JP2018010140A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus that can reduce the occurrence of image defects despite the fact that a transfer voltage can be set by a user, and does not unnecessarily reduce a setting range of the transfer voltage.SOLUTION: An image forming apparatus comprises: an intermediate transfer body; secondary transfer means; bias application means that applies a secondary transfer bias to the secondary transfer means; current detection means that can detect a current flowing in the secondary transfer means; and a control part that controls the bias application means so that the secondary transfer bias applied by the bias application means is not set beyond a predetermined upper limit voltage (step S17). The control part changes the upper limit voltage on the basis of a current detected by the current detection means when a test bias is applied to the secondary transfer means during a non-image formation period and the applied test bias (step S12).SELECTED DRAWING: Figure 5

Description

  The present invention relates to an image forming apparatus that forms an image on a recording material by an electrophotographic method or an electrostatic recording method.

  2. Description of the Related Art Conventionally, an electrophotographic image forming apparatus has been widely applied as a copying machine, a printer, a plotter, a facsimile, and a multifunction machine having a plurality of these functions. In this type of image forming apparatus, a configuration is known in which a toner image formed on a photosensitive drum as an image carrier is primarily transferred to an intermediate transfer belt as an intermediate transfer member and then secondarily transferred onto a recording material (sheet). ing. As this intermediate transfer belt, a belt in which a conductive agent is added to a resin material and adjusted to a desired electric resistance value has been proposed.

  In such an image forming apparatus, in order to cope with various transfer materials, the sheet classification is defined by conditions such as sheet type, for example, sheet thickness, basis weight, and surface property. Further, as an image forming apparatus, an apparatus that designates a sheet section of a sheet to be used by a user for each sheet tray and changes transfer conditions and fixing conditions according to the designated sheet section is generally known. As an example of changing the transfer condition in this case, for example, there is a case where the sheet resistance value is greatly different from the reference value and the toner is not properly transferred. In this case, for example, to adjust the shared voltage of the sheet and change the setting to match the resistance value of the sheet, or to avoid image defects at the leading edge or the trailing edge of the sheet, In some cases, the bias setting at the rear end is made weaker than that at the center. Further, even if the sheets belong to the same sheet category, for example, the physical properties such as the electric resistance of the sheets are different, and optimal transfer conditions and fixing conditions may not be obtained.

  In order to cope with such a situation, an image forming apparatus having an adjustment function that allows a user to set transfer conditions and fixing conditions on a user mode screen is known (see Patent Document 1). An example of the transfer condition here is a secondary transfer voltage. In this case, in the user mode, the user freely sets the secondary transfer voltage within the high voltage guaranteed range of the high voltage power supply, and the secondary transfer voltage as set is output from the high voltage power supply.

JP 2013-174875 A

  However, in the above-described image forming apparatus disclosed in Patent Document 1, in the user mode, the user can freely set the secondary transfer voltage within the high voltage guaranteed range of the high voltage power source, and the secondary transfer voltage as set can be set from the high voltage power source. Is output. For this reason, there is a possibility that the secondary transfer voltage set by the user is excessively set with respect to the impedance of the secondary transfer portion for reasons such as usage environment and durability. In this case, the secondary transfer voltage is excessively applied from the high voltage power source to the secondary transfer unit, and a current value exceeding the guaranteed high voltage range may flow to the secondary transfer unit.

  On the other hand, a protection circuit is generally provided on the high-voltage board in order to prevent heat generation and ignition or to avoid a failure due to discharge inside the board. For this reason, when a current value exceeding the high voltage guarantee range as described above flows into the secondary transfer portion, the protection circuit operates. Examples of the protection operation include an operation of switching to an intermittent oscillation operation so that overcurrent continuously flows and does not generate heat, and an operation of turning off a high-voltage output.

  However, when such a protection circuit is operated, the toner image on the intermediate transfer belt is not properly transferred to the sheet, and there is a problem that an image defect such as loss of image information is finally caused. In order to avoid image defects due to such an excessive voltage setting, it may be possible to uniformly set an upper limit for the secondary transfer voltage setting range that can be selected by the user. Here, even in the same sheet category, there are sheet types with different resistance values of several digits or more, or even in the same sheet brand, the resistance value varies in units of digits depending on the amount of moisture depending on the sheet management state. End up. For this reason, if the upper limit of the secondary transfer voltage is set according to the case where the resistance value is the minimum, if the resistance value is much larger than that, the upper limit of the secondary transfer voltage that does not cause image defects is exceeded. It will set a much lower limit. As a result, the setting range of the secondary transfer voltage becomes unnecessarily narrow, and a state where the optimum transfer condition cannot be selected may occur.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus in which the transfer voltage can be set by the user while the occurrence of image defects can be suppressed and the transfer voltage setting range is not unnecessarily narrowed.

