JP2023098238A - image forming device - Google Patents

Abstract

An object of the present invention is to provide a configuration capable of suppressing uneven charge removal of a sheet.
A fixing device (7) heats and presses a sheet onto which a toner image has been transferred at a secondary transfer portion (N2) to fix the toner image on the sheet. The static elimination device 9 applies a static elimination voltage to the sheet on which the toner image is fixed by the fixing device 7 to eliminate static electricity from the sheet. The control unit 110 controls the static elimination voltage applied to the static elimination device 9 . Further, the control unit 110 can change the static elimination voltage while one sheet is passing through the static elimination device 9 .
[Selection drawing] Fig. 3

Description

The present invention relates to an image forming apparatus such as a copier, a printer, a facsimile machine, and a multifunction machine having a plurality of these functions.

In an image forming apparatus, a toner image formed by an image forming unit is transferred onto a sheet by a transfer unit, and after the toner image is fixed on the sheet by a fixing unit, the sheet is stacked on a discharge tray or the like. At this time, the sheets may stick to each other due to the electrostatic force between the sheets. For this reason, a configuration has been proposed that includes a neutralization section that applies a voltage to a sheet on which a toner image is fixed by a fixing section to neutralize the sheet (Patent Document 1).

In the case of the configuration described in Japanese Patent Application Laid-Open No. 2002-200303, a charge amount determination unit is provided to determine the amount of charge charged on the sheet in the transfer operation by the transfer unit, and the static elimination unit cancels the charge amount determined by the charge amount determination unit. The sheet after the transfer is neutralized with a neutralization charge.

JP 2019-168484 A

However, in the case of the configuration described in Patent Document 1, the static elimination parameter is set constant for one sheet passing through the static elimination section. Therefore, when the charge amount of the sheet varies in the conveying direction due to the transfer operation or the fixing operation, the sheet cannot be uniformly charged. That is, there is a possibility that the sheet may remain unevenly charged.

SUMMARY OF THE INVENTION An object of the present invention is to provide a configuration capable of suppressing uneven charge removal of a sheet.

The image forming apparatus of the present invention includes a transfer section that transfers a toner image onto a sheet, a fixing section that heats and presses the sheet to which the toner image has been transferred by the transfer section to fix the toner image on the sheet, and the fixing section. a static elimination unit that applies a static elimination voltage to a sheet on which a toner image is fixed by a unit to eliminate static electricity from the sheet; and a control unit that controls the static elimination voltage applied to the static elimination unit, wherein the control unit comprises , the static elimination voltage can be changed while one sheet is passing through the static elimination section.

Further, the image forming apparatus of the present invention has a transfer section for transferring a toner image onto a sheet, and a pair of rotating members for holding and conveying the sheet, and heats and presses the sheet onto which the toner image has been transferred in the transfer section. a fixing section for fixing the toner image on the sheet; a charge removing section for applying a charge removing voltage to the sheet on which the toner image is fixed by the fixing section to remove charge from the sheet; a control unit for controlling voltage, wherein the control unit performs one revolution after at least one of the pair of rotating bodies comes into contact with the leading edge of the sheet while the sheet is passing through the fixing section. When the area in the conveying direction of the sheet is divided into a first area until the sheet passes through and a second area where the sheet remains, the first area and the second area are formed while the sheet is passing through the static elimination unit. and can change the static elimination voltage.

According to the present invention, it is possible to suppress the uneven charge removal of the sheet.

1 is a cross-sectional view of a schematic configuration of an image forming apparatus according to a first embodiment; FIG. FIG. 2 is a cross-sectional view of a schematic configuration of an image forming unit according to the first embodiment; 1 is a schematic configuration diagram of a static eliminator according to a first embodiment; FIG. 4A and 4B are schematic diagrams showing charged states of a sheet before and after a transfer portion, and (b) charged states of a sheet before and after a fixing portion, respectively; (a) A schematic configuration diagram of a static eliminator according to a comparative example, (b) a schematic diagram showing a charged state of a sheet before and after static elimination. With the static eliminator according to the comparative example, (a) a case where a uniformly charged sheet is statically eliminated, (b) a case where a sheet with an uneven charge amount is statically eliminated with a high static elimination parameter, and (c) a case where the charge amount is uneven is eliminated. 4A and 4B are schematic diagrams each showing a case where a certain sheet is statically eliminated with a low static elimination parameter. FIG. 4 is a flowchart of static elimination parameter control according to the first embodiment; (a) Schematic diagram showing a case where a sheet having an uneven charge amount is neutralized by control according to the embodiment, (b) Graph showing measurement results of the surface potential of a sheet having an uneven charge amount, (c) Present implementation 4 is a graph showing an example of static elimination parameter settings for the form; FIG. 2 is a schematic configuration diagram of a static eliminator according to a second embodiment; The schematic block diagram regarding the static elimination apparatus which concerns on 3rd Embodiment. 10 is a flowchart of static elimination parameter control according to the third embodiment; 7A and 7B are graphs showing (a) a current detection result of the transfer unit, (b) a current detection result of the fixing unit, and (c) an example of static elimination parameter setting;

