JP2013213990A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus that has a configuration that eliminates a power source dedicated for primary transfer to reduce costs, and can change potential of an intermediate transfer body even if the intermediate transfer body is connected to voltage maintenance means.SOLUTION: Voltage maintenance means can be connected between an intermediate transfer body and an earth, and includes a plurality of circuit parts including Zener diodes. By changing preset voltage of the voltage maintenance means, potential of belts and potential of a photoreceptor drum can be changed, and thereby transfer efficiency can be stabilized irrespective of use environment.

Description

  The present invention relates to an image forming apparatus such as a copying machine or a printer using an electrophotographic system.

  In an electrophotographic image forming apparatus, in order to support various recording materials, a toner image is transferred from a photosensitive member to an intermediate transfer member (primary transfer), and then transferred from the intermediate transfer member to a recording material (secondary transfer). Thus, an intermediate transfer method for forming an image is known.

  Patent Document 1 describes a conventional configuration of an intermediate transfer system. That is, Patent Document 1 has a configuration in which a primary transfer roller is provided for primary transfer of a toner image from a photosensitive member to an intermediate transfer member, and a power source dedicated to primary transfer is connected to the primary transfer roller. Further, Patent Document 1 has a configuration in which a secondary transfer roller is provided for secondary transfer of a toner image from an intermediate transfer member to a recording material, and a power supply dedicated to secondary transfer is connected to the secondary transfer roller. is there.

  Patent Document 2 has a configuration in which a small power source is connected to the secondary transfer inner roller, and another large power source is connected to the secondary transfer outer roller. Japanese Patent Application Laid-Open No. H11-260260 describes that primary transfer for transferring a toner image from a photosensitive member to an intermediate transfer member is performed by applying a voltage to a secondary transfer inner roller with a small power source.

JP 2003-35986 A JP 2006-259640 A

  However, if a power supply dedicated for primary transfer is arranged, the cost may increase. That is, a method for reducing the cost by omitting the power source dedicated for primary transfer is desired.

  In order to solve the above problems, the present invention provides an image carrier, an intermediate transfer member on which a toner image is primarily transferred from the image carrier, and a recording material sandwiched between the intermediate transfer member and the toner from the intermediate transfer member. A transfer member for secondary transfer of the image; a secondary transfer electric field for secondary transfer of the toner image from the intermediate transfer member to the recording material; and a primary transfer electric field for primary transfer of the toner image from the image carrier to the intermediate transfer member. Are arranged between a power source for applying a voltage to the transfer member, and the intermediate transfer body and the ground, and a voltage higher than a predetermined voltage is applied to the transfer member, thereby the intermediate transfer. Voltage maintaining means for flowing current so that a voltage drop between the body and the ground maintains a set voltage equal to or lower than the predetermined voltage, and has a changing means capable of changing the set voltage. To do.

  According to the present invention, the potential of the intermediate transfer member can be changed even if the intermediate transfer member is connected to the voltage maintaining unit in a configuration in which a power supply dedicated to primary transfer is omitted for cost reduction.

The figure explaining the basic composition in Embodiment 1 The figure which shows the relationship between the transfer potential and electrostatic image potential in Embodiment 1. Zener diode IV characteristics Block diagram in the first embodiment The figure explaining the change of the charge amount of the toner by the absolute water content of the use environment The figure explaining the relationship between the primary transfer part transfer contrast and the residual transfer density of the image forming apparatus in the conventional intermediate transfer system The figure explaining the relationship between the primary transfer part transfer contrast and the residual transfer density of the image forming apparatus in the configuration without one transfer high pressure Block diagram in the first embodiment Flowchart in the first embodiment Flowchart in the second embodiment The figure explaining that the charge amount of toner rises by the number of continuous prints Block diagram in the third embodiment Flowchart in the third embodiment

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In addition, what attached | subjected the same code | symbol in each drawing has the same structure or effect | action, The duplication description about these was abbreviate | omitted suitably.

(Embodiment 1)
[Image forming apparatus]
FIG. 1 shows an image forming apparatus according to the present embodiment. The image forming apparatus employs a tandem system in which the image forming units of the respective colors are arranged independently and in tandem. Further, an intermediate transfer system is employed in which a toner image is transferred from an image forming unit of each color to an intermediate transfer member and then transferred from the intermediate transfer member to a recording material.

  The image forming units 101a, 101b, 101c, and 101d are image forming units that form yellow (Y), magenta (M), cyan (C), and black (K) toner images, respectively. These image forming units are arranged in the order of the image forming units 101a, 101b, 101c, and 101d from the upstream side in the moving direction of the intermediate transfer belt 56, that is, the order of yellow, magenta, cyan, and black.