  The image forming apparatus of the present invention is formed on the image carrier by forming a transfer portion between the image carrier carrying an electrostatic image and the image carrier, and applying a transfer bias. A transfer unit that transfers a toner image to a recording material at the transfer unit; a bias application unit that applies the transfer bias to the transfer unit; a current detection unit that can detect a current flowing through the transfer unit; and the bias application unit And a control unit that controls the bias applying unit so that the transfer bias applied by the control unit does not exceed a predetermined upper limit voltage, and the control unit applied a test bias to the transfer unit during non-image formation. The upper limit voltage is sometimes changed based on the current detected by the current detection means and the applied test bias.

  In addition, the image forming apparatus of the present invention forms a primary transfer portion between an image carrier that carries an electrostatic image and the image carrier, and a primary transfer bias is applied to the image carrier. A secondary transfer portion is applied between the intermediate transfer member that primarily transfers the toner image formed on the intermediate transfer member and the intermediate transfer member in contact with the intermediate transfer member, and a secondary transfer bias is applied. As a result, a secondary transfer unit that secondary-transfers the toner image primarily transferred to the intermediate transfer member onto a recording material at the secondary transfer unit, and the secondary transfer bias is applied to the secondary transfer unit. A bias applying means; a current detecting means capable of detecting a current flowing through the secondary transfer means; and the bias applying means so that the secondary transfer bias applied by the bias applying means does not exceed a predetermined upper limit voltage. Control unit to control The control unit includes the upper limit based on a current detected by the current detection unit when a test bias is applied to the secondary transfer unit during non-image formation and the applied test bias. The voltage is changed.

  According to the present invention, the control unit, based on the current detected by the current detection unit when the test bias is applied to the transfer unit or the secondary transfer unit during non-image formation, and the applied test bias, Change the upper voltage limit. For this reason, the upper limit voltage that can be applied by the bias applying means can be changed to an appropriate magnitude based on the test bias and the detected current. Thereby, it is possible to suppress the occurrence of image defects while the transfer voltage can be set by the user, and to prevent the setting range of the transfer voltage from being unnecessarily narrowed.

1 is a cross-sectional view illustrating a schematic configuration of an image forming apparatus according to an embodiment. 1 is a schematic control block diagram of an image forming apparatus according to an embodiment. 1 is a schematic diagram illustrating an operation panel of an image forming apparatus according to an embodiment. 4 is a graph showing a relationship between an applied voltage and a flowed current in a secondary transfer unit of the image forming apparatus according to the embodiment, where (a) is when Vb is acquired and (b) is when Vlim is acquired. 6 is a flowchart illustrating a procedure for setting a secondary transfer bias during image formation in the image forming apparatus according to the embodiment.

  Hereinafter, embodiments of the present invention will be described in detail with reference to FIGS. In the present embodiment, a tandem type full color printer is described as an example of the image forming apparatus 1. However, the present invention is not limited to the tandem type image forming apparatus 1, and may be an image forming apparatus of another type, and is not limited to a full color, and may be a monochrome or a mono color. Alternatively, the present invention can be implemented for various uses such as a printer, various printing machines, a copying machine, a FAX, and a multifunction machine.

  As illustrated in FIG. 1, the image forming apparatus 1 includes an apparatus main body 10, a sheet feeding unit (not illustrated), an image forming unit 40, a sheet discharge unit (not illustrated), a control unit 11, and a temperature / humidity sensor 70. And an operation panel 30. The image forming apparatus 1 can form a four-color full-color image on a recording material in response to an image signal from an unillustrated document reading device, a host device such as a personal computer, or an external device such as a digital camera or a smartphone. . Note that the sheet S as a recording material is formed with a toner image, and specific examples include plain paper, a synthetic resin sheet that is a substitute for plain paper, cardboard, and an overhead projector sheet.

