JP6066578B2 - Image forming apparatus - Google Patents

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, there is a risk of increasing costs. There is a demand for a method of suppressing the cost reduction by omitting a power supply dedicated to primary transfer.

In order to solve the above-described problems, the present invention provides an image carrier, an intermediate transfer member onto which a toner image is transferred from the image carrier, and a recording material sandwiched with the intermediate transfer member to record from the intermediate transfer member. A transfer member for transferring a toner image to the power source, a power source for applying a voltage to the transfer member, a Zener diode connected between the intermediate transfer member and the ground, and a current detection circuit for detecting a current flowing through the Zener diode And a control unit that controls a transfer voltage applied to the transfer member, and the power source is configured to be able to apply a voltage to the Zener diode via the intermediate transfer member, and a voltage is applied to the Zener diode. When applied, a primary transfer electric field for transferring a toner image from the image carrier to the intermediate transfer member is formed, and a voltage is applied to the transfer member. And in the is configured as a secondary transfer electric field for transferring the toner image formed on the recording medium from the intermediate transfer member, wherein, in the non-image-forming test bias to said transfer member And when the toner image is secondarily transferred from the intermediate transfer member to the recording material based on the detection result of the current detection circuit when the test bias is applied, a current of a predetermined value or more is applied to the Zener diode. The transfer voltage is controlled to flow .

  According to the present invention, in order to reduce the cost, a power supply dedicated for primary transfer is omitted and the intermediate transfer member is connected to a Zener diode, so that both primary transfer and secondary transfer can be achieved even when a minimum voltage is applied. be able to.

1 is a diagram illustrating a basic configuration of an image forming apparatus. It is a figure which shows the relationship between a transfer potential and an electrostatic image potential. It is a figure which shows the IV characteristic of a Zener diode. It is a figure which shows the block diagram of control. It is a figure which shows the relationship between an inflow current and an applied voltage. It is a figure which shows the relationship between a belt potential and an applied voltage. 2 shows a flowchart of the first 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 from the upstream side in the moving direction of the intermediate transfer belt 7 in the order of the image forming units 101a, 101b, 101c, and 101d, that is, in order of yellow, magenta, cyan, and black.

  Each of the image forming units 101a, 101b, 101c, and 101d includes photoreceptor drums 1a, 1b, 1c, and 1d as photoreceptors (image carriers) on which toner images are formed. The primary chargers 2a, 2b, 2c, and 2d are charging units that charge the surfaces of the photosensitive drums 1a, 1b, 1c, and 1d. The exposure devices 3a, 3b, 3c, and 3sd have a laser scanner, and expose the photosensitive drums 1a, 1b, 1c, and 1d 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 4a, 4b, 4c, and 4d includes a container that stores toner of each color of yellow, magenta, cyan, and black, and the electrostatic images on the photosensitive drums 1a, 1b, 1c, and 1d are transferred to the toner. It is a developing means that uses and develops.

  The toner images formed on the photosensitive drums 1a, 1b, 1c, and 1d are primarily transferred to the intermediate transfer belt 7 by primary transfer portions N1a, N1b, N1c, and N1d. In this way, the four color toner images are superimposed and transferred onto the intermediate transfer belt 7. The primary transfer will be described in detail later.

  The photosensitive drum cleaning devices 6a, 6b, 6c and 6d remove residual toner remaining on the photosensitive drums 1a, 1b, 1c and 1d without being transferred by the primary transfer portions N1a, N1b, N1c and N1d.

The intermediate transfer belt 7 is a movable intermediate transfer member to which toner images are transferred from the photosensitive drums 1a, 1b, 1c, and 1d. In the present embodiment, the intermediate transfer belt 7 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 7 is formed so that the volume resistivity of the base layer is 10 ² to 10 ⁷ 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 coat having a volume resistivity of 10 ¹³ to 10 ¹⁶ Ω · cm in the thickness direction 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 0.5 to 10 um. Of course, it is not intended to limit to these numerical values.

