmed.

  On the other hand, as shown in Table 1, basically, if the number of reciprocating movements of the toner is increased, the transferability of the toner is improved. Table 3 shows the image density of the concave portions of the paper when the K single-color all-solid image was transferred to embossed paper called “Rezac 66” manufactured by Tokushu Paper Co., Ltd. using the printer of the first embodiment. The evaluation results are shown in 9 ranks of 0 (the larger the numerical value, the better the image quality). Note that an alternating voltage of −1.2 kV and Vpp of 7 kV were used for Voff. Table 3 also shows the results for DC (= -3 kV). From this, it can be confirmed that the higher the frequency, the higher the image density of the recess. Table 3 also shows the results of evaluating the bleeding when the K single-color line image is transferred to the embossed paper in 9 ranks of 1.0 to 5.0 (the higher the numerical value, the better the image quality). It can be seen that the higher the value, the worse the bleeding.

  The printer according to the first embodiment is equipped with a toner high image quality mode that is a normal print mode and a toner save mode that reduces the toner adhesion amount during development from the normal print mode in order to suppress toner consumption. Yes. As a method of the toner save mode, there is a method (for example, Patent Document 3) in which a masking pattern in dot units is overlapped with image data to obtain a logical product, and the number of pixels to be written is reduced to reduce toner consumption. Table 3 also shows the evaluation results of the recessed portion image density and bleeding in the toner save mode. Even in the toner save mode, the higher the frequency, the higher the concave image density. However, in order to obtain a sufficient image density, the frequency needs to be higher than in the normal print mode. The bleeding was the same as that in the normal printing mode.

  From the results in Table 3, it can be seen that, in the toner save mode in which the set toner adhesion amount during development is small, it is more advantageous in terms of image quality to suppress bleeding by setting the frequency low. Therefore, in the printer of the first embodiment, the frequency is controlled in accordance with the set toner adhesion amount during development, and the lower the frequency is set as the set toner adhesion amount during development decreases.

  In the printer of the second embodiment, the peak-to-peak voltage Vpp, the offset voltage Voff, or the current may be controlled according to not only the frequency but also the set toner adhesion amount during development. Basically, as the set toner adhesion amount at the time of development increases, the necessary Vpp, Voff or current also increases. Therefore, by controlling not only the frequency but also Vpp and Voff, a better transfer image is realized. However, as described above, when the set toner adhesion amount is small, the effect of the AC electric field is reduced. Therefore, Vpp is set to a low value in order to suppress bleeding. In other words, by controlling the applied voltage or current so that the ratio of Vpp to the absolute value of Voff decreases as the set toner adhesion amount decreases, the reciprocating motion of the toner is weakened to suppress bleeding of lines and characters. In addition, the transferability of the high adhesion amount image can be ensured.

On the other hand, when the set toner adhesion amount during development is large, it is necessary to sufficiently generate the reciprocating motion of the toner, and the voltage is set so that Voff and Vpp satisfy the relationship of the following equation. That is, when an AC voltage with a duty cycle of 50% such as a sine wave is used (Voff = Vave), an alternating electric field is formed with Vpp larger than four times the absolute value of Voff. By generating the reciprocating motion of the toner in this way, high transferability can be ensured.
Vpp> 4 × | Voff |

  Further, as the printer of the third embodiment, when the set toner adhesion amount during development is smaller than that in the normal printing mode, a voltage or current having only a DC component is applied without superimposing the AC component on the voltage or current. Is also effective in suppressing blurring of characters and line images. By applying only the direct current component, it is possible to secure the density at the convex portion of the recording paper and to suppress bleeding of lines and characters. When only the direct current component is applied, the image density can be improved by increasing the absolute value of the direct current voltage compared to the absolute value of the direct current component when the alternating current component is superimposed. That is, also in the third embodiment, the applied voltage or current is controlled so that the ratio of Vpp to the absolute value of Voff decreases as the set toner adhesion amount during development decreases.

  When only a DC component is applied, the current may be controlled instead of controlling the voltage. This is because if the current is controlled, it is difficult to be affected by the resistance variation of the transfer member or the recording paper.

  As described above, in the printers of the first to third embodiments, the secondary transfer bias is applied to the core of the secondary transfer back roller 33 and the core of the nip forming roller 36 is grounded. Yes. Therefore, Vave, which is the time average value of the potential between both rollers, is the same value as the offset voltage Voff, which is the DC component of the secondary transfer bias. When a DC voltage is applied to the core metal of the nip forming roller 36 without grounding, the DC voltage applied to the metal core of the secondary transfer back roller 33 and the DC voltage applied to the core metal of the nip forming roller 36 Is handled as the offset voltage Voff. That is, even when a DC voltage is applied to the core of the nip forming roller 36 instead of grounding the core of the nip forming roller 36, Vave and the offset voltage Voff have the same value.

In order to generate a potential difference including a direct current component and an alternating current component between a nip forming member such as the nip forming roller 36 and a back contact member such as the secondary transfer back surface roller 33, the following method is used. is there.
(1) A superimposed voltage is applied to the nip forming member, and the back contact member is grounded.
(2) A superimposed voltage is applied to the nip forming member, and a DC voltage is applied to the back contact member.
(3) An AC voltage consisting only of an AC component is applied to the nip forming member, and a DC voltage is applied to the back contact member.
(4) The nip forming member is grounded and a superimposed voltage is applied to the back contact member.
(5) A DC voltage is applied to the nip forming member, and a superimposed voltage is applied to the back contact member.
(6) A DC voltage is applied to the nip forming member, and an AC voltage consisting only of an AC component is applied to the back contact member.

