Charge adjustment device and image forming apparatus

Abstract

This provides a configuration that allows for stable charge adjustment of the sheet, regardless of the sheet width or the resistance change of the charge adjustment roller 910 in the charge adjustment section. [Solution] The charge adjustment device 9 adjusts the charge on the sheet on which the toner image has been fixed by the fixing device 7. The control unit applies a predetermined voltage from the charge adjustment high voltage power supply 90 to the charge adjustment device 9 when the sheet is not being transported to the charge adjustment nip section N formed by the charge adjustment roller 910 and the opposing roller 92. At that time, the control unit executes a mode to determine the relationship between current and voltage from the current value detected by the current detection circuit 93 and the predetermined voltage. Based on the relationship obtained in the mode and the length of the sheet in the sheet width direction obtained by the operation unit, the control unit determines the voltage to be applied from the charge adjustment high voltage power supply 90 to the charge adjustment device 9 when the sheet passes through the charge adjustment nip section N.

Description

[Technical Field]

[0001] This invention relates to image forming apparatus such as photocopiers, printers, facsimile machines, and multifunction devices having multiple functions thereof. [Background technology]

[0002] In an image forming apparatus, the toner image formed in the image forming unit is transferred to a sheet in the transfer unit, the toner image is fixed to the sheet in the fixing unit, and then the sheet is loaded onto an output tray or the like. At this time, the sheets may stick together due to electrostatic force between them. For this reason, a configuration has been proposed that includes a charge adjustment unit that applies a voltage to the sheet on which the toner image has been fixed by the fixing unit to adjust the charge of the sheet (Patent Document 1).

[0003] Patent Document 1 describes a charge adjustment unit comprising a pair of conductive rubber rollers arranged opposite each other and a power supply for applying voltage to the conductive rubber rollers, and is configured to apply voltage to a sheet passing through the nip portion of the pair of conductive rubber rollers. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2016-122156 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] However, with the configuration described in Patent Document 1, it is difficult to control the charge supplied to the sheet when the width of the sheet or the electrical resistance of the conductive rubber roller (the rotating body of the charge adjustment unit) changes. This is because, when a constant current is supplied from the power source to the roller, the current flowing through the sheet and the current flowing outside the sheet fluctuate depending on the width of the sheet and the electrical resistance of the roller. If the current flowing through the sheet is not appropriate, it is not possible to suppress the sheets from sticking together.

[0006] For example, if the resistance of the sheet is higher than expected due to factors such as the amount of moisture in the surrounding environment, current may flow only to areas without the sheet in the nip area, rather than to areas with the sheet, resulting in insufficient charge to suppress sticking. Also, if the resistance of the conductive rubber roller is higher than expected due to deterioration of the material's conductivity, the current in areas without the sheet decreases, and an excessive amount of charge is supplied to areas with the sheet, which may cause sticking.

[0007] The present invention aims to provide a configuration that can stably adjust the charge of a sheet regardless of the sheet's width or the resistance change of the rotating body of the charge adjustment unit. [Means for solving the problem]

[0008] The present invention provides an image forming apparatus comprising: a transfer unit for transferring a toner image onto a sheet; a fixing unit for heating and pressurizing the sheet onto which the toner image has been transferred in the transfer unit to fix the toner image onto the sheet; a charge adjustment unit having a pair of rotating bodies for gripping and transporting the sheet onto which the toner image has been fixed in the fixing unit, and for adjusting the charge on the sheet when a voltage is applied; a power supply for applying a voltage to the charge adjustment unit; a current detection unit for detecting the current flowing through the charge adjustment unit; an information acquisition unit capable of acquiring information on the length of the sheet in the sheet width direction perpendicular to the sheet transport direction; and a control unit for controlling the power supply, wherein the power supply is controlled at a constant voltage, and the control unit applies a predetermined voltage from the power supply to the charge adjustment unit when the sheet is not being transported to the nip portion formed by the pair of rotating bodies, and at that time executes a mode for determining the relationship between current and voltage from the current value detected by the current detection unit and the predetermined voltage, and determines the voltage to be applied from the power supply to the charge adjustment unit when the sheet passes through the nip portion based on the relationship obtained in the mode and the length of the sheet in the sheet width direction acquired by the information acquisition unit.

[0009] The present invention provides an image forming apparatus comprising: a transfer unit for transferring a toner image onto a sheet; a fixing unit for heating and pressurizing the sheet onto which the toner image has been transferred in the transfer unit to fix the toner image onto the sheet; a charge adjustment unit having a pair of rotating bodies for gripping and transporting the sheet onto which the toner image has been fixed in the fixing unit, and for adjusting the charge on the sheet by applying a voltage; a power supply for supplying current to the charge adjustment unit; a voltage detection unit for detecting the voltage applied to the charge adjustment unit; an information acquisition unit capable of acquiring information on the length of the sheet in the sheet width direction perpendicular to the sheet transport direction; and a control unit for controlling the power supply, wherein the power supply is controlled by a constant current, and the control unit supplies a predetermined current from the power supply to the charge adjustment unit when the sheet is not being transported to the nip portion formed by the pair of rotating bodies, and at that time executes a mode for determining the relationship between current and voltage from the voltage value detected by the voltage detection unit and the predetermined current, and determines the voltage to be applied from the power supply to the charge adjustment unit when the sheet passes through the nip portion based on the relationship obtained in the mode and the length of the sheet in the sheet width direction acquired by the information acquisition unit. [Effects of the Invention]

