JP6468496B2 - Image forming apparatus - Google Patents

Description

  The present invention provides a transfer bias output for outputting a transfer bias for forming a transfer electric field in a transfer nip formed by contact between an image carrier and a nip forming member, which is composed of a superimposed bias in which an AC bias and a DC bias are superimposed. The present invention relates to an image forming apparatus including a unit.

  Conventionally, as this type of image forming apparatus, the one described in Patent Document 1 is known. In this image forming apparatus, a toner image formed on the surface of a drum-shaped photoconductor by a well-known electrophotographic process is transferred to an intermediate transfer belt, and then recorded at a secondary transfer nip formed by contact between the belt and a secondary transfer roller. Secondary transfer to the 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 light and shade pattern is generated when a sufficient amount of toner is not transferred to the concave portion on the paper surface and the image density of the concave portion becomes lighter than that of the convex portion. Therefore, in the image forming apparatus described in Patent Document 1, a superimposed bias obtained by superimposing an AC bias and a DC bias is applied to the secondary transfer roller as the secondary transfer bias. By using the superimposed bias as the secondary transfer bias, it is possible to obtain a sufficient image density in the concave portion on the surface of the recording sheet and suppress the occurrence of a light and dark pattern.

  However, the inventors of the present invention have not been able to obtain a sufficient image density at the concave portion on the surface of the recording sheet unless the peak-to-peak voltage value of the AC component of the secondary transfer bias is appropriately controlled according to the transfer influence factor. It has been found that a shading pattern is generated. The transfer influencing factors are factors that affect the transferability of the toner image from the intermediate transfer belt to the recording sheet in the secondary transfer nip. The temperature, the humidity, the thickness of the recording sheet, the depth of the recess on the sheet surface, the image Examples thereof include a toner adhesion amount per unit area on the surface of the carrier. Further, the present inventors have found that in the following configuration, a sufficient image density cannot be obtained at the concave portion of the recording sheet in a low temperature and low humidity environment. That is, the AC bias is subjected to constant voltage control or constant current control so that the peak-to-peak voltage value of the AC component becomes the same value as the predetermined output target value, and the output target value changes according to the transfer influence factor such as temperature. And a DC component is output by constant current control or constant voltage control. It has also been found that the reason why a sufficient image density cannot be obtained at the concave portion of the sheet surface in such a configuration is as follows. That is, in a low-temperature and low-humidity environment, the secondary transfer bias feed peak value (Vt in FIG. 22) becomes too large, causing reverse charging of the toner particles due to discharge in the concave portion of the recording sheet surface in the secondary transfer nip. Because. Due to the reverse charging, white spot-like white spots are generated at the concave portions on the sheet surface in the image, which causes insufficient image density on the concave portions on the sheet surface.

  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 and an image that can suppress the occurrence of a light and shade pattern that is uneven on the surface of a recording sheet regardless of the environment. It is to provide a forming method.

To achieve the above object, the present invention provides an image carrier that carries a toner image on its surface, a nip forming member that forms a transfer nip in contact with the surface, the image carrier, and the nip formation. A transfer bias output means for outputting a transfer bias for forming a transfer electric field composed of an alternating electric field with the member, and a toner image on the surface of the image carrier with respect to the recording sheet sandwiched between the transfer nips transferring the, and, the temperature sensor the transfer bias output unit, a transfer bias, an image forming apparatus and outputs a made of superimposed bias obtained by superposing an AC bias on a DC bias, to obtain the temperature information As the temperature acquired by the temperature sensor is lower, the peak-to-peak voltage value of the AC bias is increased and the DC bias is It is characterized in that to reduce the relative value.

In the present invention, the direct current flowing between the image carrier and the recording sheet in the transfer nip while changing the output target value of the peak-to-peak voltage value of the AC bias according to the transfer influence factor such as temperature and humidity. Properly control the amount of.
With such control, the peak-to-peak voltage value is reduced to a value that does not cause discharge in the concave portion of the recording sheet in the transfer nip in a high-temperature and high-humidity environment where it is not necessary to increase the peak-to-peak voltage value of the AC bias so much. An effective transfer electric field is formed between the surface of the image carrier and the projections and depressions on the surface of the recording sheet. Thereby, it is possible to obtain a sufficient image density in both the convex portion and the concave portion on the surface of the recording sheet, and to suppress the generation of the light and shade pattern of the image that is uneven on the surface of the recording sheet.
On the other hand, in a low-temperature and low-humidity environment where the peak-to-peak voltage value of the AC bias needs to be relatively large, an attempt has been made to obtain sufficient image density at both the convex and concave portions of the recording sheet surface, as already described. As described above, the feed peak value (Vt) becomes too large, and white spots due to discharge are generated in the image portion on the concave portion of the recording sheet surface. This makes the shading pattern stand out. Therefore, in the present invention, as the peak-to-peak voltage value of the AC bias is increased, the output target value of the DC bias is decreased, thereby reducing the feed peak value Vt and reducing the return peak value (Vr in FIG. 22). ) Is made larger. As a result, the return peak value (Vr) that allows the toner particles in the recesses on the surface of the recording sheet to return to the image carrier surface while keeping the feed peak value (Vt) at a value that can suppress the occurrence of white spots due to discharge. Secure. Experiments have shown that as the peak-to-peak voltage value is increased, the DC bias value can be decreased in order to cause the toner particles to leave the surface of the image carrier in the transfer nip. Therefore, even if the DC bias output target value is decreased as the peak-to-peak voltage value is increased, the image density is not insufficient at the convex portions or concave portions on the surface of the recording sheet. By reducing the output target value of the DC bias, the amount of DC current flowing through the recording sheet is reduced, so the image density is reduced at both the concave and convex portions on the surface of the recording sheet. The phenomenon that the image density of the concave portion becomes thinner than the concave portion is suppressed. Therefore, it is possible to suppress the occurrence of the light and shade pattern of the image that is uneven on the surface of the recording sheet.
As described above, according to the present invention, it is possible to suppress the generation of a light and shade pattern that is uneven on the surface of a recording sheet regardless of the environment.

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 an enlarged configuration diagram illustrating a second example of a supply form of a secondary transfer bias in the printer. FIG. 6 is an enlarged configuration diagram illustrating a third example of a supply form of a secondary transfer bias in the printer. FIG. 6 is an enlarged configuration diagram illustrating a fourth example of a supply form of a secondary transfer bias in the printer. FIG. 7 is an enlarged configuration diagram illustrating a fifth example of a secondary transfer bias supply form in the printer. FIG. 9 is an enlarged configuration diagram illustrating a sixth example of a secondary transfer bias supply form in the printer. FIG. 9 is an enlarged configuration diagram illustrating a seventh example of a supply form of a secondary transfer bias in the printer. FIG. 10 is an enlarged configuration diagram illustrating an eighth example of a supply form of a secondary transfer bias 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. The schematic block diagram which shows the transcription | transfer 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. FIG. 4 is an enlarged schematic diagram illustrating the behavior of toner at a late transfer stage in the secondary transfer nip. The graph which shows the relationship between IDmax value of a recessed part, and the frequency f of an alternating current component. The wave form diagram which shows an example of the secondary transfer bias which comprises the alternating current component which consists of a rectangular wave. FIG. 2 is a block diagram showing a part of an electric circuit of the printer according to the embodiment. The block diagram which shows a part of image forming unit 1 in the printer which concerns on a 1st modification. FIG. 9 is a configuration diagram showing a part of an image forming unit 1 and a paper transport belt unit in a printer according to a second modification. FIG. 10 is a configuration diagram illustrating a part of an image forming unit for each color and a transfer unit in a printer according to a third modification. The principal part block diagram which shows the principal part of the printer which concerns on a 4th modification.

Hereinafter, as an image forming apparatus to which the present invention is applied, before describing an embodiment of an electrophotographic color printer (hereinafter simply referred to as a printer), a printer according to a reference embodiment that is helpful in understanding the present invention will be described. explain.
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. The image forming apparatus also includes a transfer unit 30 serving 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.

  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). And a charging device 6K, a developing device 8K, and the like. The image forming units 1Y, 1M, 1C, and 1K are held on a common holder and are integrally attached to and detached from the printer main body, so that these devices are exchanged at the same time. Yes.

  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 reference form, the toner is uniformly charged to the same negative polarity as the normal charging polarity of the toner. As the charging bias, one in which an AC voltage is superimposed on a DC voltage is employed. The charging roller 7K is formed by coating a metal cored bar with a conductive elastic layer made of a conductive elastic material. Instead of a method in which a charging member such as a charging roller is brought into contact with or close to the photoreceptor 2K, a method using a charging charger may be adopted.

  The surface of the photoreceptor 2K uniformly charged by the charging device 6K is optically scanned by a laser beam emitted from an optical writing unit to 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. Then, 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, the Y, M, and C image forming units 1Y, M, and C also have Y, M, and Y on the photoreceptors 2Y, M, and C in the same manner as the K image forming unit 1K. , C toner images are 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. . The transfer unit 30 includes a driving roller 32, a secondary transfer back roller 33, a cleaning backup roller 34, and the like in addition to the intermediate transfer belt 31 serving as an image carrier. It also has four primary transfer rollers 35Y, 35M, 35C, 35K, a nip forming roller 36, a belt cleaning device 37, a potential 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 6.0 to 13 [Log Ωcm], preferably 7.5 to 12.5 [Log Ωcm] (using a HRS probe at Mitsubishi Chemical Hiresta UP MCP HT450, applied voltage 100 V, 10 msec. Measured under value conditions). Further, the surface resistance is 9.0 to 13.0 [LogΩ / □], preferably 10.0 to 12.0 [LogΩ / □] (HIRESTA-UP MCP HT450 manufactured by Mitsubishi Chemical uses an HRS probe). , Measured under conditions of an applied voltage of 500 V and a value of 10 msec).

  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 constituted by an elastic roller having a metal core and a conductive sponge layer fixed on the surface thereof. One bicycle roller 35Y, 35M, 35C, 35K is shifted to the downstream side in the belt movement direction by about 2.5 [mm] with respect to the shaft center of each of the photoreceptors 2Y, 2M, 2C, and 2K. It is arranged. 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 N in which the front surface of the intermediate transfer belt 31 and the nip forming roller 36 are in contact with each other is formed. While the nip forming roller 36 is grounded, a secondary transfer bias is applied to the secondary transfer back roller 33 by a secondary transfer bias power source 39. As a result, a secondary transfer electric field is formed between the secondary transfer back roller 33 and the nip forming roller 36 to electrostatically move the negative polarity toner from the secondary transfer back roller 33 side toward the nip forming roller 36 side. Is done.