  Each of the image forming units 101a, 101b, 101c, and 101d includes photoreceptor drums 50a, 50b, 50c, and 50d as photoreceptors (image carriers) on which toner images are formed. The primary chargers 51a, 51b, 51c, and 51d are charging units that charge the surfaces of the photosensitive drums 50a, 50b, 50c, and 50d. The exposure devices 52a, 52b, 52c, and 52d include a laser scanner, and expose the photosensitive drums 50a, 50b, 50c, and 50d charged by the primary charger. As the output of the laser scanner is turned on / off based on the image information, an electrostatic image corresponding to the image is formed on each photosensitive drum. That is, the primary charger and the exposure unit function as an electrostatic image forming unit that forms an electrostatic image on the photosensitive drum. Each of the developing devices 53a, 53b, 53c, and 53d includes a container that stores toner of each color of yellow, magenta, cyan, and black, and the electrostatic images on the photosensitive drums 50a, 50b, 50c, and 50d are converted into toner. It is a developing means that uses and develops.

  The toner images formed on the photosensitive drums 50a, 50b, 50c, and 50d are primarily transferred to the intermediate transfer belt 56 by primary transfer portions N1a, N1b, N1c, and N1d. In this way, the four color toner images are transferred onto the intermediate transfer belt 56 in an overlapping manner. The primary transfer will be described in detail later.

  The photosensitive drum cleaning devices 55a, 55b, 55c, and 55d remove residual toner remaining on the photosensitive drums 50a, 50b, 50c, and 50d without being transferred by the primary transfer portions N1a, N1b, N1c, and N1d.

The intermediate transfer belt 56 is a movable intermediate transfer body to which a toner image is transferred from the photosensitive drums 50a, 50b, 50c, and 50d. In this embodiment, the intermediate transfer belt 56 has a two-layer configuration of a base layer and a surface layer. The base layer is on the inner surface side (stretching member side) and is in contact with the stretching member. The surface layer is the outer surface side (image carrier side) and is in contact with the photosensitive drum. The base layer is made of a resin such as polyimide or polyamide, PEN or PEEK, or various rubbers containing an appropriate amount of an antistatic agent such as carbon black. The base layer of the intermediate transfer belt 56 is formed so that the volume resistivity of the base layer is 10 ⁶ to 10 ⁸ Ω · cm. As the base layer in the present embodiment, a film-like endless belt made of polyimide and having a center thickness of about 45 to 150 μm is used. Further, an acrylic coating having a volume resistivity of 10 ¹³ to 10 ¹⁶ Ω · cm is applied as a surface layer. That is, the resistance of the base layer is lower than the resistance of the surface layer. The thickness of the surface layer is 1 to 10 um. Of course, it is not intended to limit to these numerical values.

  The inner peripheral surface of the intermediate transfer belt 56 is stretched by various rollers 60, 61, 62, and 63 as stretch members. The idler rollers 60 and 61 stretch an intermediate transfer belt 56 that extends along the arrangement direction of the photosensitive drums 50a, 50b, 50c, and 50d. The tension roller 63 is a tension roller that applies a constant tension to the intermediate transfer belt 56. Further, the tension roller 63 also functions as a correction roller that prevents the intermediate transfer belt 56 from meandering. The belt tension with respect to the tension roller 63 is configured to be about 5 to 12 kgf. By applying this belt tension, a nip is formed between the intermediate transfer belt 56 and the photosensitive drums 50a to 50d as primary transfer portions N1a, N1b, N1c, and N1d. The secondary transfer inner roller 62 functions as a driving roller that is driven by a motor excellent in constant speed and circulates and drives the intermediate transfer belt 56.

  The recording material is stored in a paper tray that stores the recording material P. The recording material P is taken out from the paper tray by a pickup roller at a predetermined timing and guided to the registration roller 66. The recording material P is sent out by the registration roller 66 to the secondary transfer portion N2 for transferring the toner image from the intermediate transfer belt to the recording material in synchronization with the conveyance of the toner image on the intermediate transfer belt.

  The secondary transfer outer roller 64 is a secondary transfer member that presses the secondary transfer inner roller via the intermediate transfer belt 56 to form the secondary transfer portion N2 together with the secondary transfer inner roller 62. The secondary transfer outer roller is arranged so as to sandwich the recording material together with the intermediate transfer belt at the secondary transfer portion. The secondary transfer power supply 210 is connected to the secondary transfer outer roller 64 and is a power supply as a voltage application unit that applies a voltage to the secondary transfer outer roller 64.

  When the recording material P is conveyed to the secondary transfer portion N2, a toner image is transferred from the intermediate transfer belt 56 to the recording material by applying a secondary transfer voltage having a reverse polarity to the toner to the secondary transfer outer roller. .

  The secondary transfer inner roller 62 is made of EPDM rubber. The diameter of the secondary transfer inner roller is set to 20 mm, the rubber thickness is set to 0.5 mm, and the hardness is set to 70 ° (Asker-C). The secondary transfer outer roller 64 is made of an elastic layer made of NBR rubber, EPDM rubber or the like and a cored bar. The diameter of the secondary transfer outer roller is formed to be 24 mm.