  The inner peripheral surface of the intermediate transfer belt 7 is stretched by rollers 10, 11, and 12 as stretching members. The roller 10 is driven by a motor as a driving source and functions as a driving roller for driving the intermediate transfer belt 7. The roller 10 is also a secondary transfer inner roller that presses against the secondary transfer outer roller 13 via an intermediate transfer belt. It functions as a tension roller that applies a constant tension to the intermediate transfer belt 7. Further, the roller 11 also functions as a correction roller that prevents the intermediate transfer belt 7 from meandering. The belt tension with respect to the tension roller 11 is configured to be about 5 to 12 kgf. By applying this belt tension, a nip is formed between the intermediate transfer belt 7 and the photosensitive drums 1a to 1d 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 7.

  The recording material is accommodated in a recording material tray for accommodating the recording material P. The recording material P is taken out from the recording material tray by a pickup roller at a predetermined timing and guided to the registration roller. The recording material P is sent out by the registration roller to the secondary transfer portion N2 that transfers 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 13 is a secondary transfer member that presses the secondary transfer inner roller 10 via the intermediate transfer belt 7 to form the secondary transfer portion N2 together with the secondary transfer inner roller 13. The secondary transfer outer roller 13 is a secondary transfer unit and holds the recording material together with the intermediate transfer belt. The secondary transfer unit high-voltage power source 22 as a secondary transfer power source is connected to the secondary transfer outer roller 13 and is a power source that can apply a voltage to the secondary transfer outer roller 13.

  When the recording material P is conveyed to the secondary transfer portion N2, a secondary transfer voltage having a polarity opposite to that of the toner is applied to the secondary transfer outer roller 13, whereby the toner image is transferred from the intermediate transfer belt 7 to the recording material. To do.

  The secondary transfer inner roller 10 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 13 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 13 is formed to be 24 mm.

  In order to remove residual toner and recording material powder remaining on the intermediate transfer belt 7 that are not transferred to the recording material at the secondary transfer portion N2 downstream of the secondary transfer portion N2 in the direction in which the intermediate transfer belt 7 moves. The intermediate transfer belt cleaning device 14 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 22 is used to electrostatically transfer the toner image from the photosensitive drum to the intermediate transfer belt 7. (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. That is, even when the secondary transfer power supply 22 applies a voltage, no current flows through the intermediate transfer belt 7 to the photosensitive drums 1a, 1b, 1c, and 1d, 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 1a, 1b, 1c, and 1d 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. 2 shows a case where the surface of the photosensitive drum 1 is charged by the charging unit 2 and becomes a potential Vd (here, −450 V) of the surface of the photosensitive drum. Further, FIG. 2 shows a case where the surface of the charged photosensitive drum is exposed by the exposure means 3 and the surface of the photosensitive drum becomes Vl (here, assumed to be −150 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 on the basis of the detection result of a potential sensor disposed in the vicinity of the photosensitive drum on the downstream side of the charging and 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 −250 V) is applied to the charged 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:
−150 (V) − (− 250 (V)) = 100 (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
−150 (V) − (− 450 (V)) = 300 (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,
300 (V)-(-150 (V)) = 450 (V)
It becomes.

  In the present embodiment, the potential sensor is arranged with an emphasis on the accuracy of detecting the potential of the photosensitive drum. However, the present invention is not intended to be limited to this configuration. Emphasizing cost reduction, the potential sensor is not disposed, and the relationship between the electrostatic latent image forming condition and the potential of the photosensitive drum is stored in the ROM in advance, and then based on the relationship stored in the ROM. It can also be configured to control the potential of the photosensitive drum.

[Zener diode]
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 the present embodiment, a Zener diode is used as a passive element disposed between the stretching roller and the ground.

  FIG. 3 shows the current-voltage characteristics of the Zener diode. A Zener diode has a characteristic that current hardly flows until a voltage equal to or higher than the Zener breakdown voltage Vbr is applied, but current rapidly flows when a voltage equal to or higher than the Zener breakdown voltage is applied. That is, in the range where the voltage applied to the Zener diode 15 is equal to or higher than the Zener breakdown voltage, the voltage drop of the Zener diode 15 causes a current to flow so as to maintain the Zener voltage.