[0010] According to the present invention, the charge of the sheet can be stably adjusted regardless of the width of the sheet or the resistance change of the rotating body of the charge adjustment unit. [Brief explanation of the drawing]

[0011] [Figure 1] A schematic cross-sectional view of the image forming apparatus according to the first embodiment. [Figure 2] A schematic cross-sectional view of the image forming unit according to the first embodiment. [Figure 3] A schematic cross-sectional view of the charge adjustment device according to the first embodiment. [Figure 4] A block diagram relating to voltage control of a charge adjustment device according to the first embodiment. [Figure 5] A flowchart of voltage control of a charge regulator according to the first embodiment. [Figure 6]Graph showing the current in the voltage control of the charge adjustment device according to the first embodiment, indicating the charge. [Figure 7] Graph showing the table of voltages applied to the charge adjustment device according to the first embodiment. [Figure 8] Schematic diagram for explaining the current flowing through the charge adjustment device according to the first embodiment. [Figure 9] Flowchart of the voltage control of the charge adjustment device according to the second embodiment.

Mode for Carrying Out the Invention

[0012] <First Embodiment> The first embodiment will be described with reference to FIGS. 1 to 8. First, the schematic configuration of the image forming apparatus of this embodiment will be described with reference to FIGS. 1 and 2.

[0013] [Image Forming Apparatus] As shown in FIG. 1, the image forming apparatus 100 of this embodiment is a laser beam printer that forms a full-color image on a sheet P (paper, OHP sheet, cloth, etc.) as a recording material using an electrophotographic method. The image forming apparatus 100 is an intermediate transfer type tandem type in which image forming units Pa, Pb, Pc, and Pd, which are yellow, magenta, cyan, and black toner image forming means, are arranged along an intermediate transfer belt 51.

[0014] The image forming units Pa, Pb, Pc, and Pd each include image carriers that carry electrostatic latent images and photosensitive drums 1a, 1b, 1c, and 1d as photoreceptors. In the image forming unit Pa, a yellow toner image is formed on the photoreceptor drum 1a and is primarily transferred to the intermediate transfer belt 51 as an intermediate transfer body. In the image forming unit Pb, a magenta toner image is formed on the photoreceptor drum 1b and is primarily transferred onto the yellow toner image on the intermediate transfer belt 51. In the image forming units Pc and Pd, cyan and black toner images are formed on the photoreceptor drums 1c and 1d, respectively, and are similarly sequentially primarily transferred with their positions overlapping the toner images on the intermediate transfer belt 51. In this embodiment, the photoreceptor drums and the intermediate transfer belt serve as image carriers that carry toner images.

[0015] The four-color toner image primarily transferred onto the intermediate transfer belt 51 is collectively secondarily transferred onto the sheet P fed to the secondary transfer unit N2 formed by the intermediate transfer belt 51 and the secondary transfer roller 56. The sheet P onto which the toner image has been secondarily transferred in the secondary transfer unit N2 is subjected to heating and pressure by the fixing device 7 serving as the fixing unit, and after the toner image is fixed on the surface, it is discharged to the outside and stacked on the discharge tray 86.

[0016] The feeding device 8 separates the sheet P drawn from the cassette 81 by the pickup roller 82 one by one with the separating device 83 and feeds it to the registration roller 84. The registration roller 84 receives and waits for the sheet P in a stopped state, and feeds the sheet P to the secondary transfer unit N2 in synchronization with the toner image on the intermediate transfer belt 51.

[0017] The intermediate transfer unit 5 winds the intermediate transfer belt 51, which is an example of an image carrier, around the driving roller 52, the support rollers 58 and 59, the tension roller 53, and the opposing roller 54, and rotates it in the direction of arrow R2. The opposing roller 54 is disposed at a position facing the secondary transfer roller 56 via the intermediate transfer belt 51. And a secondary transfer unit N2 is formed that nips the sheet between the outer peripheral surface of the intermediate transfer belt 51 wound around the opposing roller 54 and the secondary transfer roller 56.

[0018] The secondary transfer roller 56 is composed of an elastic layer of ion-conductive foamed rubber (NBR rubber) and a core metal, has an outer diameter of 24 mm, a roller surface roughness Rz of 6.0 μm or more and 12.0 μm or less, a resistance value measured in an N / N (23°C, 50% RH) environment, 1E+5 to 1E+7 Ω when 2 kV is applied, and the hardness of the elastic layer is about 30 to 40 in Asker-C hardness, and a transfer roller is used.

[0019] Furthermore, a secondary transfer high-voltage power supply D2 with a variable supply bias is connected to the secondary transfer roller 56, and a secondary transfer bias is applied. When performing secondary transfer, a high-voltage positive (positive polarity) transfer voltage (secondary transfer bias) is applied to the secondary transfer roller 56, thereby electrostatically attracting the negatively charged toner image to the sheet. As a result, the toner image supported on the intermediate transfer belt 51 is secondary transferred to the sheet P as it passes through the secondary transfer section N2.

[0020] The fixing device 7 forms a heating nip section by pressing a pressure roller 73 against a fixing roller 72, which is positioned around a lamp heater 71. Then, in the heating nip section, the sheet P, onto which the toner image has been transferred in the secondary transfer section N2, is heated and pressurized to fix the toner image to the sheet P. After the fixing process, the sheet P is discharged outside the machine by a discharge roller 85, which acts as a discharge section, and loaded onto a discharge tray 86 or the like.

[0021] The belt cleaning device 57 rubs a cleaning blade against the intermediate transfer belt 51 to remove residual transfer toner, paper dust, etc. remaining on the surface of the intermediate transfer belt 51 from which the sheet P has been separated after passing through the secondary transfer section N2.