<First embodiment>
A first embodiment will be described with reference to FIGS. 1 to 8. FIG. First, a schematic configuration of an image forming apparatus according to this embodiment will be described with reference to FIGS. 1 and 2. FIG.

[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 electrophotography. The image forming apparatus 100 is of an intermediate transfer 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 .

The image forming units Pa, Pb, Pc, and Pd are provided with photosensitive drums 1a, 1b, 1c, and 1d as image bearing members for carrying electrostatic latent images and photosensitive members, respectively. In the image forming portion Pa, a yellow toner image is formed on the photosensitive drum 1a and primarily transferred onto an intermediate transfer belt 51 as an intermediate transfer member. In the image forming portion Pb, a magenta toner image is formed on the photosensitive drum 1b and is primarily transferred over the yellow toner image on the intermediate transfer belt 51 . In the image forming units Pc and Pd, a cyan toner image and a black toner image are formed on the photosensitive drums 1c and 1d, respectively, and are sequentially primary-transferred so as to be superimposed on the toner image on the intermediate transfer belt 51 in the same manner. Incidentally, in this embodiment, the photosensitive drum and the intermediate transfer belt have a role as an image carrier that carries a toner image.

The four-color toner images primarily transferred to the intermediate transfer belt 51 are collectively secondarily transferred onto the sheet P fed to the secondary transfer portion N2 formed by the intermediate transfer belt 51 and the secondary transfer roller 56. be. The sheet P to which the toner image has been secondarily transferred at the secondary transfer portion N2 is subjected to heat and pressure by a fixing device 7 as a fixing portion, and after the toner image is fixed on the surface, the sheet P is discharged to the outside (not shown). It is loaded on the output tray.

The feeding device 8 separates the sheets P pulled out from the cassette 81 by the pickup roller 82 one by one by the separation device 83 and feeds them to the registration rollers 84 . The registration rollers 84 receive and wait the sheet P in a stopped state, and send the sheet P to the secondary transfer portion N2 in timing with the toner image on the intermediate transfer belt 51 .

The intermediate transfer unit 5 stretches an intermediate transfer belt 51, which is an example of an image carrier, over a drive roller 52, support rollers 58 and 59, a tension roller 53, and a counter roller 54, and rotates it in the direction of arrow R2. The opposing roller 54 is arranged at a position facing the secondary transfer roller 56 with the intermediate transfer belt 51 interposed therebetween. A secondary transfer portion N2 is formed at which the sheet is nipped by the outer peripheral surface of the intermediate transfer belt 51 stretched over the counter roller 54 and the secondary transfer roller 56 .

A power source D2 is connected to the secondary transfer roller 56 to apply a secondary transfer bias. When the secondary transfer is performed, a high positive transfer voltage (secondary transfer bias) is applied to the secondary transfer roller 56 to transfer the negatively charged toner image. It is electrostatically attracted to the sheet. As a result, the toner image carried on the intermediate transfer belt 51 is secondarily transferred onto the sheet P passing through the secondary transfer portion N2.

The fixing device 7 has a fixing roller 72 and a pressure roller 73, which are a pair of rotating bodies (fixing members) that nip and convey a sheet. Then, the sheet onto which the toner image has been transferred at the secondary transfer portion N2 is heated and pressurized to fix the toner image on the sheet. That is, the fixing device 7 presses the pressure roller 73 against the fixing roller 72 centering the lamp heater 71 to form a heating nip portion. Then, the sheet P to which the toner image has been transferred at the secondary transfer portion N2 is heated and pressed at the heating nip portion to fix the toner image onto the sheet P. FIG. After that, the sheet P that has undergone the fixing process is discharged out of the apparatus by discharge rollers 85 as a discharge unit and stacked on a discharge tray 86 .