  In order to remove residual toner and paper dust remaining on the intermediate transfer belt 56 without being transferred to the recording material by the secondary transfer portion N2 on the downstream side of the secondary transfer portion N2 in the moving direction of the intermediate transfer belt 56. An intermediate transfer belt cleaning device 65 is provided.

[Formation of primary transfer electric field in a single high pressureless system]
In the present embodiment, a power supply dedicated to primary transfer is omitted for cost reduction. Therefore, in this embodiment, the secondary transfer power source 210 is used to electrostatically transfer the toner image from the photosensitive drum to the intermediate transfer belt 56. (Hereinafter, this configuration is referred to as a high-pressure-less system.)

  However, in the configuration in which the roller that stretches the intermediate transfer belt is directly connected to the ground, even when the secondary transfer power supply 210 applies a voltage to the secondary transfer outer roller 64, almost no current flows to the stretch roller side. Current may not flow to the photosensitive drum side. In other words, even when the secondary transfer power supply 210 applies a voltage, no current flows to the photosensitive drums 50a, 50b, 50c, and 50d via the intermediate transfer belt 56, and between the photosensitive drum and the intermediate transfer belt, The primary transfer electric field for transferring the toner image does not work.

  Therefore, in order to make the primary transfer electric field action work in the reversing high pressureless system, a passive element is arranged between all of the stretching rollers 60, 61, 62, 63 and the ground so that a current flows to the photosensitive member side. Is desirable.

  As a result, the potential of the intermediate transfer belt becomes high, and a primary transfer electric field works between the photosensitive drum and the intermediate transfer belt.

In order to form a primary transfer electric field in a system without a high-voltage transfer, it is necessary for the current to flow along the circumferential direction of the intermediate transfer belt by applying a voltage from the secondary transfer power supply 210. However, if the resistance of the intermediate transfer belt itself is high, the voltage drop in the intermediate transfer belt in the moving direction (circumferential direction) in which the intermediate transfer belt moves increases. As a result, current may not easily flow to the photosensitive drums 50a, 50b, 50c, and 50d along the circumferential direction of the intermediate transfer belt. Therefore, it is desirable that the intermediate transfer belt has a low resistance layer. In this embodiment, in order to suppress a voltage drop in the intermediate transfer belt, the surface resistivity of the base layer of the intermediate transfer belt is formed to be 10 ² Ω / □ or more and 10 ⁸ Ω / □ or less. In this embodiment, the intermediate transfer belt has a two-layer structure. This is because disposing a high resistance layer on the surface layer makes it easier to suppress the current flowing in the non-image area and further enhance transferability. Of course, the intention is not limited to this configuration. A single-layer structure can be used, and a three-layer structure or more can also be used.

  Next, the primary transfer contrast, which is the difference between the potential of the photosensitive drum and the potential of the intermediate transfer belt, will be described with reference to FIG.

  FIG. 5 shows a case where the surface of the photosensitive drum 1 is charged by the charging unit 2 and becomes a potential Vd (here, −600 V) on the surface of the photosensitive drum. 5A shows a case where the surface of the charged photosensitive drum is exposed by the exposure unit 3 and the surface of the photosensitive drum becomes Vl (here, −250 V).

  The potential Vd is a potential of a non-image portion where no toner is adhered, and the potential Vl is a potential of an image portion where the toner on the photosensitive drum is adhered. Vitb indicates the potential of the intermediate transfer belt.

  The surface potential of the drum is controlled based on the detection result of the potential sensor 206 disposed in the vicinity of the photosensitive drum on the charging and downstream side of the exposure unit and upstream of the developing unit.

  The potential sensor detects the non-image part potential and the image part potential on the surface of the photosensitive drum, controls the charging potential of the charging unit based on the non-image part potential, and controls the exposure light amount of the exposure unit based on the image part potential To do.

  By this control, the surface potential of the photosensitive drum can be set to an appropriate value for both the image portion potential and the non-image portion potential.

  A developing bias Vdc (in this case, the DC component is −400 V) is applied to the charging potential on the photosensitive drum by the developing device 4, and the negatively charged toner is developed on the photosensitive drum side.

The development contrast Vca, which is the potential difference between the photosensitive drum Vl and the development bias Vdc, is:
Vca = −250 (V) − (− 400 (V)) = 150 (V)
It becomes. The electrostatic image contrast Vcb, which is the potential difference between the image portion potential Vl and the non-image portion potential Vd, is
Vcb = −250 (V) − (− 600 (V)) = 350 (V)
It becomes. The primary transfer contrast Vtr, which is the potential difference between the image portion potential Vl of the photosensitive drum and the potential Vitb (here, 300 V) of the intermediate transfer belt,
Vtr = 100 (V) − (− 250 (V)) = 350 (V)
It becomes.

[Voltage maintaining means]
In the transfer high pressureless system, the primary transfer is determined by the primary transfer contrast which is a potential difference between the potential of the intermediate transfer belt and the potential of the photosensitive drum. Therefore, in order to stably form the primary transfer contrast, it is desirable to keep the potential of the intermediate transfer belt constant.