[0022] The image forming units Pa, Pb, Pc, and Pd are configured almost identically, except that the toner colors used by the developing units 4a, 4b, 4c, and 4d attached to the photosensitive drums 1a, 1b, 1c, and 1d, respectively, differ (yellow, magenta, cyan, and black). In the following description, the image forming unit Pa will be explained with reference to Figure 2, and the other image forming units Pb, Pc, and Pd will be described by replacing the 'a' at the end of the symbols with 'b', 'c', and 'd', respectively.

[0023] As shown in Figure 2, the image forming unit Pa has a charging roller 2a, exposure device 3a, developing device 4a, primary transfer roller 55a, and cleaning device 6a arranged around the photosensitive drum 1a. The photosensitive drum 1a has an organic photoconductor layer (OPC) with negative charge polarity formed on the outer surface of an aluminum cylinder and rotates in the direction of arrow R1 at a process speed of 100 mm / sec to 700 mm / sec (for example, 240 mm / sec).

[0024] The charging roller 2a, which is a charging component, is formed by covering the surface of a metallic central shaft with a resistive elastic layer, and rotates in pressure against the photosensitive drum 1a. The power supply D3 applies a DC voltage superimposed on an AC voltage to the charging roller 2a, thereby charging the surface of the photosensitive drum 1a to a uniform negative potential.

[0025] The exposure apparatus 3a scans a laser beam, which is ON-OFF modulated with scan line image data obtained by unfolding a yellow separated color image, using a rotating mirror to write an electrostatic image of the image onto the surface of the charged photosensitive drum 1a.

[0026] The developing device 4a agitates a two-component developer, which is a mixture of non-magnetic toner and magnetic carrier, to charge the non-magnetic toner to a negative polarity and the magnetic carrier to a positive polarity. The charged two-component developer is supported in a pile-like manner on the developing sleeve 41a, which rotates in the counter direction around the fixed magnetic pole 42a and rubs against the photosensitive drum 1a. The power supply D4 applies a developing voltage to the developing sleeve 41a, which is a negative polarity DC voltage superimposed with an AC voltage, to move the toner to the exposure area of ​​the photosensitive drum 1a, which has become relatively more positive polarity than the developing sleeve 41a, and inverts the electrostatic image during development.

[0027] The primary transfer roller 55a, which is a primary transfer member, is pressed against the photosensitive drum 1a so as to sandwich the intermediate transfer belt 51, forming a primary transfer section N1a between the photosensitive drum 1a and the intermediate transfer belt 51. The power supply D1a is a transfer output unit that applies voltage to the primary transfer roller 55a, and applies a positive polarity DC voltage of 500V to 7000V (for example, +900V) to the primary transfer roller 55a as a primary transfer bias. As a result, the toner image, which is negatively charged and carried on the photosensitive drum 1a, is primary transferred to the intermediate transfer belt 51 passing through the primary transfer section N1a.

[0028] The primary transfer roller 55a has a resistance value of 1 × 10 when 2000V is applied. 2 ~10 8 A semiconducting material with a resistance of Ω was used. Specifically, an ion-conductive sponge roller with an outer diameter of φ16 mm and a core diameter of φ8 mm, formed by a blend of nitrile rubber and ethylene-epichlorohydrin copolymer, was used. The resistance value of the primary transfer roller 55a was 1 × 10⁻¹⁰ when applied at a voltage of 2 kV in an environment of 23°C and 50% RH. 6 ~10 8 It's around Ω.

[0029] The cleaning device 6a rubs its cleaning blade against the photosensitive drum 1a to remove any remaining toner that has adhered to the surface of the photosensitive drum 1a after it has passed through the primary transfer section N1a.

[0030] In recent years, the variety of sheet types has increased, and the range of sheet thicknesses and electrical resistivity has also expanded, leading to the adoption of intermediate transfer methods. Furthermore, to avoid changes in the amount of charge supplied to the toner image due to differences in the image ratio in the main scanning direction and the width of the sheet, constant voltage control is employed in the transfer section (secondary transfer section in the above example) that transfers the toner image to the sheet. In addition, changes in ambient conditions such as temperature and humidity, or the accumulation of image formation, cause changes in the electrical resistance of the intermediate transfer belt and transfer rollers, as well as the film thickness of the surface layer of the photosensitive drum. In response to these changes, Active Transfer Voltage Control (ATVC) is performed to determine the control value of the constant voltage control prior to image formation in order to optimize the voltage applied to the transfer rollers during image formation.

[0031] ATVC control is a control method that applies multiple different test voltages to the secondary transfer roller 56 when there is no sheet in the secondary transfer section N2, detects the current using a current sensing sensor at each transfer voltage to determine the relationship between the transfer voltage and the current, and sets the transfer voltage (secondary transfer bias) to be applied to the secondary transfer section N2 based on this. The control of the entire image forming apparatus 100, including this ATVC control, is performed by the control unit 110 (Figure 1).

[0032] The control unit (control circuit) 110, as shown in Figure 4 later, includes a CPU (Central Processing Unit) 111, RAM (Random Access Memory) 112, and ROM (Read Only Memory) 113. The CPU 111 controls each part while reading programs corresponding to control procedures stored in the ROM 113. The RAM 112 stores working data and input data, and the CPU 111 controls the system by referring to the data stored in the RAM based on the aforementioned programs, etc.