  Utilizing such current-voltage characteristics of the Zener diode, the potential of the intermediate transfer belt 7 is kept constant.

  That is, in this embodiment, the Zener diode 15 is disposed as a passive element between all the stretching rollers 10, 11, 12 and the ground.

  In addition, during the primary transfer, the secondary transfer power supply 22 applies a voltage so that the voltage drop of the Zener diode 15 maintains the Zener breakdown voltage. As a result, the belt potential of the intermediate transfer belt 7 can be kept constant during the primary transfer.

  In the present embodiment, 12 Zener diodes 15 having a Zener breakdown voltage standard value Vbr of 25 V are arranged in series between the stretching roller and the ground. That is, in the range where the voltage applied to the Zener diode maintains the Zener breakdown voltage, the potential of the intermediate transfer belt is kept constant at the total Zener breakdown voltage of each Zener diode, that is, 25 × 12 = 300V.

  Of course, the present invention is not intended to be limited to a configuration using a plurality of Zener diodes. A configuration in which only one Zener diode is used may be employed.

  Of course, the surface potential of the intermediate transfer belt is not intended to be limited to 300V. It is desirable to set appropriately according to the type of toner used and the characteristics of the photosensitive drum.

  Thus, when a voltage is applied by the secondary transfer power source 210, the potential of the Zener diode is maintained at a predetermined potential, and a primary transfer electric field is formed between the photosensitive drum and the intermediate transfer belt. Further, similarly to the conventional configuration, when a voltage is applied by the secondary transfer high-voltage 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, 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 outer roller, and the stretching roller inflow current detection circuit 205 (zener diode current detection means) This is a circuit for detecting 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 22, 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 recording material 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.

[Judgment function]
In the present embodiment, a step for determining the lower limit voltage of the voltage applied by the secondary transfer power source for making the surface potential of the intermediate transfer belt equal to or higher than the zener voltage is executed. This will be described with reference to FIG.

  In this embodiment, in order to determine the lower limit voltage, a stretch roller inflow current detection circuit (zener diode current detection means) that detects a current flowing into the ground via the zener diode 15 is used. The tension roller inflow current detection circuit is connected between the Zener diode and the ground. That is, all the stretching rollers are grounded via the Zener diode and the stretching roller inflow current detection circuit.

  As shown in FIG. 3, the Zener diode has a characteristic that almost no current flows when the voltage drop of the Zener diode is less than the Zener breakdown voltage. Therefore, when the tension roller inflow current detection circuit does not detect current, it can be determined that the voltage drop of the Zener diode is less than the Zener breakdown voltage. When the tension roller inflow current detection circuit detects the current, it can be determined that the voltage drop of the Zener diode maintains the Zener breakdown voltage.

  First, the charging voltages of all the stations Y, M, C, and Bk are applied, and the surface potential of the photosensitive drum is controlled to the non-image portion potential Vd.

  Next, the secondary transfer power supply applies a test voltage. The test voltage applied by the secondary transfer power supply is increased linearly or stepwise. In FIG. 5, V1, V2, and V3 are raised in stages. When the voltage applied by the secondary transfer power supply is V1, the stretching roller inflow current detection circuit does not detect current (I1 = 0 μA). When the voltages applied by the secondary transfer power supply are V2 and V3, the stretching roller inflow current detection circuit detects I2 μA and I3 μA, respectively. Here, from the correlation between the applied voltage and the detected current when the tension roller inflow current detection circuit detects the current, the current inflow start voltage V0 corresponding to the case where the current starts to flow is calculated. That is, the current inflow start voltage V0 is calculated by performing linear interpolation from the relationship of I2, I3, V2, and V3.

  By setting a voltage higher than V0 as the voltage applied by the secondary transfer power supply, the voltage drop of the Zener diode can maintain the Zener breakdown voltage.