  Further, the waveform of the AC voltage is not limited to a sine wave. In particular, in the case of a sine wave, Vave, which is a time average value of potentials at the secondary transfer back surface roller 33 and the nip forming roller 36, becomes equal to the offset voltage Voff which is a DC component of the secondary transfer bias (see FIG. 3). However, with such an AC voltage, the feed peak value Vt inevitably increases with respect to the return peak value Vr necessary for the reciprocating motion. For this reason, discharge traces are likely to occur particularly when the amount of toner adhering is large or the resistance of the recording paper is high. Therefore, it is necessary to make adjustments by reducing Voff.

  Therefore, as shown in FIGS. 4 to 10, the present inventors make the area of the waveform on the side of returning the toner sandwiching the DC component Voff smaller than the area of the waveform on the side of transferring the toner. The feed peak value Vt higher than necessary was not applied. Here, Vave, which is the time average value of the potential difference between the secondary transfer back surface roller 33 and the nip forming roller 36, is closer to the transfer direction than the offset voltage Voff.

  Here, the time on the side where the toner is transferred to the recording paper from the offset voltage Voff in one cycle of the AC voltage waveform is defined as “transfer time”. Further, the time on the side where the toner is returned to the intermediate transfer belt 31 from the offset voltage Voff in one cycle of the AC voltage waveform is defined as “return time”. The electric field formed within the transfer time is an electric field in the transfer direction in which the toner charged to the normal polarity is transferred from the intermediate transfer belt 31 side to the recording paper P side. The electric field formed within the return time is an electric field in the return direction that returns the toner from the recording paper P side to the intermediate transfer belt 31 side.

  The AC voltage in FIG. 4 is a trapezoidal wave, and the transfer time for forming the electric field in the transfer direction in one cycle is longer than the return time for forming the electric field in the return direction. The AC voltage in FIG. 5 is a rectangular wave, and the transfer time is also longer than the return time. 6 to 10 have various waveforms, the transfer time is similarly longer than the return time.

  In this way, an image better than a sine wave can be realized by the effect that the ratio of the return time in one cycle is made smaller than the transfer time, and Vve can be increased while lowering Vt. In other words, by lowering the toner return time (duty cycle), it is possible to prevent Vt from becoming unnecessarily high, and to achieve a high image density by giving a high Vave.

  For example, in the case of the printer having the configuration of the first embodiment, a rectangular wave AC voltage with Vt = −3 kV, Vr = + 2.0 kV, and return time duty cycle = 16% can be used to obtain an image without white spots. Can be obtained.

2T, Y, M, C, K photoconductor (image carrier)
8T, Y, M, C, K Developing device 31 Intermediate transfer belt (intermediate transfer member)
33 Secondary transfer back roller (opposing member)
36 Nip forming roller (transfer member)
39 Secondary transfer bias power supply (transfer bias output means)
P Recording paper (transfer material)

JP 2012-063746 A JP 2012-42835 A JP-A-7-111588

Claims (6)

An intermediate transfer body on which a toner image on the image carrier developed by the developing device is primarily transferred, and a ground or voltage for secondary transfer of the toner image carried on the intermediate transfer body to a transfer material. A transfer member to which is applied, a counter member that is opposed to the transfer member via the intermediate transfer member, and to which a voltage is applied or grounded, and a transfer bias output means,
In the image forming apparatus in which a potential difference due to an alternating current is formed between the transfer member and the opposing member by the transfer bias output unit.
The electric field formed by the potential difference includes an electric field in a transfer direction for transferring toner charged to a normal polarity from the intermediate transfer member side to the transfer material side, and the toner from the transfer material side to the intermediate transfer. An electric field in the return direction to return to the body side,
The time average value of the potentials of the transfer member and the counter member is the same as the offset voltage, which is the central value of the maximum value and the minimum value of the potential of the transfer member and the counter member, or closer to the transfer direction than the offset voltage The secondary transfer is performed under conditions, and the AC frequency applied between the transfer member and the opposing member is controlled in accordance with the set toner adhesion amount during development. An image forming apparatus characterized in that the frequency of the alternating current is lowered.   The image forming apparatus according to claim 1, wherein in addition to the AC frequency, a voltage or a current to be applied is controlled according to a set toner adhesion amount at the time of the development.   The applied voltage or current is controlled so that the ratio of the absolute value of the peak-to-peak voltage of the AC component to the absolute value of the time average value of the potential decreases as the set toner adhesion amount during the development decreases. The image forming apparatus according to claim 2.   4. The image forming apparatus according to claim 2, wherein a voltage or current having only a direct current component is applied when the set toner adhesion amount at the time of development is smaller than that in the normal printing mode. .   5. An AC voltage having a time average value of the offset voltage equal to that of the potential and having a peak-to-peak voltage of an AC component larger than four times the absolute value of the offset voltage is used. The image forming apparatus according to claim 1.   5. An AC voltage having a transfer time for forming the electric field in the transfer direction in one cycle longer than a return time for forming the electric field in the return direction is used. The image forming apparatus according to one item.

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