[0033] [Charge adjustment device] In this embodiment, in order to prevent the sheets loaded on the discharge tray 86 from sticking together due to electrostatic force, a charge adjustment device 9 is positioned downstream of the fuser 7 in the sheet transport direction and upstream of the discharge roller 85 (Figure 1), as shown in Figure 3. The charge adjustment device 9 adjusts the charge on the sheets on which the toner image has been fixed by the fuser 7 when a voltage is applied.

[0034] In Figure 3, the dashed line shows the transport path of the sheet P, and the charge adjustment device 9 is located downstream of the fixing device 7. The charge adjustment device 9 has a pair of rotating bodies, a charge adjustment roller 910 and an opposing roller 92, which grip and transport the sheet. The charge adjustment roller 910 is connected to a high-voltage charge adjustment power supply 90, which serves as the power source, and presses against the opposing roller 92 to form a charge adjustment nip section N. The opposing roller 92 is grounded. When the sheet P passes through the charge adjustment nip section N formed by the opposing roller 92 and the charge adjustment roller 910, the charge adjustment after passing through the charge adjustment nip section N is performed by a static elimination current controlled by the high-voltage charge adjustment power supply 90.

[0035] The charge adjustment roller 910 has a metal core (rotating shaft) 910a as a conductive shaft portion, and an elastic layer 910b as an outer peripheral portion containing an ion-conductive material formed on the outer circumference of the core 910a. Similarly, the opposing roller 92 has a metal core (rotating shaft) 92a as a conductive shaft portion, and an elastic layer 92b as an outer peripheral portion containing an ion-conductive material formed on the outer circumference of the core 92a.

[0036] These charge adjustment rollers 910 and opposing rollers 92 are semiconducting rollers, and the elastic layers 910b and 92b are formed from an ion-conducting material created by a blend of nitrile rubber and ethylene-epichlorohydrin copolymer. In addition, although semiconducting rollers with an outer diameter of φ20 mm and a core diameter of φ16 mm were used for the charge adjustment rollers 910 and opposing rollers 92, the materials for the charge adjustment rollers 910 and opposing rollers 92 are not limited to these.

[0037] The opposing roller 92 is positioned to sandwich the sheet between itself and the charge adjustment roller 910. Specifically, both ends of the core metal of either the core metal 910a of the charge adjustment roller 910 or the core metal 92a of the opposing roller 92 are biased toward the other roller by a spring member, causing the elastic layers 910b and 92b to press against each other and form a nip portion N. Therefore, the sheet that has passed through the fixing device 7 passes through the nip portion N formed between the charge adjustment roller 910 and the opposing roller 92.

[0038] [Voltage control of charge regulator] Next, the voltage control of the charge adjustment device 9 in this embodiment will be explained using Figures 4 and 5. As shown in Figure 4, the image forming apparatus 100 in this embodiment has an environmental sensor 75, an operating unit 76, a charge adjustment high-voltage power supply 90, a current detection circuit 93, and a voltage detection circuit 94, all of which are connected to the control unit 110.

[0039] The environmental sensor 75, acting as an environmental detection unit, detects the temperature and humidity inside the main body (inside the device) of the image forming apparatus 100. The operation unit 76, acting as an information acquisition unit, is equipped with a touch panel that allows for the display and input of information, enabling the user to input various types of information and set various conditions. It can also display the status of the image forming apparatus 100. In this embodiment, by inputting sheet information via the operation unit 76, the user can acquire information such as the length (sheet width) of the sheet in the sheet width direction perpendicular to the sheet transport direction.

[0040] The charge adjustment high-voltage power supply 90 is connected to the charge adjustment device 9 as described above and applies voltage to the charge adjustment device 9. Specifically, the charge adjustment high-voltage power supply 90 is connected to the core metal 910a of the charge adjustment roller 910 and applies voltage to the core metal 910a. In this embodiment, the charge adjustment device 9 is controlled by a constant voltage. The current detection circuit 93, acting as a current detection unit, detects the current flowing through the charge adjustment device 9. The voltage detection circuit 94, acting as a voltage detection unit, detects the voltage applied to the charge adjustment device 9.

[0041] In this embodiment, the upper and lower limits of the static discharge current when the sheet P passes through the charge adjustment nip section N are determined based on information such as the environmental sensor 75, the conditions specified by the operation unit 76, and the electrical resistance of the charge adjustment nip section N detected before the sheet P passes through the charge adjustment nip section N. Then, as the sheet P is transported through the charge adjustment nip section N, the control unit 110 controls the high voltage output from the charge adjustment high voltage power supply 90 at a constant voltage so that the static discharge current flowing through the charge adjustment nip section N remains within the range of the aforementioned upper and lower limits, while detecting the static discharge current. Furthermore, in order to perform this control, the high voltage board of the charge adjustment high voltage power supply 90 has a current detection circuit 93 for detecting the static discharge current and a voltage detection circuit 94 for detecting the output high voltage.

[0042] Specifically, the control unit 110 can execute a mode (first mode) in which, when no sheet is being transported to the charge adjustment nip section N, a predetermined voltage is applied from the charge adjustment high-voltage power supply 90 to the charge adjustment device 9, and the relationship between current and voltage is determined from the current value detected by the current detection circuit 93 and the predetermined voltage. In this mode, the control unit 110 applies a plurality of different voltages as the predetermined voltage, and when each voltage is applied, the control unit 110 determines the relationship between current and voltage from the plurality of current values ​​detected by the current detection circuit 93 and the plurality of applied voltage values.

[0043] Then, the control unit 110 determines the voltage to be applied from the charge adjustment high-voltage power supply 90 to the charge adjustment device 9 when the sheet passes through the charge adjustment nip section N, based on the relationship obtained by the mode and the length of the sheet in the sheet width direction (sheet width) obtained by the operation unit 76.