The belt cleaning device 57 rubs a cleaning blade against the intermediate transfer belt 51 to remove transfer residual 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 portion N2. to remove

In the image forming units Pa, Pb, Pc, and Pd, the toner colors used in the developing devices 4a, 4b, 4c, and 4d attached to the photosensitive drums 1a, 1b, 1c, and 1d are different from yellow, magenta, cyan, and black. Other than that, they are configured almost identically. In the following, the image forming unit Pa will be described with reference to FIG. 2, and the other image forming units Pb, Pc, and Pd will be described by replacing the suffix a in the description with b, c, and d. shall be

As shown in FIG. 2, the image forming section Pa includes a charging roller 2a, an exposure device 3a, a developing device 4a, a primary transfer roller 55a, and a cleaning device 6a around the photosensitive drum 1a. The photosensitive drum 1a has an organic photoconductor layer (OPC) with a negative charging polarity formed on the outer peripheral surface of an aluminum cylinder, and rotates in the direction of arrow R1 at a process speed of 240 mm/sec.

The charging roller 2a, which is a charging member, is formed by covering the surface of a metallic center shaft with a resistive elastic layer, and is driven to rotate while being pressed against the photosensitive drum 1a. The power source D3 applies a DC voltage superimposed with an AC voltage to the charging roller 2a to uniformly charge the surface of the photosensitive drum 1a to a negative potential.

The exposure device 3a scans with a rotating mirror a laser beam obtained by ON-OFF modulating scanning line image data representing a yellow separation color image, and writes an electrostatic image of the image on the surface of the charged photosensitive drum 1a.

The developing device 4a stirs a two-component developer in which a non-magnetic toner is mixed with a magnetic carrier, and charges the non-magnetic toner negatively and the magnetic carrier positively. The charged two-component developer is carried in a brushed state on the developing sleeve 41a rotating counter to the photosensitive drum 1a around the fixed magnetic pole 42a, and rubs against the photosensitive drum 1a. The power supply D4 applies a developing voltage obtained by superimposing an AC voltage on a DC voltage of negative polarity to the developing sleeve 41a, thereby moving the toner to the exposed portion of the photosensitive drum 1a which has a positive polarity relative to that of the developing sleeve 41a. to reversely develop the electrostatic image.

A 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 portion N1a between the photosensitive drum 1a and the intermediate transfer belt 51. The power source D1a is a transfer output unit that applies a voltage to the primary transfer roller 55a, and applies a positive DC voltage of +900 V as a primary transfer bias to the primary transfer roller 55a. As a result, the negatively charged toner image carried on the photosensitive drum 1a is primarily transferred onto the intermediate transfer belt 51 passing through the primary transfer portion N1a.

As the primary transfer roller 55a, a semiconductive roller having a resistance value of 1×10 ² to 10 ⁸ Ω when 2000 V is applied is used. Specifically, an ion-conductive sponge roller made of a blend of nitrile rubber and ethylene-epichlorohydrin copolymer and having an outer diameter of φ16 mm and a core diameter of φ8 mm was used. The resistance value of the primary transfer roller 55a is about 1×10 ⁶ to 10 ⁸ Ω when the applied voltage is 2 kV under the environment of temperature 23° C. and humidity 50% RH.

The cleaning device 6a rubs the cleaning blade against the photosensitive drum 1a to remove transfer residual toner adhering to the surface of the photosensitive drum 1a that has passed through the primary transfer portion N1a.

In recent years, the variety of sheets has become abundant, and the thickness and electrical resistivity of sheets are wide, so an intermediate transfer method is adopted. In addition, in order 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, the width of the sheet, etc., the transfer unit (secondary transfer unit in the above example) is used to transfer the toner image onto the sheet. part) employs constant voltage control. Furthermore, the electrical resistance of the intermediate transfer belt and the transfer roller, the film thickness of the surface layer of the photosensitive drum, and the like change due to changes in the atmospheric environment such as temperature and humidity, or accumulation of image formation. Along with these changes, ATVC control (Active Transfer Voltage Control) is executed to determine a control value for constant voltage control prior to image formation in order to optimize the voltage applied to the transfer roller during image formation. be.