  The image forming unit 40 can form an image on the sheet S fed from the sheet feeding unit based on the image information. The image forming unit 40 includes image forming units 50y, 50m, 50c, and 50k, toner bottles 41y, 41m, 41c, and 41k, exposure devices 42y, 42m, 42c, and 42k, an intermediate transfer unit 44, and a secondary transfer unit. 45 and a fixing unit 46. The image forming apparatus 1 according to the present embodiment is compatible with full color, and the image forming units 50y, 50m, 50c, and 50k are yellow (y), magenta (m), cyan (c), black ( Each of the four colors k) is provided separately with the same configuration. For this reason, in FIG. 1, each color component is shown with a color identifier after the same symbol. However, in FIG. 2 and the specification, there may be a case where only a symbol is used without adding a color identifier. .

  The image forming unit 50 includes a photosensitive drum (image carrier) 51 that forms a toner image, a charging roller 52, a developing device 20, a pre-exposure device 54, and a regulating blade 55. The image forming unit 50 is unitized as a process cartridge and is configured to be detachable from the apparatus main body 10.

  The photosensitive drum 51 is rotatable and carries an electrostatic image used for image formation. In the present embodiment, the photosensitive drum 51 is a negatively chargeable organic photoconductor (OPC) having an outer diameter of 30 mm, and is driven to rotate in the direction of the arrow at a process speed (peripheral speed) of, for example, 210 mm / sec. The photosensitive drum 51 has an aluminum cylinder as a base, and has three layers of an undercoat layer, a photocharge generation layer, and a charge transport layer, which are sequentially applied and laminated as a surface layer on the surface thereof.

  The charging roller 52 uses a rubber roller that contacts and rotates following the surface of the photosensitive drum 51, and uniformly charges the surface of the photosensitive drum 51. As shown in FIG. 2, a charging bias power source 60 is connected to the charging roller 52. The charging bias power supply 60 applies a DC voltage as a charging bias to the charging roller 52 and charges the photosensitive drum 51 via the charging roller 52. The exposure device 42 is a laser scanner, and emits laser light according to the separated color image information output from the control unit 11.

  The developing device 20 develops the electrostatic image formed on the photosensitive drum 51 by applying a developing bias with the stored toner. The developing device 20 has a developing sleeve 24. A developing bias power supply 61 for applying a developing bias is connected to the developing sleeve 24.

  The toner image developed on the photosensitive drum 51 is primarily transferred to an intermediate transfer belt 44b described later. The surface of the photosensitive drum 51 after the primary transfer is neutralized by the pre-exposure device 54. The regulating blade 55 is a counter blade type, and is an elastic blade mainly composed of urethane whose free length is 8 mm, and is in contact with the photosensitive drum 51 with a predetermined pressing force.

  As shown in FIG. 1, the intermediate transfer unit 44 is wound around a plurality of rollers such as a driving roller 44a, a driven roller 44d, and primary transfer rollers 47y, 47m, 47c, and 47k, and carries a toner image. Intermediate transfer belt 44b. The primary transfer rollers 47y, 47m, 47c, and 47k are disposed to face the photosensitive drums 51y, 51m, 51c, and 51k, respectively, and contact the intermediate transfer belt 44b.

  The primary transfer roller 47 faces the photosensitive drum 51 and is provided inside the intermediate transfer belt 44b. The primary transfer roller 47 is formed of a metal roller whose material is SUM (sulfur and sulfur composite free cutting steel) or SUS (stainless steel). The primary transfer roller 47 has a straight shape in the thrust direction, and the diameter of the roller is about 6 to 10 mm. A primary transfer bias power supply 62 (see FIG. 2) for applying a primary transfer bias is connected to the primary transfer roller 47, and a voltage having a polarity opposite to the charging polarity of the toner is applied from the primary transfer bias power supply 62. As a result, a primary transfer contrast that is a potential difference between the surface potential of the photosensitive drum 51 and the potential of the primary transfer roller 47 is formed. By forming a predetermined primary transfer contrast in each primary transfer portion, the toner images on the respective photosensitive drums 51 are sequentially electrostatically attracted to the intermediate transfer belt 44b and superimposed on the intermediate transfer belt 44b. An image is formed.

  The intermediate transfer belt (intermediate transfer member) 44b forms a primary transfer portion with the photosensitive drum 51, and a primary transfer bias is applied to cause the toner image formed on the photosensitive drum 51 to be primary at the primary transfer portion. Transcript. By applying a positive primary transfer bias to the intermediate transfer belt 44b by the primary transfer roller 47, the respective negative toner images on the photosensitive drum 51 are sequentially transferred to the intermediate transfer belt 44b. A tension of about 29 to 118 N (about 3 to 12 kgf) is applied to the intermediate transfer belt 44 b.