[0044] In particular, in this embodiment, the control unit 110 determines a target current value to flow to the charge adjustment device 9, and upper and lower limits of the current value that may flow in the area where the sheet is located while the sheet is held in the charge adjustment nip section N, based on the information detected by the environmental sensor 75. Next, it determines a first voltage at which the target current value flows in the area of ​​the charge adjustment nip section N where there is no sheet, based on the relationship obtained in the above mode, and also determines a second voltage related to the electrical resistance of the sheet based on the information detected by the environmental sensor 75. Then, when the sheet is transported to the charge adjustment nip section N, a third voltage obtained by adding the above first voltage and second voltage is applied to the charge adjustment device 9, and the current value at that time is detected by the current detection circuit 93.

[0045] Furthermore, the control unit 110 calculates the current value in the area where the sheet is located in the charge adjustment nip section N based on the current value when the third voltage described above is applied, the relationship obtained in the mode described above, and the sheet width obtained by the operation unit 76. Then, the control unit 110 determines the voltage to be applied from the charge adjustment high-voltage power supply 90 to the charge adjustment device 9 when the sheet passes through the charge adjustment nip section N, such that the current value in the area where the sheet is located falls within the range of the upper and lower limits described above.

[0046] The following describes an example of the control flow described above using the flowchart in Figure 5. First, when the control unit 110 receives job information from the operation unit 76, it starts the image forming process by the image forming apparatus 100 (S101). The job information specified by the operation unit 76 includes image information specified by the user, the size (width, length) of the sheet P on which the image will be printed, information related to the thickness of the sheet P (thickness or basis weight), and information related to the surface properties of the sheet P, such as whether the sheet P is coated paper or not. The CPU 111 of the control unit 110 writes this information to the RAM 112 (S102).

[0047] The control unit 110 reads environmental information such as temperature and humidity inside the machine using an environmental sensor 75 (S103). The ROM 113 in the control unit 110 has a correlation between the environmental information and the target current Itarget of the static elimination current. Based on the aforementioned environmental information, the CPU 111 in the control unit 110 determines the target current Itarget corresponding to the environment when performing charge adjustment in the charge adjustment device 9, and writes this to the RAM 112 (S104). The reason the target current is changed according to the environmental information is that the amount of charge required to cancel the amount of charge on the sheet after passing through the fixing device 7 changes depending on the environment.

[0048] Furthermore, when the sheet P passes through the charge adjustment nip section N, the current range that may flow through the area where sheets can be prevented from sticking together (hereinafter, for convenience, also referred to as the sheet area or paper-feeding section) is determined by referring to a table, as shown in Figure 6, which is created in the ROM 113 based on the results of a preliminary study, according to the ambient environment information acquired in S103 (S105). The current range that may flow through the paper-feeding section is the value between the upper limit current (upper value) Imax and the lower limit current (lower value) Imin.

[0049] Furthermore, the current range that can suppress adhesion between sheets varies not only depending on the surrounding environment but also on the thickness and surface properties of the sheets P. Therefore, it is desirable that the table of current ranges that can be applied to the paper feed section also changes depending on information related to the thickness of the sheets P (basis weight) and information related to the surface properties of the sheets P.

[0050] Furthermore, before the sheet P on which the toner image is fixed reaches the charge adjustment nip section N, the control unit 110 supplies a predetermined static elimination voltage (a predetermined voltage) from the charge adjustment high-voltage power supply 90 while the opposing roller 92 and the charge adjustment roller 910 are in contact, and reads the electrical resistance of the charge adjustment nip section N, or in other words, the voltage-current relationship (S106). That is, it executes the mode described above (first mode).

[0051] The electrical resistance of the charge adjustment nip section N may be expressed by a polynomial of second order or higher, rather than by a linear relationship between the current and voltage. Therefore, in this embodiment, a predetermined static elimination voltage is supplied for electrical resistance detection in three or more stages so that the voltage-current relationship before the sheet P reaches the charge adjustment nip section N can be expressed by a polynomial, and the current flowing at that time is detected. In this embodiment, the coefficients a, b, and c of the following quadratic equation are calculated from the approximate relationship between the current I and voltage V at three points. I=aV 2 +bV+c

[0052] The CPU 111 calculates a static elimination voltage value (first voltage) Vb to be output from the charge adjustment high-voltage power supply 90 in order to allow the target current Itarget to flow, when the sheet P is not being transported to the charge adjustment nip section N (hereinafter, for convenience, this will also be referred to as "non-transport"). This is determined from the target current Itarget written to the RAM 112 in S104 and the voltage-current relationship obtained in S106.

[0053] Furthermore, the ROM 113 contains a table, as shown in Figure 7, that calculates the static elimination voltage Vp to be output by adding the electrical resistance of sheet P to the amount of moisture contained in the atmosphere inside the main unit, which is determined from the ambient environment information.The CPU 111 then calculates the static elimination voltage (second voltage) Vp to be output by adding the electrical resistance of sheet P to the amount of moisture contained in the atmosphere inside the main unit, which is determined from the job information acquired in S102 and the ambient information acquired in S103.The initial value of the static elimination voltage (static elimination high voltage, third voltage) Vj to be applied when sheet P reaches the charge adjustment nip section N is written to the RAM 112 as Vb+Vp, which is the sum of Vb and Vp, in preparation for the timing when sheet P reaches the charge adjustment nip section N (S107).