In the ATVC control, a plurality of different test voltages are applied to the secondary transfer roller 56 when there is no sheet in the secondary transfer portion N, current is detected by a current detection sensor at each transfer voltage, and the difference between the transfer voltage and the current is detected. This is control for determining the relationship and setting the transfer voltage (secondary transfer bias) to be applied to the secondary transfer portion N based on the relationship. Overall control of the image forming apparatus 100, including such ATVC control, is performed by the control unit 110 (FIG. 1).

The control unit 110 has a CPU (Central Processing Unit), a ROM (Read Only Memory), and a RAM (Random Access Memory). The CPU controls each part while reading a program corresponding to the control procedure stored in the ROM. The RAM stores work data and input data, and the CPU performs control by referring to the data stored in the RAM based on the above-described programs and the like.

[Static elimination of sheet]
Here, in the case of the present embodiment, in order to prevent the sheets stacked on the discharge tray 86 from sticking to each other due to electrostatic force, as shown in FIG. On the other hand, a static eliminator 9 as a static eliminator is arranged on the upstream side of the discharge roller 85 (FIG. 1). The static elimination device 9 applies a static elimination voltage to the sheet on which the toner image is fixed by the fixing device 7 to eliminate static electricity from the sheet. In FIG. 3 and the like, the symbols "-" and "+" described on the surface of the sheet P schematically indicate the charged state of the surface, with "-" indicating negative polarity and "+". indicates positive polarity.

As shown in FIG. 3, the static eliminator 9 includes static elimination rolls (static elimination members) 91 and 92 that are configured as a pair of rotating bodies and are arranged in contact with each other. is urged by the spring member to press against the opposing static elimination roll 91 to form a nip. A static elimination power supply (static elimination high voltage) 90 is connected to the opposing static elimination roll 91 , and a static elimination bias Vd as a static elimination parameter is applied from the static elimination power supply 90 .

In the present embodiment, the static eliminator 9 adopts a system in which the static eliminator contacts the sheet, but is not limited to this, and adopts a system in which the static eliminator is arranged in a non-contact manner with respect to the sheet. It is of course possible to

[Change in Sheet Charge Amount in Fixing Device]
Next, the process in which charge fluctuation occurs while the sheet P passes through the fixing device 7 will be described with reference to FIGS. As shown in FIG. 4A, each surface of the sheet is charged because the secondary transfer bias is applied when the image is transferred to the sheet at the secondary transfer portion N2. For example, when the image-forming particles (toner) are negatively charged particles, a negative bias is applied to the opposite roller 54 in FIG. An image (toner image) is transferred to a sheet by applying a potential difference.

At this time, discharge charge migration and charge injection corresponding to the respective potential differences occur on the respective surfaces of the sheet on the downstream side of the secondary transfer portion N2. As a result, the image forming surface is negatively charged, and the back surface of the sheet is positively charged. For example, when the secondary transfer bias is set constant for the entire surface of a sheet and the sheet also has a uniform resistance in the conveying direction, the front and back sides of the sheet P are formed as shown in FIG. The surface is uniformly charged. Further, when the sheet P has a high volume resistivity, that is, when it has insulating properties, a phenomenon occurs in which the charges on the respective surfaces of the sheet shown on the right side of FIG. Because it is difficult to use, it maintains a high charge amount, and electrostatic sticking tends to be a problem.

Next, FIG. 4B is a schematic diagram showing changes in the charge amount when the sheet passes through the fixing device 7 after passing through the transfer section. Here, the pressure roller 73 as a fixing pressure member has a diameter of 30 mm. The resistance is as low as about 1×10 ³ Ω·cm, and the metal core 74 is grounded through the resistance. Therefore, part of the positive charges distributed on the back surface of the sheet is transferred to the pressure roller 73 particularly in the first rotation of the pressure roller 73 . Therefore, the leading edge side of the sheet P that has passed through the fixing device 7 has a low charging amount, and the trailing edge side has a high charging amount.

Especially in recent years, there is a demand for printing on various sheets, and there is an increasing demand for image formation on high-resistance sheets that are almost electrically insulating, such as transparent films, OHP paper, and synthetic paper. As an example of a high-resistance sheet, an insulating PET (polyethylene terephthalate) sheet having a thickness of 75 μm was actually used, and the electrification charge of the sheet was measured when it passed through the secondary transfer portion N2 and the fixing device 7 described above. FIG. 8B shows the result of actually measuring the charge amount on the back surface of the sheet in detail along the conveying direction. From this measurement result, it was found that the charge amount was low in an area of about 90 mm, which corresponds to one cycle of the pressure roller 73, on the leading edge of the sheet.