  FIG. 6 shows the relationship between the voltage applied by the secondary transfer power source and the belt potential of the intermediate transfer belt at this time.

  For example, in this embodiment, the Zener voltage of the Zener diode is set to 300V. Therefore, when the potential of the intermediate transfer belt is less than 300V, no current flows through the Zener diode, and when the belt potential of the intermediate transfer belt reaches 300V, current starts to flow through the Zener diode. Even if the voltage applied by the secondary transfer power supply is further increased, the belt potential of the intermediate transfer belt is controlled to be constant.

  That is, in the range of less than V0 where the current flow into the Zener diode starts to be detected, the belt potential cannot be controlled with a constant voltage when the secondary transfer bias changes. As long as the secondary transfer bias changes, the belt potential can be controlled at a constant voltage within a range exceeding V0 where the current flow into the Zener diode starts to be detected.

  In this embodiment, before and after the current inflow start voltage is used as the test voltage, but it is not intended to be limited to this configuration. By setting a large predetermined voltage as the test voltage in advance, the test voltage can be configured to exceed the current inflow start voltage. Such a configuration has an advantage that the determination step can be omitted.

  In the present embodiment, the determination function for calculating the current inflow start voltage V0 is executed with emphasis on enhancing the accuracy of calculating the current inflow start voltage. Of course, the intention is not limited to this configuration. Instead of executing the determination function for calculating the current inflow start voltage V0 with an emphasis on suppressing the downtime from becoming longer, the current inflow start voltage V0 may be stored in the ROM in advance. .

[Adjustment function for secondary transfer]
In the present embodiment, a function called ATVC (Active Transfer Voltage Control) for applying an adjustment voltage is executed in order to optimize the secondary transfer electric field for transferring the toner image to the recording material. This is an adjustment function for adjusting for secondary transfer, and is executed when the recording material does not pass through the secondary transfer portion when the recording material is not passing through. With ATVC, the correlation between the voltage applied by the secondary transfer power supply and the current flowing through the secondary transfer portion can be grasped.

  The adjustment function is performed by the CPU circuit unit 150 controlling the secondary transfer power source when the recording material does not pass through the secondary transfer unit. That is, the CPU circuit unit 150 functions as an execution unit (adjustment unit) that executes an adjustment function for secondary transfer.

  In ATVC as an adjustment function, a plurality of adjustment voltages Va, Vb, and Vc under constant voltage control are applied by a secondary transfer voltage power source. In ATVC, the currents Ia, Ib, and Ic that flow when the adjustment voltage is applied are detected by the secondary transfer portion current detection circuit 204 (secondary transfer current detection means). As a result, the correlation between voltage and current can be grasped.

[Secondary transfer target current setting]
Based on the correlation between the applied adjustment voltages Va, Vb, and Vc and the measured currents Ia, Ib, and Ic, voltages for supplying the secondary transfer target current It necessary for the secondary transfer Vi is calculated. The secondary transfer target current It is set based on the matrix shown in Table 1.

  Table 1 is a table stored in a storage unit provided in the CPU circuit unit 150. This table sets the secondary transfer target current It according to the absolute water content (g / kg) in the atmosphere. The reason for this will be described. As the amount of water increases, the charge amount of the toner decreases. Therefore, the secondary transfer target current is set so as to decrease as the moisture amount increases. That is, as the moisture amount increases, the secondary transfer target current It decreases. The absolute moisture amount is calculated by the CPU circuit unit 150 from the temperature detected by the temperature / humidity sensor 207 and the relative humidity. In this embodiment, the absolute moisture amount is used, but it is not intended to be limited to this. Humidity can be used in place of the absolute water content.

  When the moisture content (g / kg) is 19 (g / kg) between 22 (g / kg) and 18 (g / kg), the lower set value of 18 is used. .