[0054] The table for determining Vp, as shown in Figure 7, was obtained through prior studies. The static elimination voltage Vp for the electrical resistance of sheet P changes not only with information related to the thickness of sheet P (basis weight) but also with the surface properties of sheet P. Therefore, it is desirable that the table also changes with information related to the surface properties of sheet P. In this embodiment, information related to the thickness of sheet P and information related to the surface properties of sheet P are included in the job information obtained in S102. However, it is also possible to have separate measuring means for detecting the thickness of sheet P and the surface properties of sheet P, and to determine Vp based on the detection results obtained from these measuring means.

[0055] From the moment the leading edge of the sheet P in the transport direction reaches the charge adjustment nip section N, until the rear end of the sheet P in the transport direction exits the charge adjustment nip section N, the static elimination current Ij is detected (S108) and the paper feeding current Ip is calculated (S109). In this embodiment, the position where the leading edge of the sheet P has advanced sufficiently is defined as 10 mm from the charge adjustment nip section N. Also, in this embodiment, the position where the rear end of the sheet P in the transport direction exits the charge adjustment nip section N is defined as 10 mm before the rear end of the sheet P exits the charge adjustment nip section N. The static elimination current Ij is detected at predetermined intervals, and in this embodiment, detection is performed every 8 msec.

[0056] Figure 8 shows a schematic diagram of the charge adjustment device 9 as viewed from the sheet transport direction. As shown in Figure 8, the static elimination current Ij detected by the current detection circuit 93 is divided into the current flowing through the paper-feeding section and the current flowing through the non-paper-feeding section. If Lr is the length (width) of the charge adjustment roller 910 in a direction approximately perpendicular to the sheet transport direction of the sheet P, and Lp is the length (sheet width) of the sheet P in a direction approximately perpendicular to the sheet transport direction of the sheet P, then the current is expressed by the following equation. Ij = Lp / Lr × Ip + (Lr - Lp) / Lr × Inp

[0057] Here, the paper-feeding current Ip is the current that flows through the portion of the charge adjustment nip section N where the sheet P is located, in the longitudinal direction approximately perpendicular to the sheet P's transport direction. Inp is the current that flows through the portion of the charge adjustment nip section N where the sheet P is not located (non-paper-feeding current).

[0058] From the above formula, the paper feed current Ip is calculated using the following formula. Ip=Lr / Lp×{Ij-(Lr-Lp) / Lr×Inp} The non-paper-feeding current Inp is calculated using the following formula, based on the longitudinal roller length, and the electrical resistance of the charge adjustment nip section N obtained in S106. Inp=aVj 2 +bVj+c

[0059] To ensure that the upper limit Imax and lower limit Imin of the static elimination current function correctly even when the width of the sheet P differs, the paper-feeding section current value Ip and the non-paper-feeding section current value Inp used here are values ​​normalized to the width Lr of the charge adjustment roller 910. The paper-feeding section current Ip can be determined based on the average value of multiple current detection results during paper feeding.

[0060] The CPU 111 then determines whether the paper-feeding section current value (sheet-containing area current value) Ip calculated in S109 is greater than or equal to the upper limit Imax, and whether it is less than or equal to the lower limit Imin (S110, S111). If the paper-feeding section current value Ip is greater than or equal to the upper limit Imax (No in S110), the CPU 111 decreases the static elimination voltage Vj by the voltage change amount ΔV per cycle and stores it in the RAM 112 (S112). On the other hand, if the paper-feeding section current value Ip is less than or equal to the lower limit Imin (No in S111), the CPU 111 increases the static elimination voltage Vj by the voltage change amount ΔV per cycle and stores it in the RAM 112 (S113).

[0061] In this embodiment, a value of 50V was used as the voltage change width ΔV per cycle. This calculation of the paper feeding section current, comparison with the upper and lower limits, and the increase or decrease of the voltage are repeated until the area reaches 10 mm from the rear end of the recording material (S114). As a result, even if the paper feeding section current value Ip is initially outside the upper limit Imax and lower limit Imin, the paper feeding section current value Ip gradually approaches the range of the upper limit Imax and Imin, and typically ends up at either the upper limit Imax or the lower limit Imin.

[0062] This modified static elimination voltage will be applied as the initial value when adjusting the charge for the next sheet P and beyond (S115). The flow from S108 to S115 is repeated using the initial value determined in S115 until the job is completed (S116). Note that if the paper feed current value Ip is within a predetermined range, i.e., less than the upper limit Imax (Yes in S110) and greater than the lower limit Imin (Yes in S111), the static elimination voltage Vj will not be changed.

[0063] In this embodiment, the sheet's charge can be stably adjusted regardless of the width of the sheet P or the resistance change of the charge adjustment roller 910 of the charge adjustment device 9. That is, by performing the series of controls described above, the current flowing through the non-paper-passing portion when the sheet P is transported to the charge adjustment nip section N can be predicted by detecting the resistance before the sheet P reaches the charge adjustment device 9. By controlling the current supplied to the sheet using the predicted current flowing through the non-paper-passing portion, the charge adjustment device 9 can adjust the sheet's charge regardless of the fluctuating resistance of the charge adjustment roller 910 and the sheet under various conditions, and it is possible to maintain a state in which sheets do not stick together.

[0064] <Second Embodiment> A second embodiment will be described with reference to Figure 9. In the first embodiment described above, the voltage control of the charge adjustment device 9 was performed with a constant voltage, but in this embodiment, it is performed with a constant current. The other configurations and operations are the same as in the first embodiment described above, so the same components are denoted by the same reference numerals, and their descriptions and illustrations are omitted or simplified. The following description will focus on the differences from the first embodiment.