  Therefore, in order to stabilize the primary transfer, a voltage maintaining unit is arranged to maintain a voltage drop between the intermediate transfer member and the ground at a set voltage when a voltage higher than a predetermined voltage is applied to the secondary transfer outer roller. .

  In this embodiment, a Zener diode is used to configure the voltage maintaining means.

  FIG. 3 shows the current-voltage characteristics of the Zener diode. The zener diode hardly conducts current until a voltage higher than the zener breakdown voltage is applied. However, when a voltage higher than the Zener breakdown voltage is applied, the current flows suddenly. That is, in the range where the voltage applied to the Zener diode 11 is equal to or higher than the Zener breakdown voltage, a current is passed so that the voltage drop of the Zener diode 11 maintains the Zener breakdown voltage.

  Utilizing such a current-voltage characteristic of the Zener diode, the potential of the intermediate transfer belt 56 is kept constant.

  That is, a circuit including a Zener diode is disposed between the intermediate transfer belt and the ground. This circuit functions as a voltage maintaining means for maintaining a set voltage by a voltage drop between the intermediate transfer belt and the ground when a voltage higher than a predetermined voltage is applied to the secondary transfer outer roller. Since the predetermined voltage needs to be a voltage that is equal to or higher than the set voltage, the set voltage is a value that is equal to or lower than the predetermined voltage.

  In the present embodiment, a circuit including the Zener diode 11 is disposed as a voltage maintaining unit between the idler rollers 60 and 61, the secondary transfer inner roller 62, and the tension roller 63 tension roller and the ground.

  In addition, during the primary transfer, the secondary transfer power supply 210 applies a voltage higher than a predetermined voltage so that the voltage drop of the Zener diode 11 maintains the Zener breakdown voltage. As a result, the belt potential of the intermediate transfer belt 56 can be kept constant during the primary transfer.

  As described above, when a voltage equal to or higher than a predetermined voltage is applied to the secondary transfer outer roller by the secondary transfer power source 210, the belt potential of the intermediate transfer belt 56 is maintained constant, so that the photosensitive drum and the intermediate transfer belt A primary transfer electric field is formed between them. Further, when a voltage is applied by the secondary transfer power source, a secondary transfer electric field is formed between the intermediate transfer belt and the secondary transfer outer roller.

[controller]
The configuration of a controller that controls the entire image forming apparatus will be described with reference to FIG. As shown in FIG. 4, the controller has a CPU circuit unit 150. The CPU circuit unit 150 includes a CPU (not shown), a ROM 151, and a RAM 152. The secondary transfer portion current detection circuit 204 is a circuit (secondary transfer current detection means) for detecting the current flowing through the secondary transfer portion, and the stretching roller inflow current detection circuit 205 detects the current flowing into the stretching roller. The potential sensor 206 is a sensor for detecting the potential of the surface of the photosensitive drum, and the temperature / humidity sensor 207 is a sensor for detecting temperature / humidity.

  Information from the secondary transfer portion current detection circuit 204, the stretching roller inflow current detection circuit 205, the potential sensor 206, and the temperature / humidity sensor 207 is input to the CPU circuit portion 150. The CPU circuit unit 150 controls the secondary transfer power source 210, the development high voltage power source 201, the exposure unit high voltage power source 202, and the charging unit high voltage power source 203 in accordance with a control program stored in the ROM 151. An environment table and a paper thickness correspondence table, which will be described later, are stored in the ROM 151 and reflected by being called by the CPU. The RAM 152 temporarily stores control data and is used as a work area for arithmetic processing associated with control.

[Change means]
In order to stabilize the primary transfer, it is desirable that the potential of the intermediate transfer belt is stabilized. Therefore, in order to stabilize the primary transfer, a voltage maintaining unit is arranged to maintain a voltage drop between the intermediate transfer member and the ground at a set voltage when a voltage higher than a predetermined voltage is applied to the secondary transfer outer roller. . However, if only one set voltage is set, the potential of the intermediate transfer belt cannot be changed. Therefore, it is desirable that the potential of the intermediate transfer belt can be changed.

  Therefore, in the present embodiment, in order to change the potential of the intermediate transfer belt, a changing unit that can change the set voltage of the voltage maintaining unit is provided.

  By the way, when the charge amount of the toner changes depending on the atmospheric environment, the proper primary transfer contrast changes. Therefore, it is desirable to change the appropriate primary transfer electric field between the intermediate transfer member and the image carrier as the atmospheric environment changes.

  Therefore, in the present embodiment, the changing unit changes the set voltage according to the atmospheric environment.

  In the present embodiment, the voltage maintaining unit can be connected between the intermediate transfer member and the ground, and includes a plurality of circuit units including Zener diodes. In the present embodiment, a circuit unit 14-1 (first circuit unit) and 14-2 (second circuit unit) are provided.