  The intermediate transfer belt 44b is an endless belt composed of a single layer. Specific materials are as follows, for example, polyimide, polycarbonate, polyvinylidene fluoride (PVDF), polyphenylene sulfide, polyethylene, polypropylene, polystyrene, polyamide, polysulfone, polyarylate. A single resin such as polyethylene terephthalate, polybutylene terephthalate, polyether sulfone, polyether nitrile, ethylene tetrafluoroethylene copolymer, polyether ether ketone, or a mixture thereof can be used.

In the present embodiment, polyimide resin or polyether ether ketone resin is used as the material of the intermediate transfer belt 44b. The thickness of the intermediate transfer belt 44b is about 60 to 70 μm. The surface resistivity of the intermediate transfer belt 44b is 1.0 × 10 ¹² Ω / □ or more and 2.0 × 10 ¹² Ω / □ or less, and the volume resistivity is 4.0 × 10 ¹¹ Ω · cm or more and 6.0 × 10. ¹¹ Ω · cm or less. The resistance was measured using a measuring instrument of Hiresta UP (manufactured by Mitsubishi Chemical) and a measurement probe of URS (guard electrode outer diameter φ 17.9 mm) (manufactured by Mitsubishi Chemical) with an applied voltage of 100 V and a charge of 10 seconds. The measurement was performed under the conditions.

  The secondary transfer unit 45 includes a secondary transfer inner roller 45a and a secondary transfer outer roller (secondary transfer means) 45b. The secondary transfer inner roller 45a is configured by providing EPDM (ethylene / propylene / diene rubber) around the cored bar. The secondary transfer inner roller 45a is formed to have a roller diameter of 20 mm and a rubber thickness of 0.5 mm, and the hardness is set to, for example, 70 ° (Asker C). The secondary transfer inner roller 45a is grounded.

Connected to the secondary transfer outer roller 45b is a secondary transfer bias power source (bias applying means) 63 for applying a secondary transfer bias to the secondary transfer outer roller 45b. The secondary transfer outer roller 45b is in contact with the intermediate transfer belt 44b to form a secondary transfer portion 45 with the intermediate transfer belt 44b, and a secondary transfer bias is applied to the primary transfer belt 44b. The transferred toner image is secondarily transferred to the sheet S by the secondary transfer unit 45. The secondary transfer outer roller 45b is configured by providing an elastic layer made of NBR (nitrile rubber) or EPDM containing an ionic conductive agent such as a metal complex around the core metal. The secondary transfer outer roller 45b is formed so that the core metal diameter is 12 mm and the roller diameter including the elastic layer is 24 mm. The resistance value of the secondary transfer outer roller 45b is set to 3.0 × 10 ^{7 to} 5.0 × 10 ⁷ Ω, and the resistance values of the secondary transfer inner roller 45a and the intermediate transfer belt 44b in the secondary transfer portion 45 are secondary. It is sufficiently smaller than the resistance value of the outer transfer roller 45b. A current detection circuit (current detection means) 64 (see FIG. 2) capable of detecting a current flowing through the secondary transfer outer roller 45b is connected to the secondary transfer outer roller 45b.

  The fixing unit 46 includes a fixing roller 46a and a pressure roller 46b. When the sheet S is nipped and conveyed between the fixing roller 46a and the pressure roller 46b, the toner image transferred to the sheet S is heated and pressurized and fixed to the sheet S. The sheet discharge unit feeds the sheet S conveyed from the discharge path after fixing, for example, discharges it from a discharge port and stacks it on a discharge tray.

  As shown in FIG. 2, the control unit 11 is configured by a computer. For example, the CPU 12, a ROM 13 for storing a program for controlling each unit, a RAM 14 for temporarily storing data, and an input / output for inputting / outputting signals to / from the outside. And a circuit (I / F) 15. The CPU 12 is a microprocessor that controls the entire control of the image forming apparatus 1 and is the main body of the system controller. The CPU 12 is connected to the sheet feeding unit, the image forming unit 40, the sheet discharging unit, and the operation panel 30 via the input / output circuit 15, and exchanges signals with each unit and controls the operation. The ROM 13 stores an image formation control sequence for forming an image on the sheet S and the like. The operation panel 30 includes a display unit 31 and an operation unit 32 (see FIG. 3), and the setting of the secondary transfer bias can be changed. The user can set the number of prints, the sheet type of the sheet stage, the size, and the like by operating the operation unit 32 of the operation panel 30. The operation unit 32 can input a value related to the target value of the output voltage.