[0065] As described above, this embodiment differs from the first embodiment only in the voltage control method of the charge adjustment device 9; the configurations of the image forming apparatus 100 and the charge adjustment device 9 are the same. Therefore, the voltage control of the charge adjustment device 9 in this embodiment will be described below.

[0066] In this embodiment, similar to the first embodiment, the control unit 110 controls the high voltage output from the charge adjustment high-voltage power supply 90 so that the static discharge current flowing through the charge adjustment nip section N falls within predetermined upper and lower limits. However, in this embodiment, the high voltage output from the charge adjustment high-voltage power supply 90 is a constant current.

[0067] In other words, in this embodiment, the control unit 110 can execute a mode (second mode) in which it supplies a predetermined current from the charge adjustment high-voltage power supply 90 to the charge adjustment device 9 when the sheet is not being transported to the charge adjustment nip section N, and at that time determines the relationship between current and voltage from the voltage value detected by the voltage detection circuit 94 and the predetermined current. Then, based on the relationship determined in this mode and the length of the sheet in the sheet width direction (sheet width) obtained by the operation unit 76, the control unit 110 determines the voltage to be applied from the charge adjustment high-voltage power supply 90 to the charge adjustment device 9 when the sheet passes through the charge adjustment nip section N.

[0068] In particular, in this embodiment, the control unit 110 determines a target current value to be supplied to the charge adjustment device 9, and upper and lower limits of the current value that may be supplied to the area where the sheet is located while the sheet is held in the charge adjustment nip section N, based on the information detected by the environmental sensor 75. Next, when the sheet is transported to the charge adjustment nip section N, the target current is supplied to the charge adjustment device 9 from the charge adjustment high-voltage power supply 90, and the voltage value at that time is detected by the voltage detection circuit 94.

[0069] Furthermore, the control unit 110 calculates the sheet-present region current value flowing in the area where the sheet is located in the charge adjustment nip section N, based on the voltage value when the target current is supplied, the relationship obtained in the mode described above, and the sheet width obtained by the operation unit 76. Then, the control unit 110 determines the current to be supplied from the charge adjustment high-voltage power supply 90 to the charge adjustment device 9 as the sheet passes through the charge adjustment nip section N, such that the sheet-present region current value falls within the range of the upper and lower limits.

[0070] The following describes an example of the control flow described above using the flowchart in Figure 9. First, when the control unit 110 receives job information from the operation unit 76, it starts the image forming process by the image forming apparatus 100 (S201) and obtains sheet information from the job information (S202). The control unit 110 reads environmental information such as temperature and humidity inside the machine using the environmental sensor 75 (S203).

[0071] The ROM 113 in the control unit 110 has a correlation between the target current Itarget of the static elimination current and the environmental information. Based on the aforementioned environmental information, the CPU 111 in the control unit 110 determines the target current Itarget corresponding to the environment when the charge adjustment device 9 performs charge adjustment, and writes this to the RAM 112 (S204). In addition, when the sheet P passes through the charge adjustment nip section N, the current range that can be flowed through the paper-feed section to suppress sticking between sheets is determined by referring to a table like the one shown in Figure 6, which was created in the ROM 113 based on the results of a preliminary study, according to the ambient environmental information acquired in S203 (S205). The current range that can be flowed through the paper-feed section is the value between the upper limit current (upper value) Imax and the lower limit current (lower value) Imin. S201 to S205 are the same as S101 to S105 in Figure 5.

[0072] Next, in this embodiment, before the sheet P on which the toner image is fixed reaches the charge adjustment nip section N, the control unit 110 supplies a predetermined static elimination current (a predetermined current) from the charge adjustment high-voltage power supply 90 while the opposing roller 92 and the charge adjustment roller 910 are in contact, and reads the electrical resistance of the charge adjustment nip section N, or in other words, the voltage-current relationship (S206). That is, the above-described mode (second mode) is executed.

[0073] Furthermore, in this embodiment, a predetermined static elimination current supplied for electrical resistance detection is supplied in three or more stages so that the voltage-current relationship before the sheet P reaches the charge adjustment nip section N can be expressed as a polynomial, and the voltage applied at that time is detected. In this embodiment, the coefficients a, b, and c of the following quadratic equation are calculated from the approximate relationship between the current I and voltage V at three points. I=aV 2 +bV+c

[0074] The CPU 111 writes the target current Itarget, which was written to the RAM 112 in S204, to the RAM 112 as the initial value of the static elimination current Ij to be applied when sheet P reaches the charge adjustment nip section N, and prepares for the timing when sheet P reaches the charge adjustment nip section N (S207).

[0075] From the moment the leading edge of the sheet P in the transport direction reaches the charge adjustment nip section N, until the rear end of the sheet P in the transport direction exits the charge adjustment nip section N, the static elimination voltage Vj is detected (S208) and the paper-feeding current Ip is calculated (S209). In this embodiment, the position where the leading edge of the sheet P has advanced sufficiently is defined as 10 mm from the charge adjustment nip section N. Also, in this embodiment, the position where the rear end of the sheet P in the transport direction exits the charge adjustment nip section N is defined as 10 mm before the rear end of the sheet P exits the charge adjustment nip section N. The static elimination voltage Vj is detected at predetermined intervals, and in this embodiment, detection is performed every 8 msec.