  Here, the voltage Vi for flowing It is a voltage for flowing It when there is no recording material in the secondary transfer portion. However, the secondary transfer is performed when the recording material exists in the secondary transfer portion. Therefore, it is desirable to consider the resistance of the recording material. Therefore, the recording material sharing voltage Vii shared by the recording material is added to the voltage Vi. The recording material sharing voltage Vii is set based on the matrix shown in Table 2.

Table 2 is a table stored in a storage unit provided in the CPU circuit unit 150. This table sets the recording material sharing voltage Vii according to the absolute moisture content (g / kg) in the atmosphere and the basis weight (g / m ² ) of the recording material. As the basis weight increases, the recording material sharing voltage Vii increases. This is because as the basis weight increases, the recording material becomes thicker, and thus the electrical resistance of the recording material increases. Further, as the absolute water content increases, the recording material sharing voltage Vii decreases. This is because when the absolute moisture content increases, the moisture content of the recording material increases, so that the electrical resistance of the recording material increases. Further, the recording material sharing voltage Vii is larger during automatic duplex printing or manual duplex printing than during simplex printing. The basis weight is a unit indicating the weight per unit area (g / m ² ) and is generally used as a value indicating the thickness of the recording material. The basis weight may be input by the user at the operation unit, or the basis weight of the recording material may be input to the storage unit that stores the recording material. Based on these pieces of information, the CPU circuit unit 150 determines the basis weight.

  The voltage (Vi + Vii) obtained by adding the recording material sharing voltage Vii to Vi for flowing the secondary transfer target current It is the secondary transfer of the secondary transfer voltage controlled at a constant voltage during the secondary transfer process following the adjustment function. The target voltage Vt is set by the CPU circuit unit 150. That is, the CPU circuit unit 150 functions as a setting unit that sets the secondary transfer voltage. As a result, an appropriate voltage value is set according to the atmospheric environment and the recording material thickness. Further, during the secondary transfer, the secondary transfer voltage is applied in a state in which the constant voltage is controlled. Therefore, even if the width of the recording material is changed, the secondary transfer is performed in a stable state.

(Setting the minimum secondary transfer voltage)
In order to suppress prolonged downtime, it is desirable to perform primary transfer and secondary transfer in parallel. However, when the primary transfer and the secondary transfer are performed in parallel, if the voltage drop of the Zener diode falls below the Zener breakdown voltage, the primary transfer may become unstable.

  Therefore, in order to achieve both primary transfer and secondary transfer, it is desirable that the voltage drop of the Zener diode maintains the Zener breakdown voltage when the recording material passes through the secondary transfer portion.

  However, the transfer voltage applied when the recording material passes through the secondary transfer portion is set within a certain range. Even when the minimum transfer voltage is applied, it is desirable that both primary transfer and secondary transfer can be achieved.

  Therefore, in this embodiment, in order to achieve both primary transfer and secondary transfer even when the minimum transfer voltage for transferring the toner image to the recording material is applied, the transfer for transferring the toner image to the recording material is performed. The minimum voltage is set such that the voltage drop across the Zener diode maintains the Zener breakdown voltage.

  In this embodiment, as shown in Table 1, when the amount of water is a predetermined maximum amount of water, the minimum transfer voltage for transferring the toner image to the recording material is applied to the secondary transfer outer roller. Applied. Note that the maximum water content here is the maximum amount in the water content range that the image forming apparatus can handle, and is set in advance before shipment.

  Further, as shown in Table 2, the minimum transfer voltage for transferring the toner image of the recording material is the secondary transfer outer roller when the thickest recording material determined in advance is transported and one-side printing is performed. To be applied. The thinnest recording material here is the maximum amount in the thickness range that the image forming apparatus can handle, and is set in advance before shipment.

  That is, in this embodiment, the minimum transfer voltage for transferring the toner image to the recording material is applied when the water content is the maximum water content and the thickest recording material is conveyed by single-sided printing. . In this case, the minimum voltage of the secondary transfer voltage is set so that the voltage drop of the Zener diode maintains the Zener breakdown voltage in order to achieve both primary transfer and secondary transfer even in the case of the minimum voltage. Is done. That is, the voltage applied to the secondary transfer outer roller is set so as to exceed the current inflow start voltage V0.