  The controller 11 is connected to a charging bias power source 60, a developing bias power source 61, a primary transfer bias power source 62, a secondary transfer bias power source 63, a current detection circuit 64, and a temperature / humidity sensor 70. The temperature / humidity sensor 70 is provided inside the apparatus main body 10 and can detect temperature and humidity. The control unit 11 can detect the temperature and humidity, or the absolute water content from the temperature / humidity sensor 70. Further, the control unit 11 determines the secondary transfer bias based on the detection result of the temperature / humidity sensor 70 based on the table stored in the ROM 13. In the table, the relationship between the temperature and humidity and the secondary transfer bias is determined and set in advance so that a desired secondary transfer current flows.

  The control unit 11 controls the secondary transfer bias power supply 63 so that the secondary transfer bias applied by the secondary transfer bias power supply 63 does not exceed a predetermined upper limit voltage (step S17 in FIG. 5). The control unit 11 also determines the high voltage upper limit Vlim based on the current detected by the current detection circuit 64 when a test bias is applied to the secondary transfer outer roller 45b during non-image formation and the applied test bias. Is changed (step S12 in FIG. 5). When the secondary transfer bias changed from the operation unit 32 exceeds the high voltage upper limit Vlim, the control unit 11 sets the secondary transfer bias to the high voltage upper limit Vlim (step S17 in FIG. 5). The control unit 11 compares the relationship between a plurality of different test biases applied during non-image formation and the current detected by the current detection circuit 64, and the voltage and current within a predetermined high-voltage guaranteed range of the secondary transfer bias power source 63. Based on the relationship, a high voltage upper limit Vlim is set. Since the secondary transfer bias may be negative, the high voltage upper limit Vlim of the absolute value of the secondary transfer bias is set here.

  In the present embodiment, during image formation, a toner image is formed on the photosensitive drum 51 based on image information input from an external terminal such as a scanner or personal computer provided in the image forming apparatus 1. When you are. On the other hand, the time of non-image formation is a time other than the time of image formation, for example, when the image formation job is not executed, at the time of pre-rotation during the image formation job, or between sheets. An image forming job is a series of operations as follows based on a print command signal (image forming command signal). In other words, after the preliminary operation (so-called pre-rotation operation) necessary for image formation is started, the preliminary operation (so-called post-rotation) necessary for ending the image formation is completed through the image formation process. It is a series of operations up to. Specifically, it refers to the period from pre-rotation (preparation operation before image formation) after receiving a print command signal (input of image formation job) to post-rotation (operation after image formation). It includes the period and the interval between sheets (during non-image formation). Further, the interval between sheets is a period corresponding to a period between a toner image formed on one sheet and a toner image formed on the next sheet when image formation is continuously performed. It is.

  Next, an image forming operation in the image forming apparatus 1 configured as described above will be described with reference to FIG.

  When the image forming operation is started, first, the photosensitive drum 51 is rotated and the surface is charged by the charging roller 52. Then, laser light is emitted from the exposure device 42 to the photosensitive drum 51 based on the image information, and an electrostatic latent image is formed on the surface of the photosensitive drum 51. When toner adheres to the electrostatic latent image, it is developed and visualized as a toner image, and transferred to the intermediate transfer belt 44b.

  On the other hand, the sheet S is supplied in parallel with the toner image forming operation, and the sheet S is conveyed to the secondary transfer unit 45 via the conveyance path in synchronization with the toner image on the intermediate transfer belt 44b. . Further, the image is transferred from the intermediate transfer belt 44b to the sheet S, and the sheet S is conveyed to the fixing unit 46, where an unfixed toner image is heated and pressed to be fixed on the surface of the sheet S. Discharged from.

  Here, the predetermined high voltage guaranteed range of the secondary transfer bias power supply 63 will be described. The secondary transfer bias output from the secondary transfer bias power source 63 is variable. The secondary transfer bias power source 63 performs high voltage output by constant voltage control having a high voltage guaranteed range. When a current that is outside this high voltage guaranteed range is set, heat is generated or the voltage is increased in the high voltage circuit. It is set to operate intermittently to avoid breakdown due to the discharge. The voltage-current information indicating the high voltage guaranteed range is stored in advance in the ROM 13 of the control unit 11.