[0076] As shown in FIG. 8 above, the applied discharge current Ij is divided into the current flowing through the paper passage portion and the current flowing through the non-paper passage portion. If the length (width) of the charge adjustment roller 910 in the direction substantially orthogonal to the conveyance direction of the sheet P is Lr, and the length (sheet width) of the sheet P in the direction substantially orthogonal to the conveyance direction of the sheet P is Lp, it is expressed as the following formula. Ij = Lp / Lr × Ip + (Lr - Lp) / Lr × Inp

[0077] Here, the paper passage portion current Ip is the current flowing through the portion where the sheet P exists in the entire area in the direction (longitudinal direction) substantially orthogonal to the conveyance direction of the sheet P in the charge adjustment nip portion N. Inp is the current (non-paper passage portion current) flowing through the portion where the sheet P does not exist in the entire longitudinal direction of the charge adjustment nip portion N.

[0078] From the above formula, the paper passage portion current Ip is calculated by the following formula. I p = Lr / Lp × {Ij - (Lr - Lp) / Lr × Inp} The non-paper passage portion current Inp is calculated by the following formula using the electrical resistance of the charge adjustment nip portion N obtained in S206 depending on the roller length in the longitudinal direction. Inp = aVj 2 + bVj + c

[0079] Here, the paper passage portion current value Ip and the non-paper passage portion current value Inp use values normalized to the width Lr of the charge adjustment roller 910 so that the upper limit value Imax and the lower limit value Imin of the discharge current function properly even when the width of the sheet P is different. Note that the paper passage portion current Ip can be obtained based on the average value of a plurality of current detection results during paper passage.

[0080] The CPU 111 then determines whether the paper-feeding section current value (sheet-containing area current value) Ip calculated in S209 is greater than or equal to the upper limit Imax, and whether it is less than or equal to the lower limit Imin (S210, S211). If the paper-feeding section current value Ip is greater than or equal to the upper limit Imax (No in S210), the CPU 111 reduces the static elimination current Ij by the current change amount ΔI per cycle and stores it in the RAM 112 (S212). On the other hand, if the paper-feeding section current value Ip is less than or equal to the lower limit Imin (No in S211), the CPU 111 increases the static elimination current Ij by the voltage change amount ΔI per cycle and stores it in the RAM 112 (S213).

[0081] In this embodiment, a value of 2 μA was used as the voltage change width ΔI per cycle. This calculation of the paper-feeding section current, comparison with the upper and lower limits, and increasing or decreasing the current are repeated until the area reaches 10 mm from the rear end of the recording material (S214). As a result, even if the paper-feeding section current value Ip is initially outside the upper limit Imax and lower limit Imin, the paper-feeding section current value Ip gradually approaches the range of the upper limit Imax and Imin, and typically ends up at either the upper limit Imax or the lower limit Imin.

[0082] This modified static elimination current will be applied as the initial value for charge adjustment for the next sheet P and subsequent sheets (S215). The flow from S208 to S215 is repeated using the initial value determined in S215 until the job is completed (S216). Note that if the paper feed current value Ip is within a predetermined range, i.e., less than the upper limit Imax (Yes in S210) and greater than the lower limit Imin (Yes in S211), the static elimination current Ij will not be changed.

[0083] In this embodiment as well, the sheet's charge can be stably adjusted regardless of the width of the sheet P or the resistance change of the charge adjustment roller 910 of the charge adjustment device 9. That is, by performing the series of controls described above, the current flowing through the non-paper-passing portion when the sheet P is transported to the charge adjustment nip section N can be predicted by detecting the resistance before the sheet P reaches the charge adjustment device 9, and the current supplied to the sheet can be controlled using the predicted current flowing through the non-paper-passing portion. This allows the charge adjustment of the sheet by the charge adjustment device 9 to be performed regardless of the resistance of the charge adjustment roller 910 and the sheet, which fluctuate under various conditions, and it is possible to maintain a state in which sheets do not stick together. [Explanation of symbols]

[0084] 7. Fixing device (fixing unit) 9...Charge adjustment device (charge adjustment section) 75. Environmental sensor (environmental detection unit) 76...Operation unit (information acquisition unit) 90...Charge-regulating high-voltage power supply (power supply) 92... Opposing roller (rotating body) 93. Current detection circuit (current detection unit) 94. Voltage detection circuit (voltage detection unit) 100...Image forming apparatus 110... Control Unit 910...Charge adjustment roller (rotating body) N2...Secondary transfer section (transfer section)

Claims

[Claim 1] A transfer unit that transfers the toner image onto the sheet, A fixing unit heats and pressurizes the sheet onto which the toner image has been transferred in the transfer unit to fix the toner image to the sheet, The unit has a pair of rotating bodies that hold and transport the sheet on which the toner image has been fixed in the fixing unit, and a charge adjustment unit that adjusts the charge on the sheet when a voltage is applied, A power supply that applies voltage to the charge adjustment unit, A current detection unit that detects the current flowing through the charge adjustment unit, An information acquisition unit capable of acquiring information on the length of the sheet in the sheet width direction perpendicular to the sheet transport direction, The system comprises a control unit for controlling the power supply, The aforementioned power supply is controlled by a constant voltage. The control unit applies a predetermined voltage from the power supply to the charge adjustment unit when the sheet is not being transported to the nip portion formed by the pair of rotating bodies, and at that time executes a mode to determine the relationship between current and voltage from the current value detected by the current detection unit and the predetermined voltage, and determines the voltage to be applied from the power supply to the charge adjustment unit when the sheet passes through the nip portion based on the relationship obtained in the mode and the length of the sheet in the sheet width direction obtained by the information acquisition unit. An image forming apparatus characterized by the following features.

Images