  It is desirable to change the primary transfer electric field as the toner charge amount changes with the moisture amount. Therefore, in order to optimize the primary transfer electric field, it is desirable to adjust the image portion potential of the photoreceptor in accordance with the change in the amount of moisture.

(Embodiment 2)
Differences from the first embodiment will be described. The description overlapping with the first embodiment will be omitted.

  In the present embodiment, the transfer voltage varies depending on the number of durable sheets. That is, as the number of durable sheets increases, the transfer voltage decreases.

  As shown in Table 3, even when the toner charge amount is the same, if the cumulative number of images formed increases, the developer deteriorates and the toner charge amount decreases. It is desirable that the secondary transfer voltage decreases as the cumulative number of images formed increases. That is, in this embodiment, when the total number of image formations is a predetermined maximum number, the minimum transfer voltage for transferring the toner image to the recording material is applied to the secondary transfer outer roller. Here, the maximum number is the number determined by the life of the drum cartridge, and is determined in advance before shipment.

  In such a configuration, it is desirable that both the primary transfer and the secondary transfer can be achieved even when the total number of images formed is the maximum number of images that can be formed in advance.

  Therefore, in this embodiment, even if the minimum voltage is set when the maximum number of images that can be formed is predetermined, the minimum voltage is set so that the voltage drop of the Zener diode maintains the Zener breakdown voltage. The As a result, both primary transfer and secondary transfer can be achieved even when the total number of image formations increases.

  In this embodiment, since the lifetime of the drum cartridge is determined to be 50k, the voltage applied to the secondary transfer outer roller exceeds the current inflow start voltage V0 when the cumulative total number is 50k. Will be set as follows.

  From the viewpoint of achieving both primary transfer and secondary transfer, when the amount of moisture is a predetermined maximum amount and the maximum number of sheets, the voltage applied to the next transfer outer roller is: It is desirable to set a value that exceeds the current inflow start voltage V0.

  FIG. 7 shows a flowchart. When the operation of the image forming apparatus is started, the pre-rotation is performed for various controls (Step 3) such as potential control of the photosensitive drum (Step 1), determination of the lower limit value V0 of the secondary transfer voltage (Step 2), and other gradation control. To do. Next, ATVC control is performed in the secondary transfer portion, and the applied voltage is determined so that the current determined as the target value flows (Step 4). Thereafter, image formation is performed (Step 5), toner patches are formed between the sheets for density control (Step 6), and when the print job is completed, gamma correction control is performed and then stopped after rotation (Step 10). The forming operation ends.

  Since the lower limit value V0 of the secondary transfer voltage changes due to the resistance fluctuation of the second outer transfer roller during continuous paper feeding, correction control (Step 9) of the lower limit value V0 of the secondary transfer voltage is performed for each set number of sheets. .

  As described above, the resistance and the zener voltage of the secondary transfer outer roller are set so that the zener breakdown voltage can be obtained with the minimum voltage of the secondary transfer voltage. For this reason, it is possible to suppress transfer defects in the secondary transfer portion while maintaining the necessary transfer contrast in the primary transfer portion under all circumstances.

  It is desirable to change the primary transfer electric field according to the charge amount of the toner according to the number of images formed. Therefore, in order to optimize the primary transfer electric field, it is desirable to adjust the image portion potential of the photoconductor according to the number of images formed.

(Embodiment 3)
The description overlapping with the first embodiment will be omitted. Different points will be described.

  In the first embodiment, as the case where the secondary transfer voltage required in the secondary transfer portion is the lowest, a combination of a new secondary transfer outer roller in an HH environment and a durable drum cartridge durable in an image with a high printing ratio. As a result, the resistance of the secondary transfer outer roller was set.

  In this embodiment, attention is paid to the process speed of the image forming apparatus (moving speed of the intermediate transfer member).