  The circuit unit 14-1 directly connects four Zener diodes 11 having a standard value of the Zener breakdown voltage Vbr of 25V so as to maintain the voltage drop between the intermediate transfer belt and the ground at the set voltage 100V. Configured (25 × 4 = 100).

  The circuit unit 14-2 directly connects twelve Zener diodes 11 having a standard value of the Zener breakdown voltage Vbr of 25V so as to maintain the voltage drop between the intermediate transfer belt and the ground at the set voltage 300V. Configured (25 × 12 = 300).

  That is, the circuit units 14-1 and 14-2 are configured to maintain different set voltages.

  In the present embodiment, the changing unit includes a switch 13 that switches the electrical connection of the intermediate transfer member to one of a plurality of circuit units, and a switching control unit that controls the operation of the switch 13. In the present embodiment, the CPU circuit unit 150 functions as a switching control unit.

  Next, the necessary transfer contrast in the primary transfer unit when the use environment (temperature / humidity) of the image forming apparatus is changed will be described.

  As shown in FIG. 5, the charge amount (tribo) of the toner varies depending on the use environment. The HH environment is a temperature of 30 ° C. and a humidity of 80% RH, the NN environment is a temperature of 23 ° C. and a humidity of 50% RH, and the NL environment is a temperature of 23 ° C. and a humidity of 5% RH. Generally, the tribo is low in a high temperature and high humidity environment, and the tribo is high in a low humidity environment.

FIG. 6 shows the residual transfer density and the residual retransfer density in a conventional configuration, that is, a configuration including a power supply dedicated to primary transfer. The horizontal axis represents the potential difference (transfer contrast) between the potential of the back surface of the intermediate transfer belt and the surface of the photosensitive drum when a high voltage is applied to the core of the primary transfer roller. The residual transfer density means that a solid solid color patch having a constant density of about 0.4 mg / cm ² is formed on the photosensitive drum in the cyan image forming unit, and then remains on the photosensitive drum without being transferred to the intermediate transfer member. Concentration. The re-transfer residual density is a magenta image forming unit that forms a single-color solid patch with a constant density of about 0.4 mg / cm ² on a drum, then performs primary transfer to an intermediate transfer member, and primary transfer of cyan. Is the density of magenta re-transferred to the photosensitive drum. When the transfer residual density is 0.1 or less, the ratio of the toner on the drum that is primarily transferred onto the intermediate transfer belt is approximately 90% or more. The range of transfer contrast where the transfer residual density is 0.1 or less is called transfer latitude. Also in the conventional image forming apparatus, it is necessary to set the transfer contrast within the range of the transfer latitude for each use environment, and this has been dealt with by changing the drum potential or the voltage applied to the primary transfer member.

  FIG. 7 shows the transfer / retransfer residual density of the image forming apparatus having the configuration without one transfer high pressure in this embodiment. Compared with the conventional image forming apparatus shown in FIG. 6, it can be seen that even if the transfer contrast is increased, the density of the retransfer residue does not increase and the transfer latitude is widened. This is because the retransfer is caused by the discharge downstream of the nip of the primary transfer portion. However, in the configuration where the intermediate transfer belt is equipotential, there is much discharge upstream of the nip, and the discharge downstream of the nip hardly occurs. It is. Therefore, in contrast to the conventional image forming apparatus, the one-transition high-pressure-less configuration hardly causes retransfer on the side where the transfer contrast is high, and the transfer latitude is widened.

  Therefore, in the present embodiment, the set voltage of the voltage maintaining means is set so as to correspond to the transfer latitude in each environment (HH environment, NN environment, NL environment) using the overlap of the transfer latitude in each environment. The In other words, in the present embodiment, the first circuit unit 14-1 is configured to maintain 100V (first set voltage) as the set voltage when a voltage is applied. The second circuit unit 14-2 is configured to maintain 300V (second set voltage) as a set voltage when a voltage is applied.

  In this embodiment, since each circuit unit has a different set voltage, the number of Zener diodes included in each circuit unit is different, but it is not intended to be limited to this configuration. Since each circuit unit has a different set voltage, the Zener diodes of each circuit unit may have different Zener breakdown voltages.

  In this embodiment, each circuit unit uses a plurality of Zener diodes, but is not intended to be limited to this configuration. Each circuit unit may use only one Zener diode.

  In the present embodiment, the first circuit unit and the second circuit unit are arranged in parallel. However, it is not intended to limit to this configuration. The first circuit portion and the second circuit portion may have a portion shared in part, and are not intended to limit the circuit form or the element characteristics. Any configuration can be used as long as the set voltage to be maintained can be changed.

  In the present embodiment, the potential of the intermediate transfer belt is switched between 100 V and 300 V depending on the amount of moisture in the usage environment, thereby supporting from the HH environment to the NL environment.

  FIG. 2 is a diagram illustrating the relationship between the potentials of the drum, the developing sleeve, and the intermediate transfer belt for each use environment. According to FIG. 2, the appropriate transfer contrast is about 350 V in an environment where the amount of water in the use environment is medium (NN environment) to high (HH environment). In this case, if the potential of the drum dark portion (Vd) is −600 V, the potential of the developing sleeve (Vdc) is set to −400 V by securing a difference of 200 V from the potential of the drum dark portion due to concern about the toner fog on the non-image area. Is set. The potential (Vl) of the bright portion of the drum where the toner image is formed during image formation is about −250 V for a solid image. In the NN environment to the HH environment, the primary transfer contrast can be set to 350V by setting the intermediate transfer belt potential to 100V.

  On the other hand, when the amount of water in the usage environment is low (NL environment), the appropriate primary transfer contrast is about 500V. Even in the NL environment, the drum dark portion potential is -600V, and the developing sleeve potential is -400V. With respect to the potential of the bright portion of the drum, since the toner tribo is higher in the NL environment than in the NN environment or the HH environment, the potential difference between the developing sleeve and the bright portion of the drum is required to be larger, and is about −200 V for a solid image. In the above NL environment, the transfer contrast can be set to 500V by setting the potential of the intermediate transfer belt to 300V.

  That is, as shown in Table 1, the set voltage is set according to the absolute water content. When the absolute water content is the first absolute water content (when the absolute water content is 5 g / m 3 or more), the first set voltage (100 V) is set as the set voltage. When the absolute moisture content is a second absolute moisture content that is less than the first absolute moisture content (when the absolute moisture content is less than 5 g / m3), a second setting that is higher than the first setting voltage as the setting voltage. The voltage (300V) is set.

  In this embodiment, the primary transfer contrast can be set in two stages of 350V and 500V. However, switching does not necessarily have to be performed in two stages, and may be performed in three or more stages. Of course, depending on the configuration of the image forming apparatus, the necessary transfer contrast varies depending on the use environment, so the primary transfer contrast is not limited to 350V and 500V.

Next, a flow until the potential of the intermediate transfer belt is changed when the use environment (temperature / humidity) of the image forming apparatus is changed will be described. In this embodiment, as shown in the environmental table of Table 1, when the absolute water content is 5 g / m ³ or more, the transfer contrast is set to 350 V, and when it is less than 5 g / m ^3, 500 V is set.

FIG. 8 shows a block diagram. The main body has a temperature / humidity sensor 207, and calculates the absolute water content from the measurement result. Control unit 150 calculated absolute water content receiving by the, 5 g / ^{m 3} potential of the intermediate transfer belt in the case more than the 100 V, the potential of the intermediate transfer belt is smaller than 5 g / ^{m 3} and a 300V Thus, feedback is made to the Zener diode selector switch 13.

FIG. 9 shows a flowchart. When the printing operation is started, the temperature and humidity sensor measures the temperature and humidity of the usage environment, the control unit calculates the absolute moisture content of the usage environment (S1), and determines whether the absolute moisture content is 5 g / m ³ or more. (S2). According to the environment table of Table 1, when the absolute water content is 5 g / m ³ or more, the Zener diode changeover switch is set so that the intermediate transfer belt potential becomes 100 V (S3). When the absolute water content is less than 5 g / m ³ , the intermediate transfer belt potential is set to 300 V by the Zener diode changeover switch (S4). Next, pre-rotation for various controls is performed (S5). Thereafter, image formation is performed (S6), and it is determined whether the job is completed (S7). When the job is finished, the job is stopped after post-rotation, and the printing operation is completed (S8). If the job has not ended, image formation is performed again.

  As described above, the transfer potential can be stabilized regardless of the use environment by enabling the belt potential to be changed in the configuration without one high-voltage transfer.

  It is desirable that the secondary transfer contrast, which is a potential difference between the photosensitive drum forming the secondary transfer electric field and the intermediate transfer, is also optimized according to the charge amount of the toner. Therefore, in the present embodiment, the secondary transfer contrast is optimized by changing the secondary transfer contrast in accordance with the change in the potential of the intermediate transfer member. In other words, when the absolute value of the set voltage of the intermediate transfer member becomes large (300 V), the absolute value of the voltage applied to the secondary transfer outer roller is increased. When the absolute value of the intermediate transfer member becomes small (100 V), the absolute value of the voltage applied to the secondary transfer outer roller is reduced. In this way, the secondary transfer contrast can be optimized.

(Embodiment 2)
Description of points common to the first embodiment will be omitted. Differences from the first embodiment will be described. In the first embodiment, the set voltage of the voltage maintaining unit is controlled according to the atmospheric environment (absolute water content). On the other hand, in the second embodiment, not only the set voltage of the voltage maintaining means but also the image portion potential is changed according to the atmospheric environment (absolute water content).

  In Embodiment 1, the transfer contrast was two levels of + 350V and + 500V. However, depending on the use environment and toner conditions, a higher level of transfer contrast may be required. In such a case, if an attempt is made to deal with only the potential of the intermediate transfer belt, a large number of Zener diodes are required, and there is a concern that the cost increases and the configuration becomes complicated. Therefore, in the present embodiment, it is possible to cope with a case where the level of the transfer contrast is finer by changing both the intermediate transfer belt potential and the photosensitive drum potential.

Table 2 shows an environmental table regarding the drum potential and the belt potential when a solid image is formed. In this embodiment, the NL environment to the HH environment are divided into four. When the absolute water content is 15 g / m ³ or more, the belt potential is set to 100 V, Vd is set to −500 V, Vback is set to 200 V, Vdc is set to −300 V, and Vl is set to −150 V, thereby setting the transfer contrast to 250 V. When the absolute water content is less than 15 g / m ^{3 and} 10 g / m ³ or more, the transfer contrast is set to 350 V by setting the belt potential to 100 V, Vd to −600 V, Vdc to −400 V, and Vl to −250 V. When the absolute water content is less than 10 g / m ^{3 and} 5 g / m ³ or more, the transfer contrast is set to 400 V by setting the belt potential to 300 V, Vd to −500 V, Vdc to −300 V, and Vl to −100 V. Finally, when the absolute water content is less than 5 g / m ³ , the transfer contrast is set to 500 V by setting the belt potential to 300 V, Vd to −600 V, Vdc to −400 V, and Vl to −200 V.

  As described above, in the present exemplary embodiment, control for changing the primary transfer contrast is performed by changing the potential of the non-image portion in a state where a predetermined set voltage is set for the intermediate transfer member. Further, control for changing the primary transfer contrast is also performed by changing both the set voltage and the non-image portion potential for the intermediate transfer member.

  In this embodiment, the transfer contrast can be set in four stages. However, the switching does not necessarily have to be four stages. Of course, depending on the configuration of the image forming apparatus, the necessary transfer contrast varies depending on the use environment, so the value of the transfer contrast is not limited.

  Next, a flow until the transfer contrast is changed when the use environment (temperature / humidity) of the image forming apparatus is changed will be described.

FIG. 10 shows a flowchart. When the printing operation is started, the temperature / humidity sensor measures the temperature and humidity of the use environment, and the control unit calculates the absolute moisture content of the use environment (S1). Based on the calculation result of the absolute water content, the transfer contrast is determined based on the environment table in Table 2, and is fed back to the Zener diode switch and the image forming unit. If the absolute water content X is 15 g / m ³ or more (S2), Vd−500V, belt potential + 100V (S3). If the absolute water content X is less than 15 g / m ^{3 and} 10 g / m ³ or more (S4), Vd−600V, belt potential + 100V (S5). If the absolute water content X is less than 10 g / m ^{3 and} 5 g / m ³ or more (S6), Vd−500V, belt potential + 300V (S7). If the absolute water content X is less than 5 g / m ³ , Vd−600V, belt potential + 300V (S8). Next, pre-rotation for various controls is performed (S9). Thereafter, image formation is performed (S10), and it is determined whether the job is completed (S11). When the job is finished, the job is stopped after post-rotation, and the printing operation is completed (S12). If the job has not ended, image formation is performed again.

  As described above, the transfer efficiency can be stabilized regardless of the use environment by making it possible to change the belt potential and the photosensitive drum potential in the configuration without one transfer and high voltage.

(Embodiment 3)
Description of points that are common to the first embodiment will be omitted, and different points will be described.

  In the first embodiment, the set voltage of the voltage maintaining unit is controlled according to the atmospheric environment. On the other hand, in the third embodiment, the set voltage of the voltage maintaining unit is controlled in accordance with the continuous number of recording materials on which images are continuously formed during one image forming job.

  That is, when the continuous number is the first number (when the number of recording materials is less than 10k), the first setting voltage (100V) is selected as the setting voltage, and the second number where the continuous number is larger than the first number. Is selected (when the number of recording materials is 10k or more), the second set voltage (300 V) higher than the first set voltage is selected as the set voltage.

  FIG. 11 is a graph showing the relationship between the number of continuous prints and toner tribos in the NN environment of the image forming apparatus. According to this, the tribo increases as the number of continuous prints increases. This increase in tribo is because the toner and carrier are rubbed against each other by continuous image formation, and triboelectric charging proceeds. In addition, when printing is repeated at an image ratio of about 5% that is generally used in the market, there is a similar tendency. The first and second embodiments are intended to cope with tribo fluctuations depending on the use environment. In the present embodiment, the transfer contrast is changed by changing the tribo fluctuation that occurs during continuous printing.

  Table 3 shows the durability table. When the number of continuously printed sheets is less than 10 k, the transfer contrast is 350 V, when the number is 10 k or more and less than 20 k, 400 V, and when 20 k or more, the transfer contrast is 450 V. In each case, set values of potentials of the intermediate transfer belt and the photosensitive drum are determined.

  In this embodiment, the transfer contrast can be set in three stages. However, the switching need not necessarily be in three stages. Of course, depending on the configuration of the image forming apparatus, the necessary transfer contrast for each number of continuous prints also changes, so the value of the transfer contrast is not limited.

  Next, a flow until the transfer contrast is changed when the sheet is continuously fed by the image forming apparatus of the present embodiment will be described.

  FIG. 12 shows a block diagram. The counter unit 301 is a print number counter 301 that counts the number of recording materials (number of prints) on which an image is formed. In response to the continuous printing number from the counter unit, the control unit 150 feeds back the Zener diode changeover switch 13 and the image forming unit to an appropriate transfer contrast based on the durability table shown in Table 3.

  FIG. 13 shows a flowchart. When the printing operation is started, pre-rotation for various controls is performed (S1). Next, the counter unit counts how many sheets have been printed continuously, and determines whether or not the transfer contrast needs to be changed (S2). When the transfer contrast needs to be changed depending on the result of the counter unit, the transfer contrast is determined based on the durability table in Table 3. If the number of continuously printed sheets is less than 10k (S3), Vd-600V, belt potential + 100V (S4). If the continuous printing number is 10 k or more and less than 20 k (S5), Vd-500V, belt potential + 300V (S6). If the number of continuous prints is 20k or more, Vd-600V, belt potential + 300V (S7). Thereafter, image formation is performed (S8), and it is determined whether the job is completed (S9). When the job is finished, the job is stopped after post-rotation, and the printing operation is completed (S10). If the job has not ended, image formation is performed again after determination by the counter unit.

  Even when the toner tribo is raised by continuous paper feeding, the tribo is lowered if the image formation is not performed for a while. In contrast to such a case, when the image formation is not performed for a certain period of time, the continuous sheet passing number of the counter unit may be reset to zero.

  As described above, the belt potential and the photosensitive drum potential can be changed in a configuration without one high-voltage transfer, so that the transfer efficiency can be stabilized even when the charge amount of the toner is changed by continuous paper feeding.

  Further, it is conceivable that a difference occurs in the toner tribo of each color of Y, M, C, and Bk due to continuous printing of a single color image or replacement of a process cartridge. If it is not possible to cope with the tribo difference of each color only by changing the potential of the intermediate transfer belt, a configuration in which the surface potential of the photosensitive drum is changed for each color is also conceivable.

  The setting potential of the intermediate transfer body can be changed according to both the continuous printing number and the atmospheric environment, or the setting potential of the intermediate transfer body can be changed only according to the continuous printing number. You can also.

  In the above embodiment, the environmental difference and the durability difference are described as the conditions for changing the toner tribo. However, the conditions affecting the tribo are not limited to the above two items, and the same can be said for the image duty.

  In the present embodiment, an image forming apparatus that forms an electrostatic image by an electrophotographic method has been described. An image forming apparatus that forms an electrostatic image by an electrostatic force method instead of the electrophotographic method can be provided.

50 Photosensitive drum 56 Intermediate transfer belt 210 Power supply for secondary transfer 64 Secondary transfer outer roller 150 CPU circuit section

Claims (7)

An image carrier;
An intermediate transfer member on which a toner image is primarily transferred from the image carrier;
A transfer member for sandwiching a recording material together with the intermediate transfer member and secondarily transferring a toner image from the intermediate transfer member;
In order to form a secondary transfer electric field for secondary transfer of the toner image from the intermediate transfer body to a recording material and a primary transfer electric field for primary transfer of the toner image from the image carrier to the intermediate transfer body. A power supply for applying a voltage to
Arranged between the intermediate transfer member and the ground, a voltage not lower than a predetermined voltage is applied to the transfer member so that a voltage drop between the intermediate transfer member and the ground has a set voltage not higher than the predetermined voltage. Voltage maintaining means for flowing current so as to maintain,
An image forming apparatus comprising a changing unit capable of changing the set voltage.   The image forming apparatus according to claim 1, wherein the changing unit changes the set voltage according to an atmospheric environment.   The atmosphere environment is an absolute moisture content, and the changing means selects the first set voltage as the set voltage when the absolute moisture content is the first absolute moisture content, and the absolute moisture content is The second set voltage higher than the first set voltage is selected as the set voltage when the second absolute moisture amount is smaller than the absolute moisture amount of 1. Image forming apparatus.   The image forming apparatus according to claim 1, wherein the changing unit changes the set voltage in accordance with a continuous number of recording materials on which toner images are continuously formed.   The changing means selects the first set voltage as the set voltage when the continuous number of recording materials on which images are continuously formed is the first number, and the continuous number of recording materials is the first. 5. The image forming apparatus according to claim 4, wherein a second set voltage higher than the first set voltage is selected as the set voltage when the number is a second number greater than the number of sheets. The voltage maintaining means includes a plurality of circuit units for maintaining different set voltages,
The image forming apparatus according to claim 1, wherein the changing unit switches an electrical connection with the intermediate transfer member to any one of the plurality of circuit units. .   The image forming apparatus according to claim 6, wherein each of the circuit units includes a different number of Zener diodes.