JP2014240881A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the occurrence of a gradation pattern when a toner image having a relatively low image area ratio is formed.SOLUTION: An image forming apparatus is configured to form an alternating electric field in a secondary transfer nip by using, as secondary transfer bias, one comprising superposition bias obtained by superposing AC voltage on DC voltage, and includes a control unit that is configured to perform movement frequency adjustment processing of increasing the frequency of reciprocating movement of toner in the secondary transfer nip by changing a control parameter that affects the frequency of the reciprocating movement as the image area ratio of a toner image to be formed becomes lower. In particular, the control unit increases the frequency f as the control parameter as the image area ratio becomes lower.

Description

  In the present invention, when a toner image on an image carrier is transferred to a recording sheet sandwiched in a transfer nip by contact between the image carrier and a contact member, the toner in the toner image is transferred to the surface of the recording sheet and the image carrier. The present invention relates to an image forming apparatus that reciprocates between a body surface and a body surface.

  Conventionally, as this type of image forming apparatus, the one described in Patent Document 1 is known. This image forming apparatus primarily transfers a toner image formed on the surface of a photoreceptor by a known electrophotographic process onto the surface of the intermediate transfer belt at a primary transfer nip formed by contact between the photoreceptor and the intermediate transfer belt. Thereafter, the toner image on the intermediate transfer belt is secondarily transferred to the recording sheet sandwiched in the secondary transfer nip by the contact between the intermediate transfer belt and the nip forming roller, thereby forming a toner image on the recording sheet.

  In such a configuration, when a recording sheet having rich surface irregularities such as Japanese paper is used, it becomes easy to generate a light and shade pattern in the image according to the surface irregularities. This shading pattern is generated when a sufficient amount of toner is not transferred to the concave portion on the sheet surface, and the image density of the concave portion becomes lighter than that of the convex portion. Therefore, the image forming apparatus described in Patent Document 1 includes an alternating electric field that reciprocally moves the toner between the intermediate transfer belt and the surface of the recording sheet in the secondary transfer nip as the secondary transfer electric field. By forming the pattern, the occurrence of a shading pattern is suppressed.

  As described in Patent Document 1, the reason why the generation of a light and shade pattern can be suppressed by forming a secondary transfer electric field composed of an alternating electric field is as follows. That is, the cycle of the reciprocating movement of the toner in the secondary transfer nip is synchronized with the cycle of the alternating electric field. In the first cycle of the alternating electric field after the toner image enters the secondary transfer nip, only a small part of the plurality of toner particles forming the toner layer on the surface of the intermediate transfer belt is contained. It leaves the toner layer and goes to the surface of the recording sheet. Then, after entering the concave portion on the surface of the recording sheet, it makes a U-turn as the polarity of the electric field is reversed, and returns to the toner layer on the intermediate transfer belt. At this time, the returned toner particles collide with the toner particles stopped in the toner layer, thereby urging the toner particles to leave the toner layer. Then, in the next one cycle, in addition to the toner particles that have made one reciprocation, the toner particles that have been urged to detach by the toner particles also detach from the toner layer and reciprocate. Further, when the toner particles return to the toner layer again, the toner particles collide with another toner particle stopped in the toner layer, and the separation of the toner particles from the toner layer is promoted. Thus, each time the toner particles are reciprocated, the number of toner particles that reciprocate is increased, so that a sufficient amount of toner particles are finally transferred into the recesses on the surface of the recording sheet. A good image density. As a result, the occurrence of a light and shade pattern is suppressed.

  However, even with such a configuration, it has been found by experiments of the present inventors that there is a possibility that the generation of a light and shade pattern may not be sufficiently suppressed when an image having a relatively low image area ratio is formed. Specifically, in the case of an image having a relatively low image area ratio, the amount of toner attached to the toner image may be relatively small. Then, since the number of toners that start reciprocation at first is smaller than usual, the rate of increase in the number of toner entering the recesses with the increase in the number of reciprocations becomes lower than usual. For this reason, the number of toner entering the recesses cannot be sufficiently increased in the secondary transfer nip, and a light / dark pattern is generated.

  The present invention has been made in view of the above background, and an object of the present invention is to provide an image forming apparatus capable of suppressing generation of a light and shade pattern when a toner image having a relatively low image area ratio is formed. Is to provide.

  To achieve the above object, the present invention provides an image carrier that carries a toner image, a toner image forming unit that forms a toner image on the image carrier, and a transfer nip that contacts the image carrier. And a transfer electric field forming means for forming a transfer electric field composed of an alternating electric field between the image carrier and the nip member, and a toner image on the image carrier is sandwiched between the transfer nips. In the image forming apparatus in which the toner in the toner image is reciprocated between the surface of the recording sheet and the surface of the image carrier when transferring to the recording sheet, the image of the toner image formed by the toner image forming means As the area ratio becomes lower, the number of movements adjustment processing for increasing the number of times of reciprocation is performed by changing a control parameter that affects the number of times of reciprocation of the toner in the transfer nip. In which characterized in that a control means.

  In the present invention, it is possible to suppress the occurrence of a grayscale pattern when a toner image having a relatively low image area ratio is formed.

1 is a schematic configuration diagram illustrating a printer according to an 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 bias output from a secondary transfer bias power source of the printer. FIG. 2 is a block diagram showing a part of an electric circuit of the printer. FIG. 3 is a schematic diagram illustrating an A3 size recording sheet P and a first example of a toner image formed thereon. FIG. 4 is a schematic diagram illustrating an A3-sized recording sheet P and a second example of a toner image formed thereon. The schematic block diagram which shows the observation experiment apparatus used for experiment. FIG. 4 is an enlarged schematic diagram illustrating the behavior of toner at an initial transfer stage in a secondary transfer nip. FIG. 5 is an enlarged schematic diagram illustrating the behavior of toner in the middle stage of transfer in the secondary transfer nip. Enlarged schematic diagram showing the behavior of toner in the late transfer stage in the secondary transfer nip The graph which shows the relationship between the frequency f of the alternating voltage in the printer, and the image area ratio in "50 line section". The graph which shows the relationship between the voltage value of Vpp and Voff in the same printer, and the image area rate of "50 line division". FIG. 5 is an explanatory diagram for explaining a timing for changing the target value of the AC voltage of the secondary transfer bias and a timing for changing the target value of the DC voltage. 6 is a graph showing a waveform of a first example of a transfer bias in which an average potential Vave is shifted to the toner transfer polarity side with respect to an offset voltage Voff. 6 is a graph showing a waveform of a second example of a transfer bias in which an average potential Vave is shifted to the toner transfer polarity side with respect to an offset voltage Voff. 14 is a graph showing a waveform of a third example of a transfer bias in which the average potential Vave is shifted to the toner transfer polarity side with respect to the offset voltage Voff. 14 is a graph showing a waveform of a fourth example of a transfer bias in which the average potential Vave is shifted to the toner transfer polarity side with respect to the offset voltage Voff. 10 is a graph showing a waveform of a fifth example of a transfer bias in which an average potential Vave is shifted to the toner transfer polarity side with respect to an offset voltage Voff. The graph which shows the waveform of the 6th example of the transfer bias which shifted average potential Vave to the toner transfer polarity side rather than offset voltage Voff. The graph which shows the waveform of the 7th example of the transfer bias which shifted average potential Vave to the toner transfer polarity side rather than offset voltage Voff.

Hereinafter, an embodiment of an electrophotographic color printer (hereinafter simply referred to as a printer) will be described as an image forming apparatus to which the present invention is applied. The electrical resistance values of various members described below are values in an environment of a temperature of 22 [° C.] and a relative humidity of 50 [%] unless otherwise specified.
First, a basic configuration of the printer according to the embodiment will be described. FIG. 1 is a schematic configuration diagram illustrating a printer according to an embodiment. In the figure, the printer according to the embodiment includes four image forming units 1Y, 1M, 1C, and 1K for forming yellow (Y), magenta (M), cyan (C), and black (K) toner images. It has. Further, a transfer unit 30 as a transfer device, an optical writing unit 80, a fixing device 90, a paper feed cassette 100, a registration roller pair 101, and the like are also provided.

  The four image forming units 1Y, 1M, 1C, and 1K use Y, M, C, and K toners of different colors as image forming materials, but other than that, they have the same configuration and are replaced when the lifetime is reached. Is done. Taking an image forming unit 1K for forming a K toner image as an example, as shown in FIG. 2, this includes a drum-shaped photosensitive member 2K as a latent image carrier, a drum cleaning device 3K, and a charge eliminating device (not shown). ), A charging device 6K, a developing device 8K, and the like. These devices are held by a common holding body and integrally attached to and detached from the printer main body, so that they can be exchanged at the same time.

  The photoreceptor 2K has a drum shape having an outer diameter of about 60 [mm] in which an organic photosensitive layer is formed on the surface of a drum base, and is rotated in a clockwise direction in the drawing by a driving unit (not shown). The charging device 6K generates a discharge between the charging roller 7K and the photosensitive member 2K while bringing the charging roller 7K to which a charging bias is applied into contact with or in proximity to the photosensitive member 2K, thereby making the surface of the photosensitive member 2K uniform. Charge like this. In the 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 photosensitive member 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 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, Y, M, C, and K toner replenishing means (not shown) for individually replenishing Y, M, C, and K toners in the second storage chamber of the developing device for Y, M, C, and K, respectively. Is provided. The printer control unit stores Vtref for Y, M, C, and K, which are target values of 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 the Y, M, C, K toner density detection sensor and the Vtref for Y, M, C, K exceeds a predetermined value, the Y, M, C, K toner is detected for the time corresponding to the difference. M, C, K toner supply means is driven. As a result, Y, M, C, and K toners are replenished into the second transfer chamber of the developing device for 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 electrostatically moving the K toner on the developing sleeve toward the electrostatic latent image acts between the developing sleeve and the electrostatic latent image on the photoreceptor 2K. Further, a non-developing potential that moves K toner on the developing sleeve toward the sleeve surface acts between the developing sleeve and the background portion of the photoreceptor 2K. By the action of the developing potential and the non-developing potential, the K toner on the developing sleeve is selectively transferred to the electrostatic latent image on the photoreceptor 2K, and the electrostatic latent image is developed into the K toner image.

  In FIG. 1, Y, M, and C image forming units 1Y, M, and C are also Y, M, and C toner images on the photoreceptors 2Y, M, and C in the same manner as the K image forming unit 1K. Is formed.

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

  Below the image forming units 1Y, 1M, 1C, and 1K, a transfer unit 30 is disposed as a transfer device that moves the endless intermediate transfer belt 31 endlessly in the counterclockwise direction in the drawing. . In addition to the intermediate transfer belt 31 as an image carrier, the transfer unit 31 includes a drive roller 32, a secondary transfer back roller 33, a cleaning backup roller 34, four primary transfer rollers 35Y, 35M, 35C, 35K, and a nip forming roller. 36, a belt cleaning device 37, a density sensor 38, and the like.

  The intermediate transfer belt 31 is stretched by a drive roller 32, a secondary transfer back roller 33, a cleaning backup roller 34, and four primary transfer rollers 35Y, 35M, 35C, and 35K disposed inside the loop. . 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]. Further, the volume resistivity is about 1e6 [Ωcm] to 1e12 [Ωcm], preferably about 1e9 [Ωcm] (measured with Mitsubishi Chemical Hiresta UP MCP HT45 under an applied voltage of 100 V). The material is made of carbon-dispersed polyimide resin.

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

  The primary transfer rollers 35Y, 35M, 35C, 35K are made of an elastic roller having a metal core and a conductive sponge layer fixed on the surface thereof, and have the following characteristics. Have. 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 3E7Ω. A primary transfer bias is applied to such primary transfer rollers 35Y, 35M, 35C, 35K by constant current control. In place of the primary transfer rollers 35Y, 35M, 35C, 35K, a transfer charger or a transfer brush 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 roller 33 and the nip forming roller 36 to electrostatically move the negative polarity toner from the secondary transfer back roller 33 side toward the nip forming roller 36 side. Is done.

  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. Control parameters such as the frequency f of the AC voltage in the voltage applied from the secondary transfer bias power source 39 to the secondary transfer back roller 33 are controlled according to the image area ratio on the intermediate transfer belt 31. This control will be described in detail later.

  Below the transfer unit 31, a paper feed cassette 100 that stores a plurality of recording papers P in a stacked state is disposed. 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 resumed at a timing at which the sandwiched recording paper P can be synchronized with the four-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 four-color superimposed toner image on the intermediate transfer belt 31 brought into close 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 and the nip pressure to be a full color toner. Become a statue. 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 has the following characteristics. That is, the outer diameter is about 24 [mm]. The diameter of the cored bar is about 16 [mm]. The surface of the metal core is covered with a conductive NBR rubber layer, and its resistance R is 1e6 [Ω] to 1e12 [Ω], preferably about 4E7 [Ω]. The resistance R is a value measured by the same method as that for the primary transfer roller.

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

  A separation device 150 for assisting paper separation is disposed downstream of the nip forming roller 36 in the paper conveyance direction. The separation device 150 applies a separation bias obtained by superimposing a DC voltage on an AC voltage while contacting the tip of the sawtooth-shaped static elimination needle with respect to the recording paper P fed from the secondary transfer nip. The separation of the recording paper P from the forming roller 36 is urged.

  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 of the nip forming roller 36 while applying the superimposed bias to the core of the secondary transfer back roller 33, the secondary transfer is performed while applying the superimposed bias to the core 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 bias is applied to the secondary transfer back surface roller 33 under the condition that 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 bias is set to the same negative polarity as that of the toner. On the other hand, when the secondary transfer back surface roller 33 is grounded and the superimposed bias is applied to the nip forming roller 36, a DC voltage having a positive polarity opposite to that of the toner is used, and the time average of the superimposed bias is used. Is set to a positive polarity opposite to that of the toner. Instead of applying the superimposed bias to the secondary transfer back roller 33 or the nip forming roller 36, a DC voltage may be applied to one of the rollers and an AC voltage may be applied to the other roller. As will be described later with reference to FIG. 3, a sine wave (sine wave) is adopted as the AC voltage. However, as will be described later, the waveform of the AC voltage superimposed on the DC voltage is rectangular even if it is a sine wave. It can be a wave, a triangular wave, or a trapezoidal wave. It is also possible to use a waveform whose duty 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.

  The density sensor 38 is disposed outside the loop of the intermediate transfer belt 31. In the entire area of the intermediate transfer belt 31 in the circumferential direction, the intermediate transfer belt 31 is opposed to the grounded driving roller 32 with a gap of about 4 mm. In this state, when the toner image primarily transferred onto the intermediate transfer belt 31 enters a position facing itself, the toner adhesion amount (image density) per unit area of the toner image is measured.

  A fixing device 90 is disposed downstream of the secondary transfer nip in the paper conveyance direction. 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. The recording paper P discharged from the fixing device 90 passes through a post-fixing conveyance path and is then discharged outside the apparatus. Thereafter, the sheet is discharged to a sheet discharge tray by a finisher (not shown) so that the image surface faces downward. It is security and privacy considerations that the image surface is directed downward by the finisher.

  FIG. 3 is a waveform diagram showing an example of the waveform of the secondary transfer bias composed of the superimposed bias output from the secondary transfer bias power source 39. In the figure, the secondary transfer bias is applied to the core metal of the secondary transfer back roller as described above. In addition, as described above, when the secondary transfer vice is applied to the core of the secondary transfer back roller, the core of the secondary transfer back roller as the first member and the core of the nip forming roller as the second member A potential difference occurs between the two. Therefore, the secondary transfer bias power supply 39 also functions as a potential difference generating unit. The potential difference is generally handled as an absolute value, but in this paper, 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 from the potential of the core metal of the secondary transfer back roller is treated as a potential difference. In the configuration using a negative polarity toner as in the embodiment, the time average value of the potential difference is set such that the potential of the nip forming roller is higher than the potential of the secondary transfer back roller when the polarity becomes negative. The toner is increased on the side opposite to the charged polarity of the toner (in this example, on the plus side). Therefore, the toner is electrostatically moved from the secondary transfer back roller side to the nip forming roller side.

  In the figure, the offset voltage Voff is the value of the DC component of the secondary transfer bias. The peak-to-peak voltage Vpp is the peak-to-peak voltage of the AC component of the secondary transfer bias. In the printer according to the 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. . In the printer according to the embodiment, as described above, the secondary transfer bias is applied to the core metal of the secondary transfer back roller, and the core metal of the nip forming roller is grounded (0 V). Therefore, the potential of the core metal of the secondary transfer back roller becomes the potential difference between both core bars as it is. The potential difference between the metal cores is composed of a DC component having the same value as the offset voltage Voff and an AC component having the same value as the peak-to-peak voltage Vpp.

  As shown in the figure, the printer according to the 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 the 36 side relatively. When the secondary transfer vice has the same negative polarity as the toner, the negative polarity toner is electrostatically pushed out 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. In the figure, the return potential peak value Vr indicates a positive peak value having a polarity opposite to that of the toner.

  Although this printer employs a sinusoidal characteristic as the AC voltage of the secondary transfer bias, the waveform of the AC voltage is not limited to a sine wave. A wave having a waveform different from a sine wave, such as a rectangular wave, a triangular wave, or a trapezoidal waveform, may be employed. Further, a waveform with a changed duty ratio may be used. By forming a secondary transfer electric field consisting of an alternating electric field whose polarity is inverted at a predetermined period by an AC voltage in the secondary transfer nip, toner particles are transferred between the surface of the intermediate transfer belt 31 and the surface of the recording paper P in the secondary transfer nip. Reciprocate between.

  FIG. 4 is a block diagram showing a part of the electric circuit of the printer. In the figure, a control unit 60 as control means has a CPU 60a (Central Processing Unit) as calculation means. Further, it also has a RAM 60c (Random Access Memory) as a nonvolatile memory, a ROM 60b (Read Only Memory) as a temporary storage means, a flash memory 60d, and the like. Various devices and sensors are connected to the control unit 60 that controls the entire apparatus, but only the main devices and sensors are shown in FIG.

  The primary transfer power supplies 81Y, 81M, 81C, 81K output a primary transfer bias to be applied to the primary transfer rollers 35Y, 35M, 35C, 35K. The secondary transfer power supply 39 outputs a secondary transfer vise for application to the secondary transfer back roller 33.

  The control unit 60 controls each unit based on a control program stored in the RAM 60c or the ROM 60b. Then, based on the image data, on the intermediate transfer belt, which is the sum of the image area ratio on the photoconductor at each sub-scanning position of the LD exposure for each color (Y, C, M, K) toner. The image area ratio is calculated.

  Further, the control unit 60 obtains an appropriate value of the frequency f for the AC voltage of the secondary transfer bias based on the calculated image area ratio, and makes the secondary transfer bias waveform that realizes the frequency f a secondary transfer bias waveform. The transfer power supply 39 is controlled.

  The operation panel 50 includes a touch panel (not shown) and a plurality of key buttons. The operation panel 50 displays an image on the screen of the touch panel and receives an input operation by an operator using the touch panel and the key buttons. An image can be displayed on the touch panel based on a control signal sent from the control unit 60.

  The secondary transfer power supply 39 outputs a secondary transfer bias by constant current control. Specifically, the same current as the current target value output from the control unit 60 is output. The control unit 60 calculates an image area ratio of a section that enters the secondary transfer nip in the entire circumferential region of the intermediate transfer belt 31. Specifically, the surface of the intermediate transfer belt 31 is 600 dpi 50 pixels (approximately every 7.5 ms) with respect to the top of the page in the sub-scanning direction (photosensitive member or belt surface moving direction). A theoretical division is made for each area. Each division (hereinafter referred to as “50 line division”) includes 50 pixel lines each consisting of a set of pixels arranged in a straight line in the main scanning direction. For each pixel line, the ratio of the number of pixels of the image portion (superposed toner image) to the total number of pixels is obtained as the image area ratio. Then, the average value of the image area ratios of the 50 pixel lines is obtained as the image area ratio in the “50 line section”.

  More specifically, in the entire area of the intermediate transfer belt 31, the area immediately before entering the secondary transfer nip, which is an area divided by a size of 50 pixels in the belt moving direction, the leading end of the “50 line section”. However, the control unit 60 calculates the image area ratio of the “50 line section” at a timing (hereinafter referred to as a calculation reference timing) when approaching the entrance of the secondary transfer nip to a predetermined distance. For example, the optical writing unit 80 writes the data on the Y photoconductor 2Y between a timing that is back by a predetermined second time from a timing that is back by a predetermined first time with respect to the calculation reference timing. Based on the number of dots, the Y image area ratio of the “50 line section” is calculated. In addition, the optical writing unit 80 writes data on the M photoconductor 2M between the timing retroactive by a predetermined third time and the timing retroactive by a predetermined fourth time with respect to the calculation reference timing. Based on the number of dots, the image area ratio of M of the “50 line section” is calculated. In addition, the optical writing unit 80 writes the data to the C photoconductor 2C between the timing that is back by a predetermined sixth time from the timing that is back by a predetermined fifth time with respect to the calculation reference timing. Based on the number of dots, the image area ratio of C of the “50 line section” is calculated. In addition, the optical writing unit 80 writes the data to the K photoconductor 2K between the timing that is back by a predetermined eighth time from the timing that is back by a predetermined seventh time with respect to the calculation reference timing. Based on the number of dots, the K image area ratio of the “50 line section” is calculated. A value obtained by accumulating the four image area ratios of Y, M, C, and K is set as the image area ratio of the “50 line section”. The next “50 line section” is adjacent to the downstream side in the belt movement direction of the “50 line section” for which the image area ratio has been obtained in this way. Regarding the image area ratio of the next “50 line section”, the timing at which the tip of the next “50 line section” approaches the predetermined distance from the entrance of the secondary transfer nip, that is, the next “50 line section”. The calculation of the image area ratio is started at the calculation reference timing.

  FIG. 5 is a schematic diagram showing an A3 size recording sheet P and a first example of a toner image formed thereon. In the secondary transfer nip, the recording paper P is conveyed in the direction of the arrow in the figure. In the printer according to the embodiment, the width of the intermediate transfer belt 31 in the width direction is slightly larger than the size in the short direction (297 mm) of the A3 size recording paper P. The secondary transfer nip is an area where the intermediate transfer belt 31 and the nip forming roller 36 are in contact with each other, and the length of the roller portion of the nip forming roller 36 is larger than the width of the intermediate transfer belt 31. . Therefore, the length of the secondary transfer nip in the belt width direction is the same as the width of the intermediate transfer belt 31, which is slightly larger than the short-side size of the A3-sized recording paper P as described above. However, the control unit 60 of the printer according to the embodiment regards the length of the secondary transfer nip in the belt width direction as the same as the size of the A3 size recording paper P in the short direction, for convenience. The image area ratio of the upper 50 line section is calculated. The secondary transfer nip width, which is the length of the secondary transfer nip in the belt moving direction, is 3 [mm].

  A strip-shaped toner image extending in the recording sheet conveyance direction is formed on the recording sheet P in FIG. The length in the recording paper conveyance direction is about 220 [mm], and extends over approximately half of the longitudinal direction of the recording paper P as shown in the figure. The toner image is a solid image composed of only one of the four colors Y, M, C, and K. The length of the toner image in the short direction is 29.7 [mm], which is 1/10 of the length of the secondary transfer nip in the belt width direction (considered to be 297 mm for convenience). . Therefore, in the recording paper conveyance direction, the image area ratio of the 50-line section in the region where the toner image extends is 10 [%]. Of the entire area of the intermediate transfer belt 31 in the circumferential direction, when the region carrying the illustrated toner image enters the secondary transfer nip, the image area ratio of 50 line sections is controlled to be 10 [%]. Calculated by the unit 60.

  FIG. 6 is a schematic diagram showing an A3 size recording sheet P and a second example of a toner image formed thereon. On the recording paper P shown in the drawing, two strip-shaped toner images extending in the recording paper conveyance direction are formed at a predetermined distance in a direction orthogonal to the conveyance direction. The lengths of these toner images in the recording paper conveyance direction are each about 220 [mm], and extend in the same region in the longitudinal direction of the recording paper P as shown in the figure. The two toner images are solid images composed of only different color toners. The lengths of the toner images in the short direction are 29.7 [mm], respectively. Therefore, in the recording paper conveyance direction, the image area ratio of 50 line sections in the region where the toner images extend is 20 [%]. Of the entire area of the intermediate transfer belt 31 in the circumferential direction, when an area carrying the illustrated toner image enters the secondary transfer nip, the image area ratio of 50 line sections is controlled to be 20 [%]. Calculated by the unit 60.

  In this printer, the image area ratio of the 50-line section is obtained as the sum of those calculated individually for each color of Y, M, C, and K. Therefore, for example, even if the two toner images in the figure are not independent from each other as shown in the figure, but are two-color superimposed toner images that are completely superimposed, the two-color superimposed toner images The image area ratio of the 50-line section with respect to is 20 [%] instead of 10 [%].

Next, an observation experiment conducted by the present inventors on the relationship between the toner adhesion amount of the toner image and the number of reciprocating movements of the toner particles will be described.
The present inventors manufactured a special observation experimental apparatus in order to observe the behavior of the toner in the secondary transfer nip.

  FIG. 7 is a schematic configuration diagram showing the observation experimental apparatus. 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 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. When the Z plate 220 is driven and controlled to raise the metal plate 215, the recording paper 214 starts to contact the toner layer 216. The metal plate 215 is further raised, and the metal plate 215 is approached until a certain gap is provided between the recording paper 214 and the toner layer 216. 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 is observed using a microscope 242 and a high-speed camera 243 disposed above the substrate 210. Since the substrate 210 includes all of the glass plate 211, the transparent electrode 212, and the transparent insulating layer 213 made of a transparent material, the toner on the lower side of the transparent substrate 210 through the transparent substrate 210 from above the transparent electrode 210. Can be observed.

  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.

  First, the microscope 242 is focused on the toner layer 216 on the transparent substrate 210, the DC voltage (corresponding to the offset voltage Voff in this example) is set to 200 [V], and the peak-to-peak voltage Vpp is set to 1000 [V]. The behavior of the toner was photographed under the conditions described above. Then, the following phenomenon was observed. That is, the toner particles in the toner layer 216 reciprocate between the transparent substrate 210 and the recording paper 214 due to an alternating electric field formed by the alternating current component of the transfer bias. The amount of toner particles to be increased increases.

  Specifically, in the transfer nip, every time one period (1 / f) of the AC component of the transfer bias arrives, the alternating electric field acts once and the toner particles reciprocate once. In the first period, as shown in FIG. 8, only the toner particles existing on the surface of the toner layer 216 are detached from the layer. Then, after entering the concave portion of the recording paper 216, it returns to the toner layer 216 again. At this time, the returned toner particles collide with the toner particles of the toner layer 216, thereby weakening the adhesion between the latter toner particles and the toner layer 216 or the transparent substrate 210. As a result, in the next cycle, as shown in FIG. 9, more toner particles are detached from the toner layer 216 than in the previous cycle. Then, after entering the concave portion of the recording paper 216, it returns to the toner layer 216 again. At this time, the returned toner particles collide with the toner particles still remaining in the toner layer 216, thereby weakening the adhesion between the latter toner particles and the toner layer 216 or the transparent substrate 210. As a result, in the next one cycle, as shown in FIG. 10, more toner particles are detached from the toner layer 216 than in the previous one cycle. In this way, the number of toner particles gradually increases each time they reciprocate.

Next, an experiment conducted by the present inventors will be described regarding the relationship between the toner adhesion amount per unit area of the toner image and the number of toner particles reciprocating in the transfer nip.
Since it is difficult to measure the weight of the toner constituting the toner layer 216 immediately after development or the toner that is moving back and forth, an index for checking the ratio of the toner that is moving back and forth As an example, the covered area of the toner on the transparent electrode 212 in the observation region was adopted. First, the area of the observation area and region area A _0, was measured coverage of the toner in the region area A ₀ of the toner layer 216 immediately after being developed on the transparent electrode 212 as an initial coverage A _i.

Since the transparent electrode 212 plays the role of a solid electrostatic latent image on the photoreceptor, the toner layer 216 is the same as the solid toner image, but the initial coverage area A _i is considerably smaller than the area area A _0. . That is, there is a region where the toner particles are not attached although it is solid. Even in an actual printer, when a solid toner image obtained by developing a solid electrostatic latent image is observed with a microscope before fixing, there is a region where toner particles are not attached (hereinafter referred to as a toner non-attached region). If the toner adhesion amount is normal, the toner particle adhesion area is expanded to the toner non-attached area by crushing the toner particles in the fixing step. On the other hand, when the toner adhesion amount is reduced, a part of the toner non-attached area remains even after the fixing process. The image density of the toner image changes according to the area of the remaining toner non-attached area.

Once the initial coverage area _Ai was measured, a transfer bias was applied to the metal plate 215 to transfer a portion of the toner layer 216 onto the recording paper 214. As the transfer bias, those having a frequency f = 500 [Hz], Vpp = 1.2 [kV], and Voff = 0 [V] were employed. After transfer, was measured coverage in the region area A ₀ by the remaining toner on the transparent electrode 212 as a residual coverage A _r. Thereafter, based on the following formula, the initial coverage ratio θ _i [%] of the toner layer immediately after development and the active toner ratio R _m [%], which is the ratio of the toner reciprocating in the transfer nip, are obtained. Calculated.
θ _i = (A _i / A ₀ ) × 100
R _m = [(A _i −A _r ) / A _i ] × 100

The active toner rate R _m was examined in this way for each of the plurality of toner layers 216 having different toner adhesion amounts by adjusting the developing bias. The results are shown in Table 1 below.

The initial coverage θ _i [%] in Table 1 reflects the toner adhesion amount per dot of each dot constituting the toner image. That is, the solid image has a higher initial coverage θ _i as the toner adhesion amount per dot increases. As shown in Table 1, the active toner rate R _m decreases as the initial coverage θ _i decreases. This means that when the same amount of toner particles is transferred to the concave portion of the surface of the recording paper P, the required number of reciprocating movements of the toner particles in the transfer nip increases as the toner adhesion amount per dot decreases. doing. Nevertheless, for example, when the toner adhesion amount per dot is made smaller than usual by implementing the toner save mode or the like, the toner particles are reciprocated only the same number of times as usual. As a result, a sufficient amount of toner cannot be transferred to the recesses on the surface of the recording paper P, and there is a possibility that a light and shade pattern may be generated.

  Actually, when the present inventors secondarily transfer a solid toner image having a smaller amount of toner adhesion per dot than usual to a Japanese paper type recording paper P using a printer testing machine, the normal toner adhesion amount In this case, a shading pattern that did not occur was generated.

Next, the “first transfer experiment” conducted by the present inventors will be described.
The inventors prepared a print tester having the same configuration as the printer according to the embodiment. Various print tests were performed using this print tester. In the print test, the AC voltage of the secondary transfer bias was set to Voff = −0.8 [kV] and Vpp = 5.0 [kV]. Further, the frequency f [Hz] of the AC voltage and the process linear velocity (linear velocity of the intermediate transfer belt 31 and the photoreceptor 2) were appropriately changed. Under the conditions of different frequencies f and process linear velocities, a black solid image for test was output on recording paper P made of plain paper (with almost no irregularities on the paper surface). The output black solid image was visually evaluated in two stages. Specifically, the case where the density unevenness (pitch unevenness) synchronized with the frequency f of the AC voltage is not visually recognized is evaluated as ◯, and the case where it is visually recognized is evaluated as ×. The results are shown in Table 2 below.

  As shown in Table 2, when the process linear velocity v is set to 282 [mm / s], the occurrence of pitch unevenness can be avoided by setting the frequency f of the AC voltage to 400 [Hz] or higher. I was able to. Moreover, when the process linear velocity v was set to 141 [mm / s], the occurrence of pitch unevenness could be avoided by setting the frequency f of the AC voltage to 200 [Hz] or higher. That is, when the process linear velocity v = 282 [mm / s] and 141 [mm / s], the lower limit values of the frequency f that can avoid the occurrence of pitch unevenness are 400 [Hz] and 200 [Hz], respectively. From this, it was found that the frequency f [Hz] needs to be 200 / 141≈1.42 times the process linear velocity v [mm / s]. Therefore, in order to obtain a good image without pitch unevenness, it is necessary to set the frequency f so as to satisfy the expression “f ≧ 1.42 × v”. Hereinafter, this equation is referred to as a “frequency condition equation”.

Next, the “second transfer experiment” conducted by the present inventors will be described.
As the recording paper P, embossed paper (Rezac 66 260 kg manufactured by Special Paper Industries Co., Ltd.) was used. The secondary transfer bias was output from the secondary transfer power source 39 at Voff = −1.2 [kV] and Vpp = 7.2 [kV] (constant voltage control). The frequency f of the AC voltage was changed as appropriate. Under this condition, a K single-color all-solid image is formed on embossed paper, and the image density of the concave portion on the paper surface in the obtained image is assigned in 10 ranks from 1.0 to 5.0 (the higher the numerical value, the better the image quality). It was evaluated with. Similar evaluation was performed by adopting only a DC voltage as a secondary transfer bias instead of a superimposed bias. These results are shown in Table 3 below.

  As shown in Table 3, the higher the frequency f of the alternating voltage, the higher the image density in the recesses on the paper surface. This is because as the frequency f is increased, the number of reciprocating movements of the toner particles between the belt surface and the paper surface concave portion in the secondary transfer nip is increased, and the number of toner particles transferred into the concave portion is increased. It is.

Next, the “third transfer experiment” conducted by the present inventors will be described.
As the output image, a halftone image having an area gradation rate = 25 [%] (vs. solid) and an area gradation rate = 50 [%] is used in place of the K single-color all-solid image. The “third transfer experiment” was performed in the same manner as the “2 transfer experiment”. The image density of the concave portion on the paper surface was ranked by setting the maximum density at area gradation ratio = 100% (solid image) to 5.0. The results are shown in Table 4 below.

  As shown in Table 4, when attention is paid to the same frequency f, the lower the area gradation ratio, the lower the rank of the image density of the concave portion on the paper surface. This is the same result as the correlation between the toner particle coating area and the number of reciprocating movements in the observation experiment described above.

  From Tables 3 and 4, in the case of a halftone image by area gradation, in order to achieve a certain level of image density at the concave portion on the paper surface, the frequency f is relatively high and the toner in the secondary transfer nip is It was found that the number of reciprocation of particles needs to be increased. However, as the frequency f is increased, a phenomenon called transfer dusting that causes toner to scatter in the non-image area around the image area is likely to occur. For this reason, it is not preferable to make the frequency f higher than necessary.

  Next, a characteristic configuration of the printer according to the embodiment will be described.

  FIG. 11 is a graph showing a relationship between the frequency f of the AC voltage and the image area ratio in the “50 line section” in the printer according to the embodiment. As shown in the figure, the control unit 60 performs a process of increasing the frequency f of the AC voltage of the secondary transfer bias as the image area ratio in the “50 line section” decreases. The current value of the DC voltage and the Vpp of the AC voltage are the same as those in the “third transfer experiment”, for example.

  In such a configuration, generation of a grayscale pattern when a halftone image is formed (when a toner image having a relatively low image area ratio) is suppressed while generation of transfer dust due to a relatively high frequency f is suppressed. be able to.

  Although an example in which the method of increasing the frequency f of the AC component has been described as a method for increasing the number of reciprocating movements of the toner particles in the secondary transfer nip, the frequency f is used as a parameter that affects the number of reciprocating movements. A different one may be adopted. For example, even if the frequency f is the same, the number of reciprocating movements can be increased as the process linear velocity v is decreased. Therefore, instead of the method of increasing the frequency f as the toner adhesion amount is decreased, a method of decreasing the process linear velocity v as the toner adhesion amount is decreased may be employed. In this case, it is possible to suppress the occurrence of a light and shade pattern when forming a halftone image while suppressing an increase in the image formation time due to the slow process linear velocity v.

  That is, the following effects can be achieved by performing the movement number adjustment process for increasing the number of reciprocating movements by changing the control parameter that affects the number of reciprocating movements. That is, it is possible to suppress the occurrence of a light and dark pattern when forming a toner image having a relatively low image area ratio while suppressing problems caused by setting control parameters so that the number of reciprocating movements is relatively high.

  Note that the controller 60 sets the lower limit of the frequency f of the AC voltage to 400 [Hz] in order to avoid the occurrence of pitch unevenness.

  FIG. 12 is a graph showing the relationship between the voltage values of Vpp and Voff and the image area ratio of the “50 line section” in this printer. In addition to changing the frequency f according to the image area ratio, the control unit 60 changes Vpp and Voff according to the image area ratio, as shown in the graph.

  As the amount of toner entering the secondary transfer nip increases, the values of Vpp and Voff necessary for transferring the toner from the belt to the paper surface increase. For this reason, basically, as the image area ratio increases, Vpp and Voff are increased so that a sufficient image density can be obtained at the concave and convex portions on the paper surface. However, as described above, when the image area ratio is low, it is necessary to increase the number of reciprocating movements of the toner particles in the secondary transfer nip. For this reason, as shown in the figure, Voff and Vpp are controlled so that the ratio of Vpp to Voff increases as the image area ratio decreases.

In order to transfer a sufficient amount of toner into the recesses on the paper surface, it is necessary to increase the return potential peak value Vr shown in FIG. 3 to some extent. For this reason, the control unit 60 controls Voff and Vpp so as to satisfy the following expressions.
“Vpp> 4 × | Voff |”

  FIG. 13 is an explanatory diagram for explaining the timing for changing the target value of the AC voltage of the secondary transfer bias and the timing for changing the target value of the DC voltage. These target values are also changed based on the image area ratio of 600 dpi “50 line sections” (approximately every 7.5 ms), similarly to the frequency f. While the “50 line section” grasped based on the pixel information passes through the exit portion of the secondary transfer nip, a combination of Voff, Vpp, and frequency f corresponding to the image area ratio of the “50 line section” is selected. The adopted secondary transfer bias is output from the secondary transfer power supply 39. In the figure, the output of Voff and Vpp is turned off at the timing when the image area ratio = 0%, but only a DC voltage may be output or a predetermined combination of Voff and Vpp may be output.

  In the present embodiment, the image area ratio is calculated for each “50 line section”, but the section for calculating the image area ratio may be more or less than 50 lines. In the case of decreasing the number, the section is made larger than the frequency f of the AC voltage. For example, the lower limit of the AC voltage is 400 [Hz], the period in this case is 0.0025 [s], and the paper movement distance during this period is 7.05E-4 [m]. Since this corresponds to 16.7 pixels of 600 [dpi], the lower limit of the size of the section is 17 pixels.

  When the number of sections for calculating the image area ratio is larger than 50 lines, a maximum of one page may be used as the section.

  In the printer according to the 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. In such a configuration, Vave, which is the time average value of the potential difference between the two rollers, becomes the same value as the offset voltage Voff, which is the DC component of the secondary transfer bias. Instead of grounding the core metal of the nip forming roller 36, a DC voltage may be applied to the core metal of the nip forming roller 36. In this case, the superimposed value of the DC voltage applied to the core metal of the secondary transfer back roller 33 and the DC voltage applied to the core metal of the nip forming roller 36 may be the same as the offset voltage Voff. Thus, even when a DC voltage is applied to the core metal of the nip forming roller 36 instead of grounding the core metal of the nip forming roller 36, Vave can be set to the same value as the offset voltage Voff.

As a method for generating a potential difference including a direct current component and an alternating current component between the nip forming member such as the nip forming roller 36 and the back contact member such as the secondary transfer back surface roller 33, the following six types are exemplified. can do.
(1) A superimposed bias is applied to the nip forming member, and the back contact member is grounded.
(2) A superimposed bias is applied to the nip forming member, and a DC bias is applied to the back contact member.
(3) An AC bias consisting of only an AC component is applied to the nip forming member, and a DC bias is applied to the back contact member.
(4) The nip forming member is grounded and a superimposed bias is applied to the back contact member.
(5) A DC bias is applied to the nip forming member, and a superimposed bias is applied to the back contact member.
(6) A DC bias is applied to the nip forming member, and an AC bias consisting only of an AC component is applied to the back contact member.

  When the waveform of the AC voltage is a sine wave, 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 made equal to the offset voltage Voff which is a DC component of the secondary transfer bias. (See FIG. 3). However, if Vt becomes too large with respect to Vr necessary for the reciprocating movement of the toner particles, white spots of the image due to discharge are likely to occur particularly when the amount of toner adhered is large or the resistance of the paper is high. Therefore, measures such as reducing Voff are necessary.

  On the other hand, as shown in FIG. 14 to FIG. 20, the area of the waveform on the toner return side is made smaller than the area of the waveform on the toner transfer side across Voff, thereby reducing Vt. Necessary Vr can be secured while suppressing. It is only necessary to realize waveforms in which “transfer time” and “return time” are respectively set to desired values. The transfer time is a time during which the secondary transfer bias becomes a value closer to transferring the toner to the recording paper side than the center voltage (Voff) in one cycle of the AC voltage. The return time is a time during which the secondary transfer bias takes a value closer to returning the toner to the belt side than the center voltage (Voff) in one cycle of the AC voltage. By making the ratio of the return time smaller than the transfer time, it becomes possible to increase Vave while lowering Vt, so that the occurrence of white spots can be suppressed compared to a sine wave. For example, a rectangular wave that realizes Vt = −3 [kV], Vr = + 2.0 [kV], and Duty = 16% may be employed.

What has been described above is merely an example, and the present invention has a specific effect for each of the following modes.
[Aspect A]
In the aspect A, an image carrier (for example, the intermediate transfer belt 31) that carries a toner image and toner image forming means (for example, the image forming units 1Y, 1M, 1C, and 1K) that form a toner image on the image carrier A writing unit 80), a nip forming member (for example, the nip forming roller 36) that contacts the image carrier to form a transfer nip, and a transfer electric field composed of an alternating electric field between the image carrier and the nip forming member. A recording sheet (for example, recording paper P) having a transfer electric field forming means (for example, a secondary transfer bias power source 39) formed therebetween, and sandwiching a toner image on the image carrier in the transfer nip (for example, secondary transfer nip). In the image forming apparatus in which the toner in the toner image is reciprocated between the surface of the recording sheet and the surface of the image carrier, the toner image forming unit forms a toner image. As the image area ratio of the image becomes lower, there is provided control means for performing a movement number adjustment process for increasing the number of reciprocating movements by changing a control parameter that affects the number of reciprocating movements of toner in the transfer nip. It is characterized by this.

  In such a configuration, when a toner image having a relatively low image area ratio is formed, the number of reciprocations of the toner in the transfer nip is increased by changing the control parameter, so that the concave portion on the surface of the recording sheet in the transfer nip. Further increase the number of toner particles transferred in. As a result, it is possible to suppress the occurrence of a light and shade pattern when a toner image having a relatively low image area ratio is formed.

  As described above, when the control parameter is changed so that the number of reciprocating movements of the toner in the transfer nip is relatively large, a problem may be caused accordingly. For example, when the frequency f of the AC component of the transfer bias is used as a control parameter that affects the number of reciprocating movements, transfer dust is more likely to occur as the frequency f is increased. In addition, when the process linear velocity v is adopted as the control parameter, the image forming time becomes longer as the process linear velocity v is decreased and the number of reciprocating movements is increased. Therefore, in the present invention, the control parameter is not fixed at a value that makes the number of reciprocating movements relatively high, but is changed so that the number of reciprocating movements is increased as the toner adhesion amount of the toner image is decreased. As a result, it is possible to suppress the occurrence of a light and dark pattern when forming a toner image having a relatively low image area ratio, while suppressing problems caused by fixing the control parameters as described above.

[Aspect B]
Aspect B is a process according to aspect A, in which the frequency of the reciprocating movement is increased by increasing the frequency of the alternating electric field as the control parameter in the movement frequency adjustment process as the image area ratio decreases. The control means is configured to be implemented.

[Aspect C]
Aspect C uses aspect A or B, wherein the transfer electric field forming means includes a power source that outputs a direct current bias and an alternating current bias to be applied to a member existing around the transfer nip, and the image area A bias control process is performed to change at least one of the DC bias voltage value, the DC bias current value, the AC bias voltage value, and the AC bias current value according to a rate. Further, the control means is configured.

[Aspect D]
Aspect D is that in aspect C, as the image area ratio becomes lower in the bias control process, the AC bias with respect to the absolute value of the time average value of the superimposed bias in which the AC bias is superimposed on the DC bias. The control means is configured to carry out processing for increasing the ratio of peak-to-peak voltage.

[Aspect E]
Aspect E uses any one of aspects A to D, as the transfer electric field forming means, comprising a power source that outputs a DC bias and an AC bias to be applied to members existing around the transfer nip, As a combination of the DC bias and the AC bias, the center value that is the center of the maximum value and the minimum value of the superimposed bias obtained by superimposing the AC bias on the DC bias, and the time average value of the superimposed bias are the same. And the power supply is configured to output a voltage whose peak-to-peak voltage of the AC bias is larger than four times the center value.

[Aspect F]
Aspect F uses any one of aspects A to E, as the transfer electric field forming means, comprising a power source that outputs a DC bias and an AC bias to be applied to members existing around the transfer nip, As a combination of the direct current bias and the alternating current bias, with respect to the polarity of the alternating electric field, the time to change the polarity to move the toner charged to the normal polarity from the image carrier side to the recording sheet side in the transfer nip is reversed. The power source is configured so as to output a signal that is longer than the time required to set the polarity.

1Y, M, C, K: Image forming unit (part of toner image forming means)
31: Intermediate transfer belt (image carrier)
36: Nip forming roller (nip forming member)
39: Secondary transfer bias power supply (transfer electric field forming means)
80: Optical writing unit (part of toner image forming means)
P: Recording paper (recording sheet)

JP 2012-63746 A

Claims (6)

An image carrier that carries a toner image, a toner image forming unit that forms a toner image on the image carrier, a nip forming member that forms a transfer nip in contact with the image carrier, and a transfer electric field comprising an alternating electric field A transfer electric field forming means for forming a toner image on the image carrier onto a recording sheet sandwiched between the transfer nips. In the image forming apparatus for reciprocating the toner of the recording sheet between the surface of the recording sheet and the surface of the image carrier,
As the image area ratio of the toner image formed by the toner image forming unit decreases, the number of movements is adjusted to increase the number of reciprocating movements by changing a control parameter that affects the number of reciprocating movements of toner in the transfer nip. An image forming apparatus comprising a control means for performing processing. The image forming apparatus according to claim 1.
As the image area ratio decreases, the control means performs the process of increasing the number of reciprocating movements by increasing the frequency of the alternating electric field as the control parameter in the movement number adjusting process. An image forming apparatus comprising: The image forming apparatus according to claim 1 or 2,
As the transfer electric field forming means, a device comprising a power source that outputs a DC bias and an AC bias to be applied to members existing around the transfer nip is used.
A bias control process for changing at least one of the DC bias voltage value, the DC bias current value, the AC bias voltage value, and the AC bias current value according to the image area ratio. An image forming apparatus characterized in that the control means is configured to be implemented. The image forming apparatus according to claim 3.
In the bias control process, as the image area ratio decreases, the ratio of the peak-to-peak voltage of the AC bias to the absolute value of the time average value of the superimposed bias obtained by superimposing the AC bias on the DC bias is increased. An image forming apparatus characterized in that the control means is configured to perform the processing. The image forming apparatus according to any one of claims 1 to 4,
As the transfer electric field forming means, a device comprising a power source that outputs a DC bias and an AC bias to be applied to members existing around the transfer nip is used.
As a combination of the DC bias and the AC bias, the center value that is the center of the maximum value and the minimum value of the superimposed bias obtained by superimposing the AC bias on the DC bias, and the time average value of the superimposed bias are the same. And the power supply is configured to output a voltage having a peak-to-peak voltage of the AC bias greater than four times the center value. The image forming apparatus according to claim 1,
As the transfer electric field forming means, a device comprising a power source that outputs a DC bias and an AC bias to be applied to members existing around the transfer nip is used.
As a combination of the direct current bias and the alternating current bias, with respect to the polarity of the alternating electric field, the time to change the polarity to move the toner charged to the normal polarity from the image carrier side to the recording sheet side in the transfer nip is reversed. An image forming apparatus characterized in that the power source is configured to output an output that is longer than the time required to set the polarity.

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