  That is, when the process speed of the image forming apparatus can be switched in multiple stages, the required current at the secondary transfer portion decreases as the process speed decreases, and the voltage applied during secondary transfer decreases. In the present exemplary embodiment, assuming that the secondary transfer voltage required in the secondary transfer unit is the lowest in the image forming apparatus in which the process speed can be switched in multiple stages, the resistance of the secondary transfer outer roller and the zener voltage Is set. That is, when the process speed is the slowest predetermined speed, the minimum voltage is applied during the secondary transfer, and the minimum voltage is set so that the voltage drop of the Zener diode maintains the Zener breakdown voltage.

  A method for determining the secondary transfer voltage in this embodiment will be described. The process speed of the image forming apparatus in the present embodiment is 135 mm / sec (constant speed mode), and can be switched between two stages of 90.0 mm / sec (2/3 speed mode) according to the basis weight of the recording material. . That is, when the process speed is 90 mm / sec, which is the slowest, the secondary transfer voltage takes the minimum voltage, and the minimum voltage is set so that the voltage drop of the Zener diode maintains the Zener breakdown voltage.

  The minimum voltage is set when the absolute moisture amount is a predetermined maximum amount in the 2/3 speed mode and the drum cartridge is a durable product. In this case, the minimum voltage is set so that the voltage drop of the Zener diode maintains the Zener breakdown voltage.

  As described above, the minimum voltage of the secondary transfer voltage is set so that the voltage drop of the Zener diode maintains the Zener breakdown voltage. Therefore, it is possible to always prevent the transfer failure in the secondary transfer portion while maintaining the necessary transfer contrast in the primary transfer portion.

  It is desirable to change the primary transfer electric field according to the process speed. Therefore, in order to optimize the primary transfer electric field, it is desirable to adjust the image portion potential of the photoreceptor in accordance with the process speed.

Claims (6)

An image carrier;
An intermediate transfer member to which a toner image is transferred from the image carrier;
A transfer member for sandwiching a recording material together with the intermediate transfer member and transferring a toner image from the intermediate transfer member to the recording material;
A power source for applying a voltage to the transfer member;
A Zener diode connected between the intermediate transfer member and ground;
A current detection circuit for detecting a current flowing through the Zener diode;
A control unit for controlling a transfer voltage applied to the transfer member,
The power supply is configured to be able to apply a voltage to the Zener diode via the intermediate transfer member, and to transfer a toner image from the image carrier to the intermediate transfer member by applying a voltage to the Zener diode. And a secondary transfer electric field for transferring a toner image from the intermediate transfer member to the recording material is formed by applying a voltage to the transfer member. Configured,
The control unit applies a test bias to the transfer member during non-image formation, and records a toner image from the intermediate transfer member based on a detection result of the current detection circuit when the test bias is applied. The image forming apparatus controls the transfer voltage so that a current of a predetermined value or more flows through the Zener diode during secondary transfer . The image carrier includes a plurality of image carriers, and the toner images formed on the image carriers are superimposed on the intermediate transfer member, and then the superimposed toner images are transferred to a recording material. The image forming apparatus according to claim 1 , wherein the image forming apparatus is configured as described above. The controller is configured to transfer the toner image from the intermediate transfer member to the recording material on the basis of a current flowing through the transfer member when a different voltage is applied to the transfer member during non-image formation. 3. The image forming apparatus according to claim 1, wherein a transfer voltage applied to the transfer member during image formation is controlled so that a current flowing through the transfer member becomes a predetermined target current. A plurality of members for stretching the intermediate transfer member, wherein the zener diode is any one of claims 1 to 3, characterized in that connected between the all and the ground of the plurality of members The image forming apparatus described in 1. The intermediate transfer member is a configuration of two or more layers, in any one of claims 1 to 4 the surface layer of a volume resistivity of said image bearing member is equal to or higher than the volume resistivity of the other layers The described image forming apparatus. When you are transferring the toner image to the intermediate transfer member from the image bearing member, any one of claims 1 to 5 the toner image to the recording material from the intermediate transfer member, characterized in that it is configured to be transferred The image forming apparatus described in item 1.