[Comparative example]
Next, a problem in the case where the sheet that has passed through the fixing section as described above is subjected to static elimination by the method of the comparative example shown in FIGS. 5A and 5B will be described. The comparative example has the configuration shown in the above-mentioned Patent Document 1, and as shown in FIG. Further, in the comparative example, a charge amount determination unit 901 is provided to determine the amount of charge charged on the sheet P in the transfer operation of the secondary transfer unit N2. As shown in FIG. 5B, the static eliminator 900 eliminates static electricity from the sheet after the transfer with a static elimination charge that offsets the charge amount determined by the charge amount determination unit 901 . In the case of such a comparative example, the static elimination parameter is set constant for one sheet, and for example, the static elimination current is controlled at a constant value while the sheet is passing through the static eliminator 900 .

In the case of the comparative example, a constant static charge per unit area is uniformly applied to each surface of the sheet. Therefore, as shown in FIG. 6A, in the case of a sheet that is uniformly charged in the conveying direction, such as the sheet P immediately after passing through the secondary transfer portion N2, the charge amount of the entire sheet P can be uniformly discharged. .

On the other hand, as shown in FIG. 5B, when there is a change in electric charge in the conveying direction after passing through the fixing device 7, the method of setting the charge removal parameter to be constant for each sheet cannot be used. The charge on the entire P cannot be uniformly discharged. Specifically, when a high static elimination parameter is adopted so that the sheet P is aligned with the trailing edge, an excessive static elimination charge is applied to the leading edge of the sheet P as shown in FIG. It becomes an electric charge and becomes a factor of electrostatic sticking after ejection. Further, when a low value for the static elimination parameter is adopted in accordance with the leading edge of the sheet P, as shown in FIG. It becomes a factor of electrical sticking.

In particular, when high-resistance/insulating sheets are stacked on the discharge tray 86 , the charged charges of the stacked sheets remain even if they are left on the discharge tray 86 . In addition, even if the charge amount on one side of the adjacent sheet surfaces is almost zero, if the charge remains on the adjacent surface of the other sheet, the adjacent sheet with the charge amount of zero will be charged due to dielectric polarization. Electrostatic sticking can occur due to polarization charges on the surface. Therefore, when stacking a high-resistance/insulating sheet on the discharge tray 86, it is required to reduce the charge on the surface of the sheet to zero as much as possible. However, in the method of the comparative example, residual charges may occur as described above.

In this way, the method of setting constant static elimination parameters for each sheet as in the comparative example has the problem that it is not possible to sufficiently prevent the electrostatic sticking of high-resistance and insulating sheets. there were.

[Static Eliminator of the Present Embodiment]
Therefore, in the present embodiment, the static elimination parameter is changed while the sheet is passing through the static elimination device 9 according to the change in the amount of charge in the sheet conveying direction. Control of static elimination parameters in this embodiment will be described with reference to FIGS.

First, as shown in FIG. 4, a user selects an image forming processing mode of the image forming apparatus 100 through an operation panel 130 as an input unit provided in the image forming apparatus 100 and capable of inputting information and selecting various modes. You can choose. That is, a mode selection switch 131 (in FIG. 4, "switch" is denoted by "SW") for instructing various image forming modes (single-sided/double-sided printing mode, standard/high-quality printing mode, etc.); A physical property instruction switch 132 is provided for instructing the physical properties (resistance, thickness, basis weight, size, etc.). Further, the control unit 110 controls the static elimination voltage (static elimination parameter) applied to the static eliminator 9 .

In particular, in this embodiment, the controller 110 can change the static elimination voltage while one sheet is passing through the static elimination device 9 . Specifically, the control unit 110 performs static elimination based on a charge amount fluctuation table 120 in which the relationship between the fixing conditions in the fixing device 7 and the charge amount fluctuation generated on the sheet by the fixing operation of the fixing device 7 is determined in advance. Change voltage. In this embodiment, the charge amount variation table 120 takes into consideration the influence of the secondary transfer portion N2 in addition to the relationship of the fixing device 7 to the sheet. That is, the charge amount fluctuation table 120 includes the relationship between the fixing condition in the fixing device 7 and the charge amount fluctuation generated on the sheet by the fixing operation of the fixing device 7, the transfer condition in the secondary transfer portion N2, and the secondary transfer It is determined based on the relationship with the charge amount variation that occurs on the sheet during the transfer operation by the portion N2.