  Below the transfer unit 31, a paper feed cassette 100 that stores a plurality of recording sheets P in a bundle of sheets is disposed. In this paper feed cassette 100, a paper feed roller 100a is brought into contact with the top recording sheet P of the paper bundle, and this recording sheet P is fed to the paper feed path by being rotated 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 sheet P fed from the paper feed cassette 100 is sandwiched between the rollers. Then, rotation driving is restarted at a timing at which the sandwiched recording sheet P can be synchronized with the four-color superimposed toner image on the intermediate transfer belt 31 in the secondary transfer nip N, and the recording sheet P is moved to the secondary transfer nip N. Send it out. The four-color superimposed toner image on the intermediate transfer belt 31 brought into close contact with the recording sheet P at the secondary transfer nip N is collectively transferred onto the recording sheet P by the action of the secondary transfer electric field or nip pressure, and recorded. Combined with the white color of the sheet P, a full color toner image is obtained. When the recording sheet P having a full-color toner image formed on the surface in this way passes through the secondary transfer nip N, the recording sheet P is separated from the nip forming roller 36 and the intermediate transfer belt 31 by curvature.

  The secondary transfer back roller 33 includes a cored bar and a conductive elastic layer coated on the surface thereof. The secondary transfer back roller 33 has the following characteristics. That is, the outer diameter is 20 to 24 [mm]. The diameter of the cored bar is about 16 [mm]. The resistance R of the conductive elastic layer coated on the surface of the cored bar is 1E6 [Ω] to 2E7 [Ω]. The resistance R is a value measured by the same method as that for the primary transfer roller. Further, the resistance of the secondary transfer back roller 33 is 6.0 to 12.0 [LogΩ], preferably 4.0 [LogΩ]. As the secondary transfer back roller 33, a stainless roller without a conductive elastic layer may be used. The resistance of the secondary transfer back roller 33 is measured as follows. That is, a voltage of 1 [kV] is applied to the roller shaft while applying a load of 5 [N] to both ends in the longitudinal direction of the roller. Then, the resistance is measured a plurality of times while the roller is rotated once per minute to obtain an average value.

  The nip forming roller 36 has the following characteristics. That is, the outer diameter is about 24 [mm]. The diameter of the cored bar is about 14 [mm]. The surface of the metal core is covered with a conductive NBR rubber layer, and its resistance R is 1E6Ω or less. The resistance R is a value measured by the same method as that for the primary transfer roller. The resistance of the nip forming roller 36 is 6.0 to 8.0 [LogΩ], preferably 7.0 to 8.0 [LogΩ]. This resistance is measured by the same method as that for the secondary transfer back roller 33.

  The secondary transfer bias power source 39 outputs a secondary transfer bias for forming a transfer electric field in the secondary transfer nip N. In the example shown in the figure, a secondary transfer bias composed of a superimposed bias in which an AC voltage is superimposed on a DC voltage is output. The output terminal of the secondary transfer bias power source 39 is connected to the core metal of the secondary transfer back surface roller 33. The potential of the metal core of the secondary transfer back roller 33 is almost the same as the output voltage value from the secondary transfer bias power supply 39. Further, the core metal of the nip forming roller 36 is grounded (ground connection).

  The secondary transfer bias power supply 39 outputs a secondary transfer bias DC voltage having the same polarity as the toner charging polarity. The secondary transfer bias output from the secondary transfer bias power supply 39 is Applied to the core of the secondary transfer back roller 33. Between the secondary transfer back surface roller 33 and the nip forming roller 36, the toner on the intermediate transfer belt 31 is electrostatically moved from the secondary transfer back surface roller 33 side to the nip forming roller 36 side, and the recording sheet P is collectively 2 Next is transferred.

  The supply form of the secondary transfer bias is not limited to the form shown in FIG. For example, as shown in FIG. 3, the secondary transfer back roller 33 may be grounded while the secondary transfer bias output from the secondary transfer bias power supply 39 is applied to the core of the nip forming roller 36. In this case, a DC voltage having the opposite polarity to the charging polarity of the toner is output as the DC voltage in the secondary transfer bias. FIG. 3 shows an example in which the charging polarity of the toner is a negative polarity and the DC voltage of the secondary transfer bias is a positive polarity opposite to the charging polarity of the toner.

  Further, as shown in FIG. 4, an AC voltage output from the secondary transfer bias power supply 39 is applied to the core of the secondary transfer back roller 33, while a DC voltage output from the secondary transfer bias power supply 39. May be applied to the core of the nip forming roller 36. In this case, as in the example shown in FIG. 3, the secondary transfer bias power supply 39 outputs a DC voltage whose polarity is opposite to the charging polarity of the toner.

  Further, as shown in FIG. 5, the AC voltage output from the secondary transfer bias power supply 39 is applied to the core of the nip forming roller 36, while the DC voltage output from the secondary transfer bias power supply 39 is 2 You may apply to the core metal of the next transfer back roller 33. In this case, a DC voltage having the same polarity as the charging polarity of the toner is output from the secondary transfer bias power supply 39.

  Further, as shown in FIG. 6, a circuit for outputting a superimposed voltage obtained by superimposing a DC voltage on an AC voltage and a circuit for outputting only a DC voltage are provided in the secondary transfer bias power source 39, and as follows. Also good. That is, any one of these two circuits may be selected by the switching circuit and applied to the secondary transfer back roller 33. In this case, the DC voltages in the two circuits are output with the same polarity as the charging polarity of the toner.

  In addition, as shown in FIG. 7, either one of the two circuits described above may be selected by a switching circuit and applied to the nip forming roller 36. In this case, both of the DC voltages in the two circuits are output with a polarity opposite to the charging polarity of the toner.

  As described above, various forms of supply of the secondary transfer bias can be adopted. However, when a plain paper having no surface irregularities is used as the recording sheet P, the secondary transfer consisting only of DC voltage is used. Even if a bias is used, image density unevenness is not caused. Therefore, in this printer, a secondary transfer bias power source 39 that can switch between a first mode that outputs only a DC voltage and a second mode that outputs both a DC voltage and an AC voltage is used. As a configuration for switching, for example, as illustrated in FIG. 8 and FIG. 9, a configuration in which output on / off is controlled by a relay switch for each of a DC voltage and an AC voltage can be exemplified.

  In the example shown in FIG. 8, the secondary transfer bias power source 39 outputs a superimposed voltage circuit for outputting a superimposed voltage applied to the secondary transfer back surface roller 33 and a DC voltage applied to the nip forming roller 36. A DC voltage circuit. The relay switch connected to the secondary transfer back roller 33 makes only one of the superimposed voltage circuit and the ground conductive by switching the contact. Further, the relay switch connected to the nip forming roller 36 conducts only one of the DC voltage circuit and the ground by switching the contact. However, the former relay switch and the latter relay switch are interlocked with each other. When the former relay switch conducts the superimposed voltage circuit to the secondary transfer back roller 33, the latter relay switch forms a nip. The roller 36 is grounded. The DC component of the superimposed voltage output from the superimposed voltage circuit has the same polarity as the charging polarity of the toner. Further, when the former relay switch conducts the ground with respect to the secondary transfer back surface roller 33, the latter relay switch conducts the DC voltage circuit to the nip forming roller 36. The DC voltage output from the DC voltage circuit has a polarity opposite to the charging polarity of the toner.

  In the example shown in FIG. 9, the secondary transfer bias power supply 39 outputs a DC voltage circuit for outputting a DC voltage applied to the secondary transfer back roller 33 and a superimposed voltage applied to the nip forming roller 36. And a superimposed voltage circuit. The relay switch connected to the secondary transfer back roller 33 conducts only one of the DC voltage circuit and the ground by switching the contact. The relay switch connected to the nip forming roller 36 conducts only one of the superimposed voltage circuit and the ground by switching the contact. However, the former relay switch and the latter relay switch are interlocked with each other. When the former relay switch conducts the DC voltage circuit to the secondary transfer back surface roller 33, the latter relay switch forms a nip. The roller 36 is grounded. The DC voltage output from the DC voltage circuit has the same polarity as the charging polarity of the toner. In addition, when the former relay switch conducts the ground to the secondary transfer back surface roller 33, the latter relay switch conducts the superimposed voltage circuit to the nip forming roller 36. The DC component of the superimposed voltage output from the superimposed voltage circuit has the same polarity as the charging polarity of the toner.

  When the recording sheet P is not a sheet with large surface irregularities such as rough paper but a sheet with small surface irregularities such as plain paper, a shading pattern that follows the uneven pattern does not appear. Therefore, in this case, the secondary transfer bias power supply 39 executes the first mode and outputs a secondary transfer bias composed only of a DC voltage. On the other hand, when a recording sheet P having a large surface unevenness such as rough paper is used, the second mode is implemented and the recording sheet P is composed of both a DC voltage and an AC voltage as a secondary transfer bias. Is output.

  Untransferred toner that has not been transferred to the recording sheet P adheres to the intermediate transfer belt 31 after passing through the secondary transfer nip N. 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 on the right side of the secondary transfer nip N in the figure. 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 sheet P fed into the fixing device 90 is sandwiched between the fixing nips in a posture in which 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 sheet P discharged from the fixing device 90 passes through a post-fixing conveyance path and is then discharged outside the apparatus.

  In this printer, when a monochrome image is formed, a support plate (not shown) supporting primary transfer rollers 35Y, 35M, 35C for Y, M, C in the transfer unit 30 is moved to perform primary transfer. The rollers 35Y, M, C, and K are moved away from the photoreceptors 2Y, M, and C. As a result, the front surface of the intermediate transfer belt 31 is separated from the photoconductors 2Y, 2M, and 2C, and the intermediate transfer belt 31 is brought into contact with only the K photoconductor 2K. In this state, of the four image forming units 1Y, 1M, 1C, and 1K, only the K image forming unit 1K is driven to form a K toner image on the photoreceptor 2K.

  FIG. 10 is a waveform diagram showing the waveform of the secondary transfer bias composed of the superimposed voltage output from the secondary transfer bias power supply 39. This secondary transfer bias is applied to the core of the secondary transfer back roller 33, and the nip forming roller 36 is grounded (the mode shown in FIG. 1 or FIG. 8). When this secondary transfer vice is applied to the core of the secondary transfer back roller, a potential difference is generated between the core of the secondary transfer back roller 33 and the core of the nip forming roller.

  In the figure, the offset voltage Voff is the value of the DC component of the superimposed voltage. Vpp is a peak-to-peak voltage value of the alternating current component of the superimposed voltage. Since the waveform of the superimposed voltage is a sine wave, the duty ratio of the AC component is 50 [%]. For this reason, the time average value of the superimposed voltage is the same value as the offset voltage Voff. In the figure, the offset voltage Voff has the same negative polarity as the charging polarity of the toner. In this printer, since the polarity of the offset voltage Voff of the secondary transfer bias applied to the secondary transfer back surface roller 33 is negative, the toner particles in the secondary transfer nip N are on the secondary transfer back surface roller 33 side. To the nip forming roller 36 side. However, the toner is not pushed out but may be pulled back to the secondary transfer back roller 33 side. However, since the time average potential is negative polarity, the toner particles are relatively pushed out to the nip forming roller 36 side. To go.

  In the figure, the return potential peak value Vr indicates a positive peak value having a polarity opposite to that of the toner in the secondary transfer bias. The feed peak value Vt indicates a negative peak value having the same polarity as that of the toner in the secondary transfer bias.

Next, a transfer experiment conducted by the present inventors will be described.
Cited Document 1 does not describe the principle that a sufficient image density can be obtained in the concave portion on the surface of the recording sheet by using a secondary transfer bias composed of a superimposed bias. Therefore, the present inventors conducted a transcription experiment to elucidate the principle. Therefore, a secondary transfer bias that forms an alternating electric field in the secondary transfer nip N is used. Then, in order to clarify the cause of obtaining a sufficient image density at the concave portion on the surface of the recording sheet and making the shading pattern that is uneven on the paper surface inconspicuous, a special transfer experiment apparatus was produced.

  FIG. 11 is a schematic configuration diagram showing the transfer experiment apparatus. This transfer experiment 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 210 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 reference 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 sheet 214 bonded 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. When the metal plate 215 is further raised, the pressure on the toner layer 216 increases, but the rise of the metal plate 215 is stopped so that the output from the weight sensor becomes a predetermined value. With the pressure 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, 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 toner behavior is 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 sheet 214. The behavior of the toner at this time is photographed by the high speed camera 243.

Since the transfer experiment apparatus shown in FIG. 11 and the printer according to the embodiment have different transfer nip structures for transferring toner to a recording sheet, the transfer electric field acting on the toner is different even if the transfer bias is the same. . In order to investigate the appropriate observation conditions, the transfer bias conditions that can provide good concave portion density reproducibility were also examined using a transfer experiment apparatus. As the recording sheet 214, what is called FC Japanese paper type “Sazanami” manufactured by NBS Ricoh Co., Ltd. was used. As the toner, a Y toner having an average particle diameter of 6.8 [μm] mixed with a small amount of K toner was used. Since the transfer experiment apparatus is configured to apply a transfer bias to the back surface of the recording sheet, the polarity of the transfer bias capable of transferring the toner to the recording sheet is opposite to that of the printer according to the embodiment (that is, , Positive polarity). As the alternating current component of the transfer bias composed of the superimposed voltage, a waveform having a sine wave was adopted. The frequency f of the AC component is set to 1000 [Hz], the DC voltage (corresponding to the offset voltage Voff in this example, the time average value is the same value) is set to 200 [V], and the peak-to-peak voltage value Vpp is set to 1000 [V]. did. Then, the toner layer 216 was transferred to the recording sheet 214 with a toner adhesion amount of 0.4 to 0.5 [mg / cm ² ]. As a result, a sufficient image density could be obtained on the concave portion on the surface of the “ripple”.

  At that time, the focus of the microscope 242 was adjusted to the toner layer 216 on the transparent substrate 210 to photograph the behavior of the toner. 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 sheet 214 due to an alternating electric field formed by an alternating current component of the transfer bias. The amount of toner particles to increase.

  Specifically, at the transfer nip, every time one period (1 / f) of the AC component of the transfer bias (secondary transfer bias in the actual machine) arrives, the alternating electric field acts once and the toner particles are applied once. Move back and forth. In the first period, as shown in FIG. 12, only the toner particles present on the surface of the toner layer 216 are detached from the layer. Then, after entering the concave portion of the recording sheet 214, 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. 13, more toner particles are detached from the toner layer 216 than in the previous cycle. Then, after entering the concave portion of the recording sheet 214, 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. 14, 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. Then, it was found that when the nip passage time has elapsed (when the time corresponding to the nip passage time has elapsed in the transfer experiment apparatus), a sufficient amount of toner has been transferred into the concave portion of the recording sheet 214.

  Next, when the behavior of the toner was photographed under the condition that the DC voltage was set to 200 [V] and the peak-to-peak voltage value Vpp of the AC voltage was set to 800 [V], the following phenomenon was observed. It was. That is, among the toner particles in the toner layer 216, those existing on the surface of the toner layer detach from the layer in the first one cycle and enter the concave portion of the recording sheet P. However, the toner particles that entered entered the recesses without going to the toner layer 216. When the next one cycle arrived, very few toner particles were newly detached from the toner layer 216 and entered into the recesses of the recording sheet 214. Therefore, when the nip passage time has elapsed, only a small amount of toner particles have been transferred into the recesses of the recording sheet P.

  The present inventors conducted further observation experiments. The return peak value Vr at which the toner particles that have entered the concave portion of the recording sheet 214 from the toner layer 216 in the first period can be returned to the toner layer 216 is a unit area on the transparent substrate 210. It was found that it depends on the toner adhesion amount. As the toner adhesion amount on the transparent substrate 210 increases, the return peak value Vr at which the toner particles in the recesses of the recording sheet 214 can be pulled back to the toner layer 216 increases.

  As described above, by using a secondary transfer bias having an AC component and a DC component, a sufficient image density can be obtained on the concave portion of the recording sheet 214. However, unless the peak-to-peak voltage value Vpp of the AC component of the secondary transfer bias is appropriately controlled in accordance with the transfer influence factor, a sufficient light density cannot be obtained at the concave portion on the surface of the recording sheet 214, and a light and shade pattern is generated. I also found out. The transfer influencing factors are factors that affect the transferability of the toner image from the intermediate transfer belt to the recording sheet in the secondary transfer nip, and include temperature, humidity, recording sheet thickness, depth of recesses on the sheet surface, belt Examples thereof include the toner adhesion amount per unit area on the surface.

  The reason why the peak-to-peak voltage value Vpp of the AC component must be appropriately controlled according to the transfer influence factor is as follows. In FIG. 10 described above, the secondary transfer bias is composed of a DC bias having a positive polarity opposite to the charging polarity of the toner with respect to an AC bias that swings with the same amplitude on the plus side and the minus side around 0V. The offset voltage Voff is superimposed. During a period in which the secondary transfer bias is positive in one cycle of the AC component, the toner particles are separated from the toner layer on the belt surface and move toward the recording sheet in the secondary transfer nip. At the timing when the secondary transfer bias reaches the feed peak value Vt, the electric field strength in the direction in which the toner particles are moved from the belt surface side toward the recording sheet side (hereinafter referred to as the feed direction) becomes maximum. If the electric field strength at this time is not increased to some extent, the toner particles cannot be transferred well from the belt surface toward the recording sheet, and a sufficient image density can be obtained not only on the concave portion of the recording sheet surface but also on the convex portion. It will disappear. For this reason, it is necessary to increase the electric field strength in the feeding direction to a value at which a sufficient image density can be obtained at least at the convex portion on the surface of the recording sheet (hereinafter, this value is referred to as “feeding direction required electric field strength”).

  On the other hand, during the period in which the secondary transfer bias has a negative polarity, the toner particles return backward from the recording sheet side toward the belt surface side. At the timing when the secondary transfer bias reaches the return peak value Vr, the electric field strength in the direction in which the toner particles are returned backward from the recording sheet side toward the intermediate transfer belt side (hereinafter referred to as the return direction) becomes maximum. At this time, the toner particles in the recesses can be returned to the toner layer unless the electrostatic force sufficient to return them to the toner layer within a half cycle is applied to the toner particles transferred into the recesses of the recording sheet. As a result, a sufficient image density cannot be obtained at the concave portion. For this reason, the electric field strength in the return direction needs to be increased to at least a value that imparts an electrostatic force that causes the toner particles transferred into the recesses on the surface of the recording sheet to return to the toner layer within a half cycle (hereinafter referred to as the electric field strength) The value is referred to as “required electric field strength in the return direction”).

  The electric field strength in the feed direction depends on the feed peak value Vt, the electric field strength in the return direction depends on the return peak value Vr, and the sum of these peak values is the same value as the peak-to-peak voltage value Vpp of the AC component. . Therefore, the peak-to-peak voltage value Vpp needs to be set as follows while the electric field strength in the feeding direction is set to just “the electric field strength required in the feeding direction”. That is, it is necessary to make the intensity of the electric field in the return direction equal to or greater than the value that makes the electric field intensity required in the return direction (hereinafter, this value is referred to as “necessary peak-to-peak”). The “necessary peak-to-peak” varies depending on the state of the transfer influence factor. For example, when the temperature or humidity, which is one of the transfer influencing factors, decreases, the electrical resistance of the secondary transfer roller to which the secondary transfer bias is applied increases, so that the low temperature and low humidity environment is higher than the high temperature and high humidity environment. The “necessary peak-to-peak” becomes larger.

  The reason why the peak-to-peak voltage value Vpp has to be appropriately controlled according to the transfer influence factor has been described by taking the temperature that is one of the transfer influence factors as an example. It is desirable to control voltage value Vpp.

  On the other hand, in order to successfully transfer the toner image from the belt surface to the recording sheet, it is necessary to pass an appropriate amount of direct current between the belt surface and the recording sheet. The value of the offset voltage Voff that allows an appropriate amount of direct current to flow varies depending on the thickness and absolute humidity of the recording sheet. The greater the thickness of the recording sheet and the lower the absolute humidity, the higher the electrical resistance of the recording sheet and the less direct current flows through the recording sheet. Therefore, if the offset voltage Voff is not increased as the thickness of the recording sheet increases or the absolute humidity decreases, the amount of direct current flowing between the belt surface and the recording sheet is insufficient, resulting in insufficient image density. It will cause.

  Therefore, the present inventors provided the following AC bias output unit and DC bias output unit in the transfer power supply. That is, an AC bias peak-to-peak voltage value Vpp is output by constant voltage control, and the output target value is changed according to a transfer influence factor such as temperature, and a DC bias is output by constant current control. DC bias output unit. In this configuration, the peak-to-peak voltage value Vpp of the AC bias is changed to an appropriate value according to the transfer influencing factor, and the DC bias is output by constant current control, so that A certain amount of direct current is passed between the recording sheet. The present inventors have thought that such a configuration makes it possible to obtain a sufficient image density at the concave portion on the surface of the recording sheet regardless of the transfer influencing factors.

  However, a sufficient image density could not be obtained at the concave portion of the recording sheet in a low temperature and low humidity environment. It has been found that the cause of this is the reverse charging of the toner by discharge. Specifically, as described above, the “necessary peak-to-peak” becomes larger in the low temperature and low humidity environment than in the high temperature and high humidity environment. If the electrical resistance of the recording sheet does not change even when the environment changes to the low temperature and low humidity side, only the increase in the electrical resistance of the secondary transfer roller causes the “necessary peak-to-peak” to become larger. However, when the environment changes to the low temperature and low humidity side, the electrical resistance of the recording sheet increases due to moisture evaporation, and thus the value of the DC voltage (offset voltage Voff) output by constant current control becomes larger. Even if the offset voltage Voff becomes larger in this way, in order to ensure the “return direction required electric field strength” for the electric field in the return direction, the peak-to-peak voltage value of the AC bias must be increased accordingly. . For this reason, the “necessary peak-to-peak” becomes larger as the environment changes to the low temperature and low humidity side. In addition to the increase in the electrical resistance of the secondary transfer roller, the electrical resistance of the recording sheet Rising is also involved. Depending on the electrical resistance of the secondary transfer roller, if the peak-to-peak voltage value Vpp is increased to the same value as “required peak-to-peak”, the feed peak value Vt becomes too large. Then, a discharge is generated in the concave portion on the surface of the recording sheet in the secondary transfer nip, and the toner particles in the concave portion are reversely charged. By this reverse charging, white spot-like white spots are generated at the concave portions of the sheet surface in the image, which causes insufficient image density on the concave portions on the sheet surface. White spot-like white spots with no toner attached thereto are a very conspicuous phenomenon, and the shading pattern that becomes uneven becomes more prominent.

Therefore, the present inventors conducted the following further printing experiment. Note that a print tester used in various print experiments described below has the same basic configuration as the printer according to the embodiment. As the secondary transfer bias power source 39, a function generator (Yokogawa Electric FG300) generates a waveform of a superimposed voltage, which is amplified by an amplifier (Trek High Voltage Amplify Model 10/40) and output.
[First printing experiment]
Various parameters of the print tester were set as follows.
・ Process linear velocity v = 282 [mm / s]
・ Constant voltage control with offset voltage Voff = −1000 [V] of secondary transfer bias ・ Constant voltage control with peak-to-peak voltage value Vpp = 7 [kV] of AC component of secondary transfer bias ・ Frequency of AC component f = 500 [Hz]

  As recording sheets, RESAK 66 (trade name) 175 kg paper (six-plate continuous quantity) manufactured by Special Paper Industries Co., Ltd. and RESAK 66 215 kg paper (six-plate continuous quantity) were used. Rezac 66 is paper having a degree of unevenness on the paper surface larger than “ripple waves”. The depth of the concave portion on the paper surface is about 100 [μm] at the maximum. For the laboratory, temperature = 23 [° C.] and humidity = 50 [%].

  Under the above conditions, an A4 size solid black image was printed on Rezac 66 175 kg paper and Rezac 66 215 kg paper, respectively. In the case of 175 kg paper, sufficient image density was obtained for each of the smooth portion having no concave portion and the concave portion on the paper surface, whereas for 215 kg paper, sufficient image density was obtained for each of the smooth portion and the concave portion. I couldn't. If the offset voltage Voff of the secondary transfer bias is simply output at a constant voltage, the necessary amount of DC current cannot be passed through the secondary transfer nip to 215 kg paper having a higher electrical resistance than 175 kg paper. It was. For this reason, it seems that sufficient image density could not be obtained in both the smooth portion and the concave portion.

[Second printing experiment]
The DC voltage of the secondary transfer bias is output from the secondary transfer bias power source 39 by constant current control. The output target value of the offset current Ioff, which is the current value of the DC component of the secondary transfer bias, was set to −47.5 [μA]. Except for these points, A4 size solid black images were printed on Rezac 66 175 kg paper and Rezac 66 215 kg paper under the same conditions as in the first printing experiment. As a result, in both 175 kg paper and 215 kg paper, sufficient image density was obtained in both the smooth portion and the concave portion on the paper surface. In either 175 kg paper or 215 kg paper having a higher electrical resistance than this, a necessary amount of direct current can be applied to the paper at the secondary transfer nip, so that a sufficient image density can be obtained in both the smooth portion and the concave portion. It was obtained.

[Third print experiment]
Various parameters of the print tester were set as follows.
・ Process linear velocity v = 176 [mm / s]
-Frequency of AC component of secondary transfer bias f = 500 [Hz]
・ Constant voltage control with various values for peak-to-peak voltage value Vpp of AC component of secondary transfer bias ・ Constant current control with various values of offset current Ioff of DC component of secondary transfer bias

  As the recording sheet, Rezac 66 175 kg paper was used. The laboratory environment was set such that temperature = 10 [° C.] and humidity = 15 [%]. A magenta halftone image and a solid blue image formed by superimposing the magenta and cyan images were printed under various peak-to-peak voltage values Vpp and DC component conditions. Then, the output halftone image and blue solid image were visually evaluated for the toner transferability to the recesses on the paper surface, the toner transferability to the smooth portions, and the occurrence of white spots in the recesses, respectively.

  The toner transfer property to the smooth portion of the paper surface is the ability to transfer the toner particles on the belt surface to the smooth portion. With respect to the toner transferability to the smooth portion, the case where the toner particles were transferred well enough to obtain a sufficient image density with respect to the smooth portion was evaluated as ◯. In addition, a case where toner particles are transferred in such an amount that an acceptable level of image density can be obtained although it is slightly thinner than the smooth portion, and a case where only toner particles below the allowable level are transferred to the smooth portion. Was evaluated as x.

  The toner transferability to the recesses on the paper surface is the ability to transfer toner particles to the recesses. With respect to the toner transferability to the concave portion, the case where the toner particles were transferred satisfactorily enough to obtain a sufficient image density with respect to the concave portion was evaluated as ◯. In addition, a case where toner particles are transferred in such an amount that is thin enough to the concave portion but an acceptable level of image density is obtained, and a case where only toner particles lower than the allowable level are transferred to the concave portion. It evaluated as x. The toner transferability to the concave portion is evaluated by excluding defective transfer of the toner particles due to white spots. Therefore, for the image where white spots are generated, the white spots in the image portions on the concave portions are determined. The evaluation was based on the image density of the area without the image.

  In addition, as for the occurrence of white spots in the recesses, ○ when no white spots have occurred, Δ when slightly white spots have occurred but acceptable level, white spots exceeding the allowable level have occurred. The case was evaluated as x.

The conditions of the peak-to-peak voltage value Vpp and the offset current Ioff adopted in the third print experiment are shown in Table 1 below. As the AC bias, a sinusoidal one as shown in FIG. On the other hand, in condition 8, as in the eighth print experiment described later, an AC bias having a rectangular wave shape and a return time ratio of 40 [%] was employed.

  As can be seen from Table 1, under condition 1, the output target value of the offset current Ioff is fixed to −40 [μA] regardless of the peak-to-peak voltage value Vpp. On the other hand, in each of the conditions 2 to 8, the output target value of the offset current Ioff is decreased as the peak-to-peak voltage value Vpp is increased.

表２において、ＯＫという評価結果は、平滑部へのトナー転写性、凹部へのトナー転写性、及び白抜けの発生状況の３項目が何れも○という評価結果になったことを示している。また、ＯＫという評価結果になっていない欄では、３項目のうち、○という評価結果が得られなかった項目だけについて評価結果が記載されており、欄内に評価結果が記載されていない項目については○という評価結果になっている。例えば、条件２のＶｐｐ＝４ｋＶにおける青ベタ画像では、凹部へのトナー転写性の評価結果が×になったが、欄内に記載されていない平滑部へのトナー転写性や白抜けの発生状況は、何れも○という評価結果になっている。また、条件２のＶｐｐ＝１０ｋＶにおける青ベタ画像では、白抜けの発生状況の評価結果が△になったが、欄内に記載されていない凹部へのトナー転写性や平滑部へのトナー転写性は、何れも○という評価結果になっている。また、条件７のＶｐｐ＝１０ｋＶにおける青ベタ画像では、平滑部へのトナー転写性の評価結果が△になったが、欄内に記載されていない凹部へのトナー転写性や白抜けの発生状況については、何れも○という評価結果になっている。 The evaluation results of the images under each condition are shown in Table 2 below.

In Table 2, an evaluation result of OK indicates that all three items of the toner transferability to the smooth portion, the toner transferability to the concave portion, and the occurrence of white spots are evaluation results of ◯. In addition, in the column where the evaluation result is not OK, among the three items, the evaluation result is described only for the item for which the evaluation result of ○ is not obtained, and the item for which the evaluation result is not described in the column Has an evaluation result of ○. For example, in the blue solid image at Vpp = 4 kV in Condition 2, the evaluation result of the toner transferability to the concave portion becomes “x”, but the toner transferability to the smooth portion not described in the column and the occurrence of white spots Each has an evaluation result of ○. In addition, in the solid blue image at Vpp = 10 kV in Condition 2, the evaluation result of the occurrence state of white spots was Δ, but the toner transferability to the concave portion and the toner transferability to the smooth portion not described in the column Each has an evaluation result of ○. In addition, in the solid blue image at Vpp = 10 kV in Condition 7, the evaluation result of the toner transferability to the smooth portion was Δ, but the toner transferability to the concave portion not described in the column and the occurrence of white spots All of the evaluation results are ○.

A combination of Table 1 and Table 2 is shown in Table 3 below. In Table 3, the column in which the image quality evaluation result is x is hatched.

As shown in Table 3, the configuration in which the offset current Ioff is decreased as the peak-to-peak voltage value Vpp is increased is better than the configuration in which the offset current Ioff is constant. . Above all, condition 8 gave the best results.
However, for example, in condition 8, a good result may be obtained without increasing the offset current Ioff to −40 [μA] under the condition that the peak-to-peak voltage value Vpp is set to 4 [kV]. For example, as in the case of the peak-to-peak voltage value Vpp = 12 [kV], good results may be obtained with −24 [μA]. That is, the offset current Ioff is not decreased as the peak-to-peak voltage value Vpp is increased, but the offset current Ioff is set to −24 [μA] regardless of the peak-to-peak voltage value Vpp. Even in this case, the same good result as in condition 8 may be obtained.

Therefore, as shown in Table 4, conditions 9, 10, and 11 were newly added, and the same print experiment was performed under each condition.

  As can be seen from Table 4, when the peak-to-peak voltage value Vpp is set to 4 [kV], the smooth portion (convex portion) on the surface of the recording sheet is required unless the offset current Ioff is increased to at least −38 [μA]. As a result, the image density of the blue solid image is insufficient. Therefore, in the configuration in which the offset current Ioff is constant regardless of the peak-to-peak voltage value Vpp, when the peak-to-peak voltage value Vpp is set to a relatively large value, the image density is sufficiently high at the smooth portion on the surface of the recording sheet. I found out that I couldn't get it.

  In condition 8, even if the offset current Ioff is decreased as the peak-to-peak voltage value Vpp is increased, the image density is not insufficient in the smooth portion of the recording sheet surface. This means that the “feeding direction required electric field strength” decreases as the peak-to-peak voltage value Vpp increases. Such a phenomenon is considered to occur because the toner particles are easily separated from the surface of the intermediate transfer belt as the peak-to-peak voltage value Vpp increases (the adhesion force between the belt and the toner particles is weakened).

  Condition 8 is the same as Condition 5 when focusing only on the potential condition, but the waveform of the AC bias is different. Specifically, condition 5 is a waveform with an AC bias having a sine wave shape (FIG. 10), whereas condition 8 has a waveform with an AC bias having a rectangular waveform and a return time ratio of 40% (FIG. 16). ing. From this, it is better to adopt a rectangular wave shape and a waveform having a return time ratio of less than 50%, rather than adopting a sinusoidal shape as the AC bias under the same potential condition. It turns out that a result is obtained. The return time ratio will be described in detail in an eighth print experiment described later.

[Fourth printing experiment]
In the resac 66, as the continuous printing quantity increases, the paper thickness increases and the depth of the concave portion on the paper surface increases. Therefore, in the secondary transfer nip N, the toner particles transferred to the concave portion and the belt surface The distance becomes larger. For this reason, the larger the amount of the Rezac 66, the larger the return peak value Vr necessary for returning the toner particles transferred to the recesses to the belt surface. Therefore, it is desirable that the peak-to-peak voltage value Vpp of the AC voltage is increased as the paper thickness is increased, in addition to being increased in accordance with the low temperature and low humidity of the environment. The former is a process for making the surface potential of the secondary transfer back roller 33 constant regardless of the increase in resistance of the secondary transfer back roller 33 due to low temperature and low humidity, whereas the latter is a process for reducing the depth of the concave portion of the paper. This is to cope with an increase in the appropriate value of the return peak value Vr due to the increase in the length.

表５において、条件１２では、ピークツウピーク電圧値Ｖｐｐにかかわらず、オフセット電流Ｉｏｆｆの出力目標値を−４０［μＡ］に固定している。これに対し、条件１３、条件１４、条件１５、条件１６ではそれぞれ、ピークツウピーク電圧値Ｖｐｐを大きくするにつれて、オフセット電流Ｉｏｆｆの出力目標値を小さくしている。 Therefore, under the conditions that the laboratory environment is set to temperature = 10 [° C.] and humidity = 15 [%] (low temperature and low humidity), various peaks and lots of resac 66 are used, and appropriate peaks are obtained. An experiment was conducted to examine the toe peak voltage value Vpp and the offset current Ioff. The results of this experiment are shown in Table 5 below.

In Table 5, in condition 12, the output target value of the offset current Ioff is fixed to −40 [μA] regardless of the peak-to-peak voltage value Vpp. On the other hand, in condition 13, condition 14, condition 15, and condition 16, the output target value of the offset current Ioff is decreased as the peak-to-peak voltage value Vpp is increased.

  As can be seen from Table 5, as the thickness of the recording sheet increases (100 kg → 175 kg), the peak-to-peak voltage value Vpp is increased to avoid insufficient image density in the smooth portion as follows. Therefore, it was possible to maintain good image quality. That is, the offset current Ioff is decreased as the peak-to-peak voltage value Vpp is increased. Specifically, while maintaining the “field strength required in the feed direction” to achieve a sufficient image density in the smooth portion and the concave portion, the increase in the feed peak value Vt was suppressed and the occurrence of white spots could be suppressed. .

[Fifth print experiment]
In order to investigate the variation of the appropriate value of the peak-to-peak voltage value Vpp with respect to the resistance change of the secondary transfer back roller 33 accompanying the environmental change, a test image was printed while changing the temperature and humidity in the laboratory. In any environment, Rezac 66 175 kg paper was used for the recording sheet. The results of this experiment are shown in Table 6 below.

  In Table 6, under condition 17, the output target value of the offset current Ioff is fixed to −38 [μA] regardless of the peak-to-peak voltage value Vpp. In contrast, in condition 18, condition 19, condition 20, and condition 21, the output target value of the offset current Ioff is decreased as the peak-to-peak voltage value Vpp is increased.

  As can be seen from Table 6, as the environment is reduced in temperature and humidity (32 ° C. 80% → 10 ° C. p15%), the peak-to-peak voltage value Vpp is increased to avoid insufficient image density in the smooth portion. By doing so, it was possible to maintain good image quality. That is, the offset current Ioff is decreased as the peak-to-peak voltage value Vpp is increased. Specifically, while maintaining the “field strength required in the feed direction” to achieve a sufficient image density in the smooth portion and the concave portion, the increase in the feed peak value Vt was suppressed and the occurrence of white spots could be suppressed. .

[Sixth printing experiment]
In the secondary transfer nip N, a printing experiment was conducted to examine the return time during which the toner particles transferred into the recesses on the paper surface can be effectively returned to the belt surface. The return time is the time during which the secondary transfer bias composed of the superimposed voltage has the polarity in the return direction within one AC cycle. On the other hand, the feed time is a time during which the secondary transfer bias is in the feed direction polarity within one AC period. The sum of the return time and the feed time is the same value as one AC cycle. The return time ratio is the ratio of the return time in one AC cycle. The return time ratio was set to 50 [%], and the frequency f of the AC component of the secondary transfer bias was appropriately changed, and the image density on the concave portion of the blue solid image was measured under each condition. FIG. 15 shows the relationship between the IDmax value of the recess and the frequency f of the AC component obtained by this experiment.

  From FIG. 15, it can be seen that when the frequency of the AC component exceeds 16000 [Hz], the IDmax value of the concave portion rapidly decreases. This means that when the AC period is less than 0.06 [msec], the IDmax value of the concave portion is drastically lowered, and in this experiment, the return time ratio = 50 [%]. Therefore, when the return time is less than 0.03 [msec], the IDmax value of the concave portion is rapidly lowered.

[Seventh print experiment]
AC component peak-to-peak voltage Vpp = 2500 [V], offset voltage Voff = −800 [V], and return time ratio = 20 [%], respectively. Under these conditions, while changing the frequency f of the AC component and the process linear velocity v, a blue solid image is output on plain paper under the conditions of the respective frequency f and process linear velocity v, and the output blue solid image Was visually observed. Then, the presence or absence of image density unevenness (pitch unevenness) that seems to be an influence of an alternating electric field in the secondary transfer nip N was evaluated. Then, under the condition of the same frequency f, it was found that as the process linear velocity v is increased, it becomes easier to generate pitch unevenness, and under the same process linear velocity v, it is easier to generate pitch unevenness as the frequency f is decreased.

  As a result, if the toner is not reciprocated between the intermediate transfer belt and the concave portion of the paper surface by a certain number of times (hereinafter referred to as reciprocation number N in the nip) in the secondary transfer nip N, pitch unevenness occurs. It shows that it will end. Under the conditions of process linear velocity v = 282 [mm / s] and frequency f = 400 [Hz], pitch unevenness was not observed, but process linear velocity V = 282 [mm / s] and frequency f = 300 [Hz]. ], Pitch unevenness was observed. The secondary transfer nip N width d, which is the length of the secondary transfer nip N in the belt moving direction, is set to 3 [mm]. Therefore, the number N of reciprocations in the nip under the condition in which no pitch unevenness is recognized is calculated as (3 mm × 400 Hz / 282 mm / s) = about 4 times, and this value can avoid the pitch unevenness at the limit. . That is, the number of reciprocations within the nip is the minimum number of reciprocations within the nip.

  Further, under the condition that the process linear velocity v = 141 [mm / s] and the frequency f = 200 [Hz], pitch unevenness was not recognized, but the process linear velocity V = 141 [mm / s] and the frequency f = Pitch unevenness was recognized under the condition of 100 [Hz]. The condition that the process linear velocity v = 141 [mm / s] and the frequency f = 200 [Hz] is the same as the condition that the process linear velocity v = 282 [mm / s] and the frequency f = 400 [Hz]. The number N of reciprocations in the nip is calculated to be about 4 times (3 mm × 200 Hz / 141 mm / s). Therefore, it can be said that an image free from pitch unevenness can be obtained by satisfying the minimum condition of “frequency f> (4 / d) * v”.

[Eighth print experiment]
As the AC component of the secondary transfer bias, a rectangular wave as shown in FIG. 16 is output from the secondary transfer bias power source 39 instead of a sine wave. The duty of this rectangular wave is not 50%. Since the rise time to the polarity in the return direction is shorter than the fall time to the polarity in the feed direction, the return time ratio is lower than 50 [%]. In the state shown in the figure, the DC component is superimposed, but since it is a rectangular wave, the return time ratio is lower than 50 [%] even if the DC component is not superimposed. As described above, when the alternating current component having a waveform in which the return time ratio is lower than 50 [%] using only the alternating current component is employed, the following effects can be obtained. That is, as in the case of using an AC component having a waveform in which the return time ratio is 50 [%] only with an AC component, such as a sine wave, the smoothing portion and the concave portion are reduced under the condition of a lower peak-to-peak voltage value Vpp. In both cases, a sufficient image density could be obtained. This is because even if the electric field in the feeding direction is relatively weak, if the falling time in the feeding direction is relatively long, a sufficient amount of toner particles can be transferred from the belt direction to the smooth portion of the paper surface. Therefore, by adopting an alternating current component having a waveform whose return time ratio is lower than 50 [%] with only the alternating current component, compared to a case where an alternating current component having a return time ratio of 50 [%] is employed, Generation of white spots can be suppressed.

  In the first print experiment and the second print experiment described above, an alternating current component having a rectangular wave as shown in FIG. 16 and having a return time ratio of 40 [%] is employed. Also in the third print experiment, the same AC component is used for condition 8 only.

[9th printing experiment]
In order to investigate the variation of the appropriate value of the peak-to-peak voltage value Vpp due to the fluctuation of the toner adhesion amount per unit area on the intermediate transfer belt in the secondary transfer nip, a plurality of test images having different image areas are used. Printing under various potential conditions. The image area ratio of the toner image in the secondary transfer nip is correlated with the toner adhesion amount per unit area on the belt in the secondary transfer nip. Therefore, grasping the image area ratio means grasping the toner adhesion amount per unit area on the belt in the secondary transfer nip.

As the recording sheet, Rezac 175 kg paper was used. Further, the DC bias of the secondary transfer bias composed of the superimposed bias is set to a constant output voltage value not by constant current control but by constant voltage control. In the test images output at the respective image area ratios, the image density of the smooth portion, the image density of the concave portion, and the white spot were visually evaluated. The potential conditions and evaluation results in this ninth print experiment are as shown in Table 7 below. In Table 7, Voff represents an offset voltage Voff as a DC bias.

  From the comparison between the condition 22 in which the peak-to-peak voltage value Vpp is constant regardless of the image area ratio and the condition 23 in which the peak to peak voltage value Vpp is changed in accordance with the image area ratio, as the image area ratio increases, It can be seen that Vpp required to transfer a sufficient amount of toner to the concave portion is increased. However, as in Condition 23, in the case of a relatively high image area ratio, the DC bias is the same value as in the case of a relatively low image area ratio even though the peak-to-peak voltage value Vpp is relatively high. In this case, white spots caused by the discharge are generated in the image. Therefore, as in the condition 24 and the condition 25, the output target value of the DC bias is lowered while the output target value of the peak-to-peak voltage value is increased as the image area ratio increases. As a result, it is possible to suppress the occurrence of white spots while obtaining a sufficient image density in the recesses on the paper surface. Although an example has been described in which the output target value in the DC bias constant voltage control is lowered as the image area ratio increases, the output target value in the DC bias constant current control may be lowered. Further, the setting of the output target value of the peak-to-peak voltage value according to the image area ratio may be manually performed by the user according to the image area ratio, or the image forming apparatus may automatically perform according to the image area ratio. You may do it.

Next, a characteristic configuration of the printer according to the embodiment will be described.
FIG. 17 is a block diagram illustrating a part of an electric circuit of the printer according to the embodiment. In the figure, a control unit 60 includes a CPU (Central Processing Unit) 60a as a calculation means, a RAM (Random Access Memory) 60c as a data storage means, a ROM (Read Only Memory) 60b, a flash memory (FM) 60d, and the like. ing. Various devices and sensors are connected to the control unit 60 that controls the entire printer, but only those related to the characteristic configuration of the printer are shown in FIG.

  The primary transfer power sources 81Y, 81M, 81C, 81K output a primary transfer bias to be applied to the primary transfer rollers (35Y, M, C, K). The secondary transfer bias power source 39 outputs a secondary transfer bias to be applied to the secondary transfer back roller (33), as already described many times. The operation panel 50 includes a touch panel (not shown) and a plurality of key buttons. The operation panel 50 can display an image on the screen of the touch panel and can accept an input operation by an operator using the touch panel and the key buttons. The operator can input what type of recording sheet is set in the paper feed cassette (100) by an input operation on the operation panel 50. Hereinafter, the type of recording sheet is referred to as sheet type. Note that the sheet type is a concept that reflects the type focused on the thickness of the sheet, such as 175 kg paper, in addition to the type focused on the state of the paper surface such as “ripple wave” or “Rezac 66”.

  The secondary transfer bias power source 39 outputs the peak-to-peak voltage value Vpp of the AC component of the secondary transfer bias by constant voltage control, while outputting the DC component offset current Ioff by constant current control. . For constant current control, it is necessary to measure the actual output value of the offset current Ioff using an ammeter, and to implement a process of increasing or decreasing the DC voltage value so that the result is the same value as the output target value. There is. The position at which the actual output value of the offset current Ioff is measured by an ammeter is a position inside the secondary transfer bias power supply 39 and upstream of the output terminal of the secondary transfer bias power supply 39 in FIG. It can be illustrated. Further, it may be in the vicinity of the electrode terminal that is slid against the core metal of the secondary transfer back roller 33 outside the secondary transfer bias power source 39.

  The ROM 60d stores a data table indicating the relationship between the temperature / humidity environment and a combination of an appropriate output target value of the peak-to-peak voltage value Vpp and an output target value of the offset current Ioff for each of various sheet types. In this data table, if the combination of temperature value and humidity value is the same in a low temperature and low humidity environment, the output target value of the peak-to-peak voltage value Vpp is increased as the sheet type has a higher electrical resistance. . On the other hand, the output target value of the offset current Ioff is reduced.

  In such a configuration, the output target value of the peak-to-peak voltage value Vpp output by the constant voltage control is changed according to the transfer influence factors such as the temperature and humidity and the electrical resistance of the recording sheet. Further, the offset current Ioff is output by constant current control to appropriately control the amount of direct current flowing between the belt surface and the recording sheet in the transfer nip. By such control, in a high temperature and high humidity environment where it is not necessary to increase the peak-to-peak voltage value Vpp of the AC bias so much, the peak-to-peak value is reduced to a value that does not cause discharge in the concave portion of the recording sheet in the secondary transfer nip. The peak voltage value Vpp is kept. Further, an effective secondary transfer electric field is formed between the belt surface and the convex portion or concave portion of the recording sheet surface. Thereby, it is possible to obtain a sufficient image density in both the convex portion and the concave portion on the surface of the recording sheet, and to suppress the generation of the light and shade pattern of the image that is uneven on the surface of the recording sheet.

  On the other hand, in a low temperature and low humidity environment where the peak to peak voltage value Vpp needs to be relatively large, the output target value of the offset current Ioff is decreased as the peak to peak voltage value Vpp is increased. As a result, the feed peak value Vt is made smaller and the return peak value Vr is made larger. In this way, the return peak value Vr is secured so that the toner particles in the recesses on the surface of the recording sheet can be returned to the surface of the image carrier while keeping the feed peak value Vt at a value that can suppress the occurrence of white spots due to discharge. To do. By reducing the output target value of the offset current Ioff, the amount of direct current flowing through the recording sheet is reduced, so the image density is reduced at both the concave and convex portions on the surface of the recording sheet. The phenomenon that the image density of the concave portion becomes thinner than the convex portion is suppressed. Therefore, it is possible to suppress the occurrence of the light and shade pattern of the image that is uneven on the surface of the recording sheet.

  As described above, in the printer according to the embodiment, it is possible to suppress the generation of the light and shade pattern that follows the surface unevenness of the recording sheet regardless of the environment. In addition, after selecting a basic peak-to-peak voltage value that increases as the environment decreases in temperature and humidity, a correction shift value that increases as the electrical resistance of the sheet type increases is selected. The output target value of the peak-to-peak voltage value may be used. By doing so, it is possible to set the peak-to-peak voltage value to a more appropriate value as compared with the case where the peak-to-peak voltage value is determined based only on the environment.

  In addition, it may replace with the structure which carries out constant voltage control of alternating current bias, and the structure which carries out constant current control of alternating current bias may be employ | adopted. In this case, if the output target value of the AC bias current output by the transfer bias output means is changed so that the peak-to-peak voltage value of the AC bias becomes a target value according to the acquisition result by the information acquisition means. Good. The relationship between the transfer bias current and the voltage changes depending on the resistance of the transfer portion, but it is possible to cope with the change of the resistance by doing as follows. For example, the relationship between the current and voltage when the humidity, which is a transfer influencing factor, is changed from high humidity to low humidity is obtained in advance by experiment or calculation. Then, the output target value of the current may be changed so that the peak-to-peak voltage value of the AC bias becomes the target value at the time of high humidity and low humidity.

  Further, instead of changing the bias value according to the humidity as a transfer influence factor, the bias value may be changed according to another transfer influence factor. For example, as described in the ninth print experiment, the output target value (absolute value) of the peak-to-peak voltage value is increased and the output target value of the DC bias (the absolute value) is increased as the image area ratio as a transfer influence factor increases. (Absolute value) may be lowered.

Next, a printer according to each modified example in which a part of the configuration of the printer according to the embodiment is changed to another configuration will be described. Unless otherwise specified below, the configuration of the printer according to each modification is the same as that of the embodiment.
[First Modification]
FIG. 18 is a configuration diagram showing a part of the image forming unit 1 in the printer according to the first modification. This printer is a single-color machine provided with only one image forming unit 1 for forming a single-color toner image. In the figure, a photosensitive member 2 as an image carrier is rotationally driven in a clockwise direction in the drawing by a driving means (not shown).

  Around the photoreceptor 2, a drum cleaning device, a charge removal device, a charging device, a developing device, and the like (not shown) having the same configuration as the printer according to the embodiment are disposed. A transfer roller 235 as a nip forming member is disposed below the photoconductor 2 and abuts against the photoconductor 2 while being urged toward the photoconductor 2 by an urging means (not shown). Is forming. The recording sheet P is fed into the transfer nip. Then, the toner image on the photoreceptor 2 is transferred onto the recording sheet P at the transfer nip.

  Examples of the transfer roller 235 include those in which a conductive foam layer made of a conductive foam is coated on the peripheral surface of the rotating shaft member, and those in which a conductive elastic layer is coated on a cored bar. .

  The recording sheet P on which the toner image is formed on the first surface after passing through the transfer nip is fixed on the first surface via a fixing device (not shown). If necessary, the toner image is transferred and fixed on the second surface by being retransmitted to the transfer nip by a reverse retransmission device (not shown).

  A transfer bias is applied to the transfer roller 235 by a transfer bias power supply 240. The transfer bias power supply 240 as the potential difference generating means has a DC power supply and an AC power supply, like the secondary transfer bias power supply 39 of the printer according to the embodiment. As in the case of the secondary transfer bias of the printer according to the embodiment, the following processing is performed in a high-temperature and high-humidity environment where it is not necessary to increase the peak-to-peak voltage value Vpp of the AC bias so much. That is, an effective transfer electric field between the photosensitive member surface and the convex portion or concave portion of the recording sheet surface while keeping the peak-to-peak voltage value Vpp at a value small enough not to cause discharge in the concave portion of the recording sheet in the transfer nip. Form. Thereby, it is possible to obtain a sufficient image density in both the convex portion and the concave portion on the surface of the recording sheet, and to suppress the generation of the light and shade pattern of the image that is uneven on the surface of the recording sheet. On the other hand, in a low temperature and low humidity environment where the peak to peak voltage value Vpp needs to be relatively large, the output target value of the offset current Ioff is decreased as the peak to peak voltage value Vpp is increased. As a result, the feed peak value Vt is made smaller and the return peak value Vr is made larger.

  In the printer according to the embodiment, the negative transfer toner is electrostatically repelled against the secondary transfer back roller 33 to which the secondary transfer bias is applied, so that the toner image is nipped from the secondary transfer back roller 33 side. It was electrostatically moved to the forming roller 36 side. On the other hand, in the printer according to the first modified example, the negative polarity toner on the photosensitive member 2 is electrostatically attracted to the transfer roller 235 to which the transfer bias is applied, so that the toner image is transferred to the photosensitive member 2. Electrostatically moved from the side to the transfer roller 235 side. For this reason, the transfer bias power supply 240 outputs a transfer bias having the same waveform as that in FIG.

[Second Modification]
FIG. 19 is a configuration diagram showing a part of the image forming unit 1 and the paper transport belt unit 210 in the printer according to the second modification. This printer is a single-color machine provided with only one image forming unit 1 for forming a single-color toner image. In the figure, a photosensitive member 2 as an image carrier is rotationally driven in a clockwise direction in the drawing by a driving means (not shown).

  Around the photoreceptor 2, a drum cleaning device, a charge eliminating device, a charging device, a developing device, and the like (not shown) having the same configuration as the printer according to the first embodiment are disposed. A paper transport belt unit 210 is disposed below the photosensitive member 2.

  The paper transport belt unit 210 stretches an endless paper transport belt 211 by a driving roller 212 and a driven roller 213 disposed inside the loop, and is rotated in response to the rotation of the driving roller 212 in the drawing. Move endlessly clockwise. A paper transport belt 211 as a nip forming member is in contact with the photosensitive member 2 to form a transfer nip. In the vicinity of the transfer nip, the transfer brush 215 and the transfer roller 214 disposed inside the loop of the paper transport belt 211 are in contact with the back surface of the paper transport belt 211. A transfer bias is applied to the transfer brush 215 and the transfer roller 214 by a transfer bias power source 240 as a potential difference generating unit.

  A recording sheet is fed by a registration roller pair 102 to a transfer nip formed by contact between the photosensitive member 2 and the paper conveying belt 211. Then, the toner image on the photoreceptor 2 is transferred onto the recording sheet P at the transfer nip.

  Examples of the transfer roller 214 include those in which a conductive foam layer made of a conductive foam is coated on the peripheral surface of the rotating shaft member, and those in which a conductive elastic layer is coated on a cored bar. . Moreover, as the transfer brush 215, what provided the brush part by the straight hair of a several conductive fiber on the surface of a conductive support body can be illustrated.

  The recording sheet P on which the toner image is formed on the first surface after passing through the transfer nip is fixed on the first surface via a fixing device (not shown). If necessary, the toner image is transferred and fixed on the second surface by being retransmitted to the transfer nip by a reverse retransmission device (not shown).

  In the same figure, out of the entire area in the circumferential direction of the paper conveying belt 211, the position shifted from the center of the transfer nip in the belt moving direction to the downstream side and the position where the belt is sandwiched between the photoreceptor 2 and Although the transfer brush 215 is in contact, it may be in contact with the center of the transfer nip. Further, although the transfer brush 215 is disposed upstream of the transfer roller 214 in the belt moving direction, the positional relationship between the two may be reversed. Further, only one of the transfer brush 215 and the transfer roller 214 may be provided.

  A transfer bias is applied to the transfer brush 215 and the transfer roller 214 by a transfer bias power supply 240. The transfer bias power supply 240 as the potential difference generating means has a DC power supply and an AC power supply, like the secondary transfer bias power supply 39 of the printer according to the embodiment. As the transfer bias, it is possible to output a DC bias consisting only of a DC voltage, or to output a superimposed bias obtained by superimposing an AC voltage on the DC voltage.

  A transfer bias is applied to the transfer roller 214 by a transfer bias power supply 240. Similar to the secondary transfer bias of the printer according to the first modification, the transfer bias power supply 240 performs the following processing in a high temperature and high humidity environment where the peak-to-peak voltage value Vpp of the AC bias does not need to be increased so much. I do. That is, an effective transfer electric field between the photosensitive member surface and the convex portion or concave portion of the recording sheet surface while keeping the peak-to-peak voltage value Vpp at a value small enough not to cause discharge in the concave portion of the recording sheet in the transfer nip. Form. Thereby, it is possible to obtain a sufficient image density in both the convex portion and the concave portion on the surface of the recording sheet, and to suppress the generation of the light and shade pattern of the image that is uneven on the surface of the recording sheet. On the other hand, in a low temperature and low humidity environment where the peak to peak voltage value Vpp needs to be relatively large, the output target value of the offset current Ioff is decreased as the peak to peak voltage value Vpp is increased. As a result, the feed peak value Vt is made smaller and the return peak value Vr is made larger.

  In the printer according to the second modification, the toner image is transferred from the photosensitive member 2 side by electrostatically attracting the negative polarity toner on the photosensitive member 2 to the transfer roller 214 to which a transfer bias is applied. Electrostatically moved to the 235 side. For this reason, as the above-described superimposing bias, one having the same waveform as that of the printer according to the first modification is employed. This superimposed bias has an average absolute value per unit time that is smaller than the absolute value of the DC bias consisting of only the DC voltage described above.

[Third Modification]
FIG. 20 is a configuration diagram showing a part of each color image forming unit and a transfer unit 300 in a printer according to a third modification. In the figure, for each color, only the photoconductors (2Y, M, C, K) are shown as image forming units. However, around the photoconductor, the image forming units shown in FIG. A drum cleaning device, a static eliminator, a charging device K, a developing device, and the like are provided.

  A transfer unit 300 is disposed below the photosensitive members 2Y, 2M, 2C, and 2K for each color. In the transfer unit 300, an endless transfer / conveying belt 301 is stretched by the following members disposed inside the loop. That is, there are four transfer rollers 302Y, M, C, K, a separation roller 307, a driving roller 303, a first driven roller 304, a second driven roller 305, an entrance roller 306, and the like. Then, the transfer conveying belt 301 is moved endlessly in the counterclockwise direction in the drawing as the driving roller 303 rotates.

  The transfer rollers 302Y, M, C, and K sandwich the transfer conveyance belt 301 between the photoreceptors 2Y, M, C, and K. As a result, the front surface of the transfer conveyance belt 301 as a nip forming member and the photoreceptors 2Y, 2M, 2C, and 2K abut, and transfer nips for Y, M, C, and K are formed. .

  Outside the loop of the transfer / conveyance belt 301, the paper suction roller 308 abuts on a place where the transfer / conveyance belt 301 is wound around the entrance roller 306 to form a paper suction nip. Further, the belt cleaning device 311 abuts on a place where the transfer conveyance belt 301 is wound around the driving roller 303 to form a cleaning nip.

  As the transfer rollers 302Y, 302M, 302C, and 302K, those in which a conductive foam layer made of a conductive foam is coated on the peripheral surface of the rotating shaft member, or a conductive elastic layer is coated on a cored bar. Can be illustrated. A transfer bias is applied to the transfer rollers 302Y, M, C, and K by a transfer bias power source 310Y, M, C, and K. As a result, a transfer electric field for electrostatically moving the toner from the photoreceptor side to the transfer roller side is formed between the transfer rollers 302Y, M, C, and K and the electrostatic latent images on the photoreceptors 2Y, M, C, and K. Is done.

  Further, a paper suction bias is applied to the paper suction roller 308 by a suction bias power source (not shown). A registration roller pair 102 is disposed in the vicinity of the paper suction roller 308, and sends a recording sheet (not shown) toward the paper suction nip at a predetermined timing. The recording sheet sandwiched in the paper adsorption nip is adsorbed on the front surface of the transfer conveyance belt 301 by electrostatic force. As the transfer conveyance belt 301 moves endlessly, the transfer conveyance belt 301 sequentially passes through the transfer nips for Y, M, C, and K. As a result, a four-color superimposed toner image is formed on the first surface of the recording sheet.

  Of the transfer nips for Y, M, C, and K, the recording sheet that has passed through the K transfer nip at the end of the transfer process faces the separation roller 307 as the transfer conveyance belt 301 moves endlessly. It starts. At this facing position, the transfer conveyance belt 301 is wound around the separation roller 307 with a large winding angle, so that the moving direction is suddenly changed. The recording sheet electrostatically attracted to the surface of the transfer / conveying belt 301 cannot follow the sudden change and is separated from the surface of the belt by the curvature.

  The toner image is fixed on the first surface of the recording sheet thus separated from the transfer conveyance belt 301 via a fixing device (not shown). Then, if necessary, the toner image is retransmitted to the registration roller pair 102 by a reverse retransmission device (not shown), and the four-color superimposed toner image is also transferred and fixed on the second surface.

  The transfer bias power supplies 310Y, 310M, 310C, and 310K as potential difference generating means each have a DC power supply and an AC power supply, like the secondary transfer bias power supply 39 of the printer according to the embodiment. As the transfer bias, it is possible to output a DC bias consisting only of a DC voltage, or to output a superimposed bias obtained by superimposing an AC voltage on the DC voltage.

  Similarly to the secondary transfer bias power supply 39 of the printer according to the embodiment, the transfer bias power supplies 310Y, M, C, and K have a DC power supply and an AC power supply. Then, like the secondary transfer bias of the printer according to the embodiment, the following processing is performed in a high temperature and high humidity environment where the peak-to-peak voltage value Vpp of the AC bias does not need to be increased so much. That is, an effective transfer electric field between the photosensitive member surface and the convex portion or concave portion of the recording sheet surface while keeping the peak-to-peak voltage value Vpp at a value small enough not to cause discharge in the concave portion of the recording sheet in the transfer nip. Form. Thereby, it is possible to obtain a sufficient image density in both the convex portion and the concave portion on the surface of the recording sheet, and to suppress the generation of the light and shade pattern of the image that is uneven on the surface of the recording sheet. On the other hand, in a low temperature and low humidity environment where the peak-to-peak voltage value Vpp needs to be relatively large, the following processing is performed. That is, as the peak-to-peak voltage value Vpp is increased, the output target value of the offset current Ioff is decreased, thereby reducing the feed peak value Vt and increasing the return peak value Vr.

  Also in the printer according to the third modification, the negative polarity toner on the photoreceptors 2Y, M, C, and K is electrostatically attracted to the transfer rollers 302Y, M, C, and K to which a transfer bias is applied. . As a result, the toner image is electrostatically moved from the photoreceptors 2Y, M, C, and K sides to the transfer rollers 302Y, M, C, and K sides. For this reason, as the above-described superimposing bias, one having the same waveform as that of the printer according to the first modification is employed. This superimposed bias has an average absolute value per unit time that is smaller than the absolute value of the DC bias consisting of only the DC voltage described above.

[Fourth Modification]
FIG. 21 is a main part configuration diagram showing main parts of a printer according to a fourth modification. In the figure, an image forming unit 1K for K has a static elimination lamp 14K, a drum cleaning device 3K, a charging device 6K, a latent image writing unit 15K, a developing device 8K, and the like around a photoconductor 2K. The latent image writing unit 15K optically writes an electrostatic latent image on the surface of the photoreceptor 2K using an LED array. The image forming units 1Y, M, and C for Y, M, and C have the same configuration as the image forming unit 1K for K.

  The intermediate transfer belt 31 of the transfer unit 30 sequentially passes through the primary transfer nips for Y, M, C, and K while being moved endlessly in the clockwise direction in the drawing. As a result, a four-color superimposed toner image is formed on the front surface of the intermediate transfer belt 31.

  Below the transfer unit 30, a paper transport belt unit 400 that moves the endless paper transport belt 401 endlessly is disposed. The paper conveying belt 401 is stretched by a driving roller 402 disposed on the inner side of the loop and a secondary transfer pressing roller 403, and rotates counterclockwise in the drawing as the driving roller 402 rotates. Can be moved endlessly. The paper transport belt unit 400 has a portion that is wound around the secondary transfer pressing roller 403 in the entire area in the circumferential direction of the paper transport belt 401, and the secondary transfer back roller 33 in the entire area in the circumferential direction of the intermediate transfer belt 31. It is in contact with the hanging part. As a result, a secondary transfer nip is formed by the contact between the front surface of the intermediate transfer belt 31 and the front surface of the paper transport belt 401 as a nip forming member.

  A secondary transfer bias is applied to the secondary transfer back roller 33 by a secondary transfer bias power source 39 having the same configuration as that of the printer according to the embodiment. On the other hand, the secondary transfer pressing roller 403 of the paper transport belt unit 400 is grounded. As a result, a secondary transfer electric field is formed between the secondary transfer back roller 33 and the secondary transfer pressure roller 403 at and around the secondary transfer nip.

  The four-color superimposed toner image on the intermediate transfer belt 31 is secondarily transferred onto the first surface of the recording sheet P sent out from the registration roller pair 102 toward the secondary transfer nip. Thereafter, the recording sheet P passes through the secondary transfer nip with the endless movement of the paper conveying belt 401 while being attracted to the front surface of the paper conveying belt 401. Then, it approaches the position facing the drive roller 402 disposed inside the loop of the paper transport belt 401. At this facing position, the paper transport belt 401 is wound around the driving roller 402 with a large winding angle, so that the moving direction is suddenly changed. The recording sheet P electrostatically attracted to the surface of the paper conveying belt 401 cannot follow the sudden change and is separated from the belt surface by curvature.

  The toner image is fixed on the first surface of the recording sheet P separated from the paper conveying belt 401 in this way via a fixing device (not shown). Then, if necessary, the toner image is retransmitted to the registration roller pair 102 by a reverse retransmission device (not shown), and the four-color superimposed toner image is also transferred and fixed on the second surface.

  The secondary transfer bias power source 39 has a DC power source and an AC power source, as in the printer according to the embodiment. Then, in a high temperature and high humidity environment where it is not necessary to increase the peak-to-peak voltage value Vpp of the AC bias, the following processing is performed. That is, an effective transfer electric field is generated between the belt surface and the convex portions and concave portions of the recording sheet surface while keeping the peak-to-peak voltage value Vpp at such a small value that does not cause discharge in the concave portion of the recording sheet in the transfer nip. Form. Thereby, it is possible to obtain a sufficient image density in both the convex portion and the concave portion on the surface of the recording sheet, and to suppress the generation of the light and shade pattern of the image that is uneven on the surface of the recording sheet. On the other hand, in a low temperature and low humidity environment where the peak-to-peak voltage value Vpp needs to be relatively large, the following processing is performed. That is, as the peak-to-peak voltage value Vpp is increased, the output target value of the offset current Ioff is decreased, thereby reducing the feed peak value Vt and increasing the return peak value Vr.

  The secondary transfer back roller 33 may be grounded and a secondary transfer bias may be applied to the secondary transfer press roller 403.

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 on its surface, a nip forming member (for example, the nip forming roller 36) that forms a transfer nip in contact with the surface, Transfer bias output means (for example, a secondary transfer bias power supply 39) for outputting a transfer bias for forming a transfer electric field composed of an alternating electric field between the image carrier and the nip forming member is sandwiched in the transfer nip. The toner image on the surface of the image carrier is transferred to the recording sheet, and the transfer bias output means outputs a transfer bias composed of a superimposed bias in which an AC bias is superimposed on a DC bias. In the image forming apparatus, the transferability of the toner image from the image carrier to the recording sheet in the transfer nip is affected. An information acquisition unit (for example, an operation panel 50, a temperature sensor 51, and a humidity sensor 52) that acquires information on a transfer influence factor that is a cause is provided, and the AC bias peak-to-peak according to the acquisition result by the information acquisition unit The transfer bias output means (for example, the secondary transfer power source 39) is configured to perform a process of reducing the output target value of the DC bias as the output target value of the voltage value is changed and the output target value increases. It is characterized by comprising.

[Aspect B]
Aspect B is a process according to aspect A in which the DC bias is output by constant current control, and the output target value in the constant current control of DC bias is reduced as the output target value of the peak-to-peak voltage value is increased. The transfer bias output means is configured to be implemented. In such a configuration, by outputting a direct current bias with constant current control, a constant direct current can flow through the recording sheet even if the electrical resistance of the recording sheet changes.

[Aspect C]
Aspect C is a process according to aspect A in which the DC bias is output by constant voltage control, and the output target value in the DC bias constant voltage control is decreased as the output target value of the peak-to-peak voltage value is increased. The transfer bias output means is configured to be implemented.

[Aspect D]
In aspect D, in aspect B or C, as the information acquisition unit, one that acquires temperature information that is information on the transfer influence factor is used, and the peak-to-peak voltage value decreases as the temperature value indicated by the temperature information decreases. The transfer bias output means is configured to perform a process of increasing the output target value. In such a configuration, even if the electrical resistance of a member to which a secondary transfer bias is applied, such as the secondary transfer back roller 33, is increased as the environment is lowered, an alternating electric field having a strength necessary for the transfer nip is generated. Can be formed.

[Aspect E]
Aspect E uses any one of the aspects B to D as the information acquisition means for acquiring humidity information as information on the transfer influence factor, and the peak toe decreases as the humidity value indicated by the humidity information decreases. The transfer bias output means is configured to perform processing for increasing the output target value of the peak voltage value. In such a configuration, even if the electrical resistance of a member to which a secondary transfer bias is applied, such as the secondary transfer back surface roller 33, is increased as the environment is reduced in humidity, an alternating electric field having a strength necessary for the transfer nip is generated. Can be formed.

[Aspect F]
Aspect F uses any one of the aspects B to E as the information acquisition means to acquire the thickness information of the recording sheet fed into the transfer nip as the transfer influence factor, and the thickness indicated by the thickness information. The transfer bias output unit is configured to perform a process of increasing the output target value of the peak-to-peak voltage value as the value increases. With such a configuration, an alternating electric field having a strength required for the transfer nip can be formed even if the electrical resistance of the recording sheet increases as the thickness of the recording sheet increases.

[Aspect G]
In aspect G, in any one of aspects B to F, as the information acquisition unit, information that acquires depth information of the recesses on the surface irregularities of the recording sheet fed into the transfer nip, which is information on the transfer influence factor, is used. In addition, the transfer bias output means is configured to perform a process of increasing the output target value of the peak-to-peak voltage value as the depth value indicated by the depth information increases. . With such a configuration, even if the appropriate value of the return peak value Vr becomes larger as the depth of the concave portion on the surface of the recording sheet becomes larger, an alternating electric field having the strength required for the transfer nip can be formed.

[Aspect H]
In aspect H, in any one of aspects B to G, as the information acquisition unit, information that acquires information on the amount of toner adhesion per unit area on the surface of the image carrier, which is information on the transfer influence factor, is used. In addition, the transfer bias output means is configured to perform a process of increasing the output target value of the peak-to-peak voltage value as the adhesion amount indicated by the toner adhesion amount information increases. is there. In such a configuration, an alternating electric field having a strength required for the transfer nip can be formed even if the appropriate value of the feed peak value Vt increases as the toner adhesion amount increases.

[Aspect I]
Aspect I is characterized in that, in the aspects A to H, the transfer bias output means is configured to output the AC bias by constant voltage control.

[Aspect J]
In the aspect J, in any one of the aspects A to I, as the transfer bias, the time average potential in one cycle of the superimposed bias is more than the center value of the maximum potential and the minimum potential in one cycle. The transfer bias output means is configured to output a value that is shifted to a side that facilitates electrostatic transfer from the image carrier to the recording sheet. The time average potential within one cycle of the superimposed bias is shifted to the side that facilitates electrostatic transfer of the toner from the image carrier to the recording sheet in the transfer nip, rather than the center value of the maximum potential and the minimum potential within one cycle. Since the return bias ratio of the AC component is less than 50 [%], the appropriate value of the peak-to-peak voltage value Vpp is smaller than when the return bias ratio is 50 [%]. Thus, the occurrence of white spots can be suppressed.

[Aspect K]
Aspect K includes a superposition bias obtained by superimposing the alternating current bias and the direct current bias as the transfer bias in any of the aspects A to J, and a plus time and a minus polarity that are times when the polarity becomes a plus polarity within one cycle. The transfer bias is output so that the negative time which becomes 0.03 [msec] or more that contributes to the reversal of the toner from the recording sheet surface to the image carrier surface in the transfer nip is output. The output means is configured. In such a configuration, it is possible to suppress the occurrence of insufficient image density in the recesses by effectively returning the toner particles transferred into the recesses on the paper surface to the belt surface.

[Aspect L]
Aspect L is the aspect of any of the aspects A to K, wherein the frequency f [Hz] of the AC bias, the nip width d [mm] that is the length of the transfer nip in the moving direction of the image carrier, and the image carrier The body surface moving speed v [mm / s] satisfies the condition of “f> (4 / d) × v”. In this configuration, as described above, it is possible to avoid the occurrence of periodic density unevenness due to the insufficient number of reciprocating movements of the toner between the sheet surface and the image carrier surface in the transfer nip.

[Aspect M]
Aspect M includes an image carrier that carries a toner image on its surface, a nip forming member that contacts the surface to form a transfer nip, and an alternating electric field between the image carrier and the nip forming member. A transfer bias output means for outputting a transfer bias for forming a transfer electric field, and transferring a toner image on the surface of the image carrier to a recording sheet sandwiched between the transfer nips, and the transfer In the image forming apparatus in which the bias output means outputs a transfer bias composed of a superimposed bias obtained by superimposing an AC bias on a DC bias, the temperature, the humidity, the thickness of the recording sheet fed to the transfer nip, and the transfer The depth of the concave portion on the surface unevenness of the recording sheet fed into the nip, and the toner attached per unit area on the surface of the image carrier Information acquisition means for acquiring at least one of the quantity information, an output target value of the peak-to-peak voltage value of the AC bias according to an acquisition result by the information acquisition means, and the output The transfer bias output unit is configured to perform a process of reducing the output target value of the DC bias as the target value increases.

[Aspect N]
Aspect N is a process according to aspect M, in which the DC bias is output by constant current control, and the output target value in the DC bias constant current control is decreased as the output target value of the peak-to-peak voltage value is increased. The transfer bias output means is configured to be implemented.

[Aspect O]
Aspect O is a process according to aspect M, in which the DC bias is output by constant voltage control, and the output target value in the DC bias constant voltage control is decreased as the output target value of the peak-to-peak voltage value is increased. The transfer bias output means is configured to be implemented.

[Aspect P]
Aspect P is characterized in that, in aspects M to O, the transfer bias output means is configured to output the AC bias by constant voltage control.

31: Intermediate transfer belt (image carrier)
36: Nip forming roller (nip forming member)
39: Secondary transfer bias power supply (transfer bias output means)
50: Operation panel (information acquisition means)
51: Temperature sensor (information acquisition means)
52: Humidity sensor (information acquisition means)
P: Recording sheet

JP 2006-267486 A

Claims (6)

An image carrier that carries a toner image on its surface;
A nip forming member that contacts the surface and forms a transfer nip;
A transfer bias output means for outputting a transfer bias for forming a transfer electric field composed of an alternating electric field between the image carrier and the nip forming member;
Transfer the toner image on the surface of the image carrier to the recording sheet sandwiched in the transfer nip,
And, the transfer bias output unit, a transfer bias, an image forming apparatus and outputs a made of superimposed bias obtained by superposing an AC bias on a DC bias,
It has a temperature sensor that acquires temperature information,
An image forming apparatus , wherein the peak-to-peak voltage value of the AC bias increases and the absolute value of the DC bias decreases as the temperature acquired by the temperature sensor decreases .   An image carrier that carries a toner image on its surface;
A nip forming member that contacts the surface and forms a transfer nip;
A transfer bias output means for outputting a transfer bias for forming a transfer electric field composed of an alternating electric field between the image carrier and the nip forming member;
Transfer the toner image on the surface of the image carrier to the recording sheet sandwiched in the transfer nip,
The transfer bias output means outputs an image composed of a superimposed bias obtained by superimposing an AC bias on a DC bias as a transfer bias,
It has a humidity sensor that acquires humidity information,
An image forming apparatus, wherein the peak-to-peak voltage value of the AC bias increases and the absolute value of the DC bias decreases as the humidity acquired by the humidity sensor decreases. The image forming apparatus according to claim 1 or 2 ,
An image forming apparatus, wherein the transfer bias output means is configured to output the AC bias by constant voltage control. The image forming apparatus according to any one of claims 1 to 3 ,
As the transfer bias, toner is electrostatically transferred from the image carrier to the recording sheet in the transfer nip with a time average potential within one cycle of the superimposed bias being larger than the center value of the maximum potential and the minimum potential within one cycle. An image forming apparatus, wherein the transfer bias output means is configured to output a value shifted to a side that facilitates the transfer. The image forming apparatus according to any one of claims 1 to 4 ,
As the transfer bias, of the plus time that is a plus polarity within one cycle of the superimposed bias and the minus time that becomes a minus polarity, the toner from the recording sheet surface to the image carrier surface in the transfer nip An image forming apparatus, wherein the transfer bias output means is configured to output the one that contributes 0.03 [msec] or more to the reverse. The image forming apparatus according to any one of claims 1 to 5 ,
About the frequency f [Hz] of the AC bias, the nip width d [mm] which is the length of the transfer nip in the image carrier surface movement direction, and the surface movement speed v [mm / s] of the image carrier. An image forming apparatus having a condition of “f> (4 / d) × v” .

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