As shown in FIG. 4A, mathematical formulas indicating the boundaries of the high-pressure guarantee range (shown by broken lines in the figure) are represented by the following two formulas.
I = a × V + b (when 0 ≦ V <C) (Formula 1)
V = C (when V ≧ C) (Formula 2)

  In the present embodiment, the high-voltage substrate in the high-voltage guaranteed range shown by the above two formulas will be described. However, the high-voltage guaranteed range varies depending on the high-voltage circuit design, and the one defined by the above-described formula is used. It is not limited.

The voltage V2tr applied to the secondary transfer unit 45 at the time of image formation is calculated by the following mathematical formula 3. The control unit 11 adds the voltage Vb determined when the sheet is not passed and the Vp given from the paper sharing voltage table, and uses the voltage V2tr when the sheet passes through the secondary transfer unit 45 of the secondary transfer outer roller 45b. To transfer the toner image on the intermediate transfer belt 44b to a sheet.
V2tr = Vb + Vp (Formula 3)

  Since the secondary transfer outer roller 45b is a sponge rubber material in which an ionic conductive agent is dispersed as described above, the secondary transfer outer roller 45b is deteriorated and varies in resistance value depending on the durability state. In addition, since the electric resistance varies depending on the temperature and humidity, it is necessary to set a voltage in accordance with the roller resistance at the time of image formation. In the present embodiment, two or more types of test bias are applied to the secondary transfer outer roller 45b during non-sheet passing during the printing operation, and the current flowing through the secondary transfer unit 45 is detected by the current detection circuit 64. As shown in FIG. 4A, the slope α and the intercept β are calculated based on the applied voltages of the two test biases and the detected current values, and the relationship of I = V × α + β (V−I straight line) ) Then, automatic transfer voltage control (hereinafter referred to as ATVC) for determining a voltage value for obtaining a desired transfer current is executed. That is, when the control unit 11 performs ATVC during the image forming operation, it is possible to set a voltage according to the roller resistance at that time, and as shown in FIG. 4A, a desired current (target current It) is set. The voltage Vb to obtain is determined. Further, the control unit 11 determines the paper sharing voltage Vp from the paper sharing table for the voltage value corresponding to the environmental information detected by the temperature / humidity sensor 70. The voltage V2tr applied to the secondary transfer unit 45 at the time of image formation can be calculated using Vb and Vp set as described above.

  A case where Vb or Vp described above is changed will be described below. As shown in FIG. 3, the display unit 31 of the operation panel 30 displays items for which the secondary transfer settings for the first and second surfaces can be adjusted for each sheet type, and the reference value (when “0” is displayed) is displayed. On the other hand, the paper sharing voltage Vp can be increased or decreased by inputting an adjustment value from the operation unit 32. For example, when the user prints a sheet with a high (low) volume resistivity, the secondary voltage is increased (or decreased) so as to increase (or decrease) the offset voltage Vpos corresponding to the paper resistance difference on the basis of a paper sharing voltage table value given in advance. It is possible to change the transfer setting. Based on the offset voltage Vpos determined by the paper sharing voltage setting input by the user, the control unit 11 reflects Vp ′ instead of Vp when determining the secondary transfer voltage as Vp ′ = Vp + Vpos.

Here, the secondary transfer high voltage upper limit (upper limit voltage) Vlim will be described. The voltage given by the intersection of the VI straight line obtained by the control unit 11 during execution of ATVC and Formula 1 (I = a × V + b) indicating the high voltage guaranteed area is calculated by the following Formula 4, and the secondary transfer high voltage Set to upper limit Vlim.
Vlim = (β−b) / (a−α) (Equation 4)

  That is, when a voltage value exceeding Vlim is set in the secondary transfer setting determined by the ATVC of the secondary transfer unit 45 and the operation unit 32, V2tr is set to Vlim and a current value outside the high voltage guaranteed range is not output. Like that. For example, since the roller resistance value of the secondary transfer outer roller 45b decreases in a high temperature and high humidity environment, the user mistakenly sets the offset voltage of the paper sharing voltage Vp that can be input through the operation unit 32. An overcurrent outside the guaranteed high voltage range may be set. In such a case, the overcurrent protection circuit operates and the secondary transfer bias power supply 63 performs intermittent output, so that the toner image on the intermediate transfer belt 44b is not optimally transferred to the sheet and reflects the intermittent output under high piezoelectricity. The striped image is output and image information is lost. Therefore, even when the voltage value outside the high voltage guaranteed range is unintentionally set by the image forming apparatus 1 according to the present embodiment, the secondary transfer bias is set so as not to set the voltage at which the protection circuit operation is executed. Therefore, such a situation that the image information is lost is avoided. Note that Vlim is set slightly lower than Vlim (for example, Vlim ′ = Vlim × 95%) in consideration of variations such as high voltage output accuracy, current detection accuracy, voltage detection accuracy, and roller resistance fluctuation. desirable.

  Next, the inter-sheet ATVC in the image forming apparatus 1 of the present embodiment will be described. The impedance of the secondary transfer unit 45 including the roller resistance of the secondary transfer outer roller 45b and the like varies depending on the use mode and environment of the image forming apparatus 1. When the user's usage environment itself changes, there are various factors that change the impedance of the secondary transfer unit 45. For example, there are cases where the number of continuous prints is large and the temperature inside the apparatus main body 10 is raised by radiant heat from the fixing unit 46. Alternatively, in the double-sided printing mode, the secondary transfer outer roller 45b may be heated because the second side passes through the secondary transfer unit 45 immediately after the sheet is heated on the first side after fixing.

  When performing control (inter-sheet ATVC) for resetting the secondary transfer impedance by applying two or more test biases in the inter-sheet control during the continuous print mode in accordance with such a change in impedance, the control unit 11 recalculates the high voltage upper limit Vlim. The control unit 11 detects a change in the second transfer impedance by the inter-sheet ATVC, and changes the secondary transfer setting when it is determined that the correction of the second transfer setting is necessary, and also changes the high voltage upper limit Vlim.

In the present embodiment, as the inter-sheet ATVC, the control unit 11 applies two or more types of test bias during the non-sheet passing timing during continuous sheet passing, and detects the current at that time. Then, the control unit 11 calculates the current Iin when Vb is applied from the relationship between the detection current and the applied voltage, determines the deviation from the desired current It, and corrects the secondary transfer bias. Then, when the deviation amount between the detected current and the target current It is equal to or larger than the predetermined amount ΔIth, the control unit 11 offsets the applied voltage so that the current value approaches a desired range. Specifically, the following determination formula is used.
When Iin> It and Iin−It> ΔIth, Vb ′ = Vb−ΔV
When Iin> It and Iin−It ≦ ΔIth, Vb ′ = Vb
When Iin <It and It−Iin> ΔIth, Vb ′ = Vb + ΔV
When Iin <It and It−Iin ≦ ΔIth, Vb ′ = Vb

  Then, the control unit 11 calculates the V-I straight line with the two types of test biases and the currents detected by the current detection circuit 64 when each bias is applied in the inter-sheet ATVC, and represents the high voltage guaranteed region. Vlim is newly calculated and updated from the intersection with.

  Next, the procedure for setting the secondary transfer bias at the time of image formation in the image forming apparatus 1 of the present embodiment will be described with reference to FIG. Note that it is assumed that the user uses the operation unit 32 to adjust and input the secondary transfer settings for the first and second surfaces in advance.

  When the image forming job is started (step S10), the control unit 11 executes ATVC during the pre-rotation and determines Vb (step S11). Then, the control unit 11 calculates the high voltage upper limit Vlim based on Formula 4, and calculates the voltage V2tr applied to the secondary transfer unit 45 during image formation based on Formula 3 (Step S12). Further, the control unit 11 determines whether or not the high voltage upper limit Vlim exceeds the voltage V2tr (step S15). If the control unit 11 determines that the high voltage upper limit Vlim exceeds the voltage V2tr, the secondary transfer bias is set to the voltage V2tr (step S16). On the other hand, when the control unit 11 determines that the high voltage upper limit Vlim does not exceed the voltage V2tr, the control unit 11 sets the secondary transfer bias to the high voltage upper limit Vlim (step S17). Then, the control unit 11 performs image formation with the set secondary transfer bias (step S18).

  The control unit 11 determines whether or not the remaining number of prints in the main image forming job is equal to or greater than a predetermined print number set in advance (step S19). If the control unit 11 determines that the remaining number of prints is equal to or greater than a predetermined print number set in advance, the control unit 11 executes an inter-sheet ATVC at a predetermined timing (step S20), updates Vb, and newly Vb ′ is calculated (step S21). Then, the control unit 11 calculates the high voltage upper limit Vlim for the next image forming process (step S12). If the controller 11 determines in step S19 that the remaining number of prints is not equal to or greater than a predetermined print number, the image forming job is terminated (step S22).

  As described above, according to the image forming apparatus 1 of the present embodiment, the control unit 11 includes the current detected by the current detection circuit 64 when a test bias is applied to the secondary transfer outer roller 45b during non-image formation, Based on the applied test bias, the high voltage upper limit Vlim is changed. Therefore, the high voltage upper limit Vlim that can be applied by the secondary transfer bias power source 63 can be changed to an appropriate size based on the test bias and the detected current. Thereby, it is possible to suppress the occurrence of image defects while the transfer voltage can be set by the user, and to prevent the setting range of the transfer voltage from being unnecessarily narrowed.

  In the image forming apparatus 1 according to the above-described embodiment, the image forming apparatus 1 includes the intermediate transfer belt 44b. After the primary transfer of each color toner image from the photosensitive drum 51 to the intermediate transfer belt 44b, the composite of each color is performed. The toner image is secondarily transferred onto the sheet S at once. However, the present invention is not limited to this, and a method of directly transferring from the photosensitive drum to the sheet conveyed by the sheet conveying belt may be employed. In this case, the image forming apparatus includes a transfer unit that forms a transfer portion with the photosensitive drum and applies a transfer bias to transfer the toner image formed on the photosensitive drum to a sheet at the transfer portion. Then, the control unit 11 changes the high voltage upper limit Vlim based on the current detected by the current detection unit when a test bias is applied to the transfer unit during non-image formation and the applied test bias. Also in this case, the high voltage upper limit Vlim that can be applied by the bias applying means can be changed to an appropriate magnitude, so that the transfer voltage can be set by the user, and the occurrence of image defects can be suppressed, and the transfer voltage setting range. Can be prevented from becoming unnecessarily narrow.

DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus, 11 ... Control part, 32 ... Operation part, 44b ... Intermediate transfer belt (intermediate transfer body), 45 ... Secondary transfer part, 45b ... Secondary transfer outer roller (secondary transfer means), 51, 51c, 51k, 51m, 51y ... photosensitive drum (image carrier), 63 ... secondary transfer bias power supply (bias applying means), 64 ... current detection circuit (current detection means), S ... sheet (recording material), Vlim ... High voltage upper limit (upper limit voltage).

Claims (5)

An image carrier for carrying an electrostatic image;
A transfer unit that forms a transfer portion with the image carrier and applies a transfer bias to transfer the toner image formed on the image carrier to a recording material at the transfer unit; and
Bias applying means for applying the transfer bias to the transfer means;
Current detection means capable of detecting a current flowing through the transfer means;
A controller that controls the bias applying means so that the transfer bias applied by the bias applying means does not exceed a predetermined upper limit voltage, and
The control unit changes the upper limit voltage based on the current detected by the current detection unit when a test bias is applied to the transfer unit during non-image formation and the applied test bias.
An image forming apparatus. An operation unit capable of changing the setting of the transfer bias;
The control unit sets the transfer bias to the upper limit voltage when the transfer bias changed from the operation unit exceeds the upper limit voltage.
The image forming apparatus according to claim 1. An image carrier for carrying an electrostatic image;
An intermediate transfer member that forms a primary transfer portion with the image carrier and applies a primary transfer bias to primarily transfer the toner image formed on the image carrier with the primary transfer portion;
A secondary transfer portion is formed between the intermediate transfer member and the intermediate transfer member, and a secondary transfer bias is applied to transfer the toner image primarily transferred to the intermediate transfer member to the secondary transfer portion. Secondary transfer means for secondary transfer to the recording material at the transfer section;
Bias applying means for applying the secondary transfer bias to the secondary transfer means;
Current detection means capable of detecting current flowing in the secondary transfer means;
A controller that controls the bias application unit so that the secondary transfer bias applied by the bias application unit does not exceed a predetermined upper limit voltage, and
The control unit changes the upper limit voltage based on the current detected by the current detection unit when a test bias is applied to the secondary transfer unit during non-image formation and the applied test bias. To
An image forming apparatus. An operation unit capable of changing the setting of the secondary transfer bias;
The control unit sets the secondary transfer bias to the upper limit voltage when the secondary transfer bias changed from the operation unit exceeds the upper limit voltage;
The image forming apparatus according to claim 3. The control unit includes a relationship between a plurality of different test biases applied at the time of non-image formation and a current detected by the current detection unit, and a relationship between a voltage and a current in a predetermined high-voltage guarantee range of the bias application unit. Setting the upper limit voltage based on
The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus.

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