Specifically, the charge amount fluctuation table 120 calculates the charge amount fluctuation that occurs in the conveying direction of the sheet P based on the transfer conditions of the secondary transfer portion N2 and the information on the transfer charge from the sheet of the fixing device 7 to the fixing member. It is predetermined. Here, for the charge amount variation table 120, items may be appropriately selected from the transfer condition of the secondary transfer portion N2 and the charge transfer amount condition of the fixing device 7. FIG.

The sequence of static elimination parameter control according to this embodiment will be described with reference to the flowchart of FIG. FIG. 7 shows the process from transferring an image onto a sheet to discharging the sheet. After the image is transferred to the sheet at the secondary transfer portion N2 (S101) and the image is fixed on the sheet at the fixing device 7 (S102), the control portion 110 reads physical property information of the sheet (S103). Then, the control unit 110 determines whether it is necessary to remove the static electricity from the sheet based on this information (S104). If the sheet does not require static elimination, such as a sheet with a low resistance value (NO in S104), the sheet is discharged without static elimination (S109).

On the other hand, if it is determined in S104 that the static electricity of the sheet needs to be eliminated (YES in S104), the control unit 110 determines whether or not it is necessary to vary the static elimination parameter (S105). As described above, in the case of a sheet having high resistance and insulating properties, it is determined that the static elimination parameter needs to be changed (YES in S105), and the control unit 110 calculates the static elimination parameter (S106). At this time, the control unit 110 refers to the charge amount variation table 120 (S107). Then, based on the calculated static elimination parameter, the control unit 110 performs static elimination on the sheet by varying the static elimination parameter with the static elimination device 9 (S108), and discharges the sheet to the discharge tray 86 (S109).

If it is determined in S105 that the static elimination parameter does not need to be varied (NO in S105), the control unit 110 changes the static elimination parameter based on the transfer conditions without considering the influence on the fixing device 7, for example. Calculate (S110). Then, based on the calculated static elimination parameter, the control unit 110 performs static elimination on the sheet with a constant static elimination parameter in the conveying direction by the static elimination device 9 (S111), and discharges the sheet to the discharge tray 86 (S109).

For example, in the present embodiment, when the resistance of the sheet is large, the amount of charge is particularly low in the first area from the leading edge of the sheet to the first rotation period of the pressure roller 73 of the fixing device 7, and the trailing edge side of the first area. The second region of has a high charge amount. Therefore, the amount of charge charged on the sheet after transfer is determined based on the transfer conditions and the resistance of the sheet, and the amount of charge transferred in the fixing device 7 is determined based on the electrical characteristics of the pressure roller 73 of the fixing device 7 and the resistance of the sheet. do.

Accordingly, as shown in FIG. 7A, when the leading edge of the sheet passes through the charge eliminator 9, the amount of charge obtained by subtracting the amount of transfer charge in the fixing device 7 from the amount of charge after transfer is The control unit 110 controls the static elimination power source 90 so as to apply a static elimination parameter that eliminates the charge amount after transfer when the trailing edge portion of the sheet passes through the static elimination unit.

In other words, the controller 110 can change the static elimination parameters as follows. First, while the sheet is passing through the fixing device 7, at least one roller (rotating body) of the fixing roller 72 and the pressure roller 73 is in contact with the leading edge of the sheet until it makes one turn as the first area. and In the present embodiment, the area from when the pressure roller 73 comes into contact with the leading edge of the sheet to when it makes one turn is defined as the first area. Also, the remaining area of the sheet is defined as a second area. When the area in the conveying direction of the sheet is thus divided into the first area and the second area, the static elimination parameter is set in the first area and the second area while the sheet is passing through the static elimination device 9. Can be changed. In the present embodiment, as shown in FIG. 8C, the second region on the rear end side of the sheet has a larger static elimination current than the first region on the front end side.

Regarding the charge amount fluctuation table 120, the charge removal parameters for portions other than the portion where the charge is transferred from the sheet to the fixing member are determined from the transfer conditions and the physical properties of the sheet as in the comparative example. In the configuration of the present embodiment, in addition to this, regarding the portion where the charge transfers from the sheet to the fixing member, for example, the larger the difference between the volume resistivity of the sheet P and the volume resistivity of the pressure roller 73, the more the transfer charge amount. Set the table value to be large. Further, since charge transfer can have a time constant, it is also possible to set table values for fixing conditions such that the slower the passage speed of the fixing device 7 is, the larger the change in the amount of charge is.

With the above control, when the charge fluctuation of the sheet occurs as shown in FIG. 8B, a low static elimination parameter is applied to the leading end of the sheet and A high neutralization parameter is set. As a result, as shown in FIG. 8A, the entire sheet P can be uniformly neutralized. That is, it is possible to suppress the uneven charge removal of the sheet. An example of the static elimination parameter Dp is a static elimination current value that is set when the static elimination power supply 90 of FIG. 4 is under constant current control.

In this embodiment, by controlling the static elimination parameter Dp set in the static elimination device in this manner, the static elimination parameter of the static elimination device 9 can be set according to the amount of charge that varies in the sheet conveying direction. sticking due to electrostatic force can be more reliably prevented.

<Second embodiment>
A second embodiment will be described with reference to FIG. In the first embodiment, the charge amount variation table 120, which is predetermined according to the electrical characteristics of the fixing device 7 and the physical properties of the sheet P, is used for static elimination parameter control. On the other hand, in the present embodiment, charge amount fluctuation based on the fixing device 7 is detected to perform static elimination parameter control. Since other configurations and functions are the same as those of the first embodiment described above, the same configurations are denoted by the same reference numerals, and explanations and illustrations are omitted or simplified. Different points will be mainly described.

In this embodiment, unlike the first embodiment, a fixing section current detection section 141 is provided. The fixing unit current detection unit 141 detects a current flowing from the sheet to at least one of the fixing roller 72 and the pressure roller 73 (rotating member) while the sheet is passing through the fixing device 7 . The fixing section current detection section 141 of this embodiment is an ammeter installed between the pressure roller 73 and the ground section.

In this embodiment, the control unit 110 changes the static elimination voltage according to the detection result of the fixing unit current detection unit 41 . That is, the change in the charge amount on the sheet in the conveying direction is changed according to the current detected by the fixing unit current detection unit 141 . Specifically, when the sheet P passes through the fixing device 7, the current fluctuation in the pressure roller 73 and the grounding portion is measured as time-series data by the fixing-portion current detection unit 141, and the time-series data (charge According to the quantity fluctuation measurement result 140), the control unit 110 changes the static elimination parameter set in the static elimination power source 90 when the same sheet P passes through the static elimination device 9 during passage.

The physical properties of the sheet may fluctuate due to manufacturing variations of the sheet, changes in the amount of moisture when the sheet is left in the cassette, and the like. Further, the degree of change in the charge amount changes depending on the environmental temperature and humidity of the fixing device 7 and the like. On the other hand, in the present embodiment, the current flowing through the pressure roller 73 is actually detected while the sheet is passing through the fixing device 7, and static elimination parameter control is performed according to the detection result. Therefore, even if the physical properties of the sheet change as described above, the static elimination parameter can be appropriately controlled.

That is, in the present embodiment, the neutralization parameter can be set by correcting the amount of charge transferred from the sheet to the fixing section without depending on the change factor of the variation in the amount of charge in the sheet conveying direction as described above. The static elimination parameter can be controlled more stably than in the embodiment.

In the present embodiment, the fixing unit current detection unit 141 is used to control the static elimination parameter. A fixing section potential detection section 142 that detects the surface potential of one roller (rotating body) may be used. For example, a fixing section potential detection section (surface potential sensor) 142 may be arranged downstream of the rotation of the pressure roller 73 of the fixing device 7, and the neutralization parameter may be controlled based on this detection result. In this case as well, by controlling the static elimination parameter in the same manner as in the case of using the fixing section current detection unit 141, the static elimination parameter can be controlled more stably than in the first embodiment. Although both the fixing section current detection section 141 and the fixing section potential detection section 142 are shown in FIG. 9, either one of them may be provided.

<Third Embodiment>
A third embodiment will be described with reference to FIGS. 10 to 12(c). In the present embodiment, charge amount variation in the secondary transfer portion N2 is also detected to perform charge elimination parameter control. Other configurations and actions are the same as those of the above-described second embodiment. Different points will be mainly described.

In this embodiment, in addition to the configuration of the second embodiment, a transfer section current detection section 143 is provided. The transfer portion current detection portion 143 detects the current flowing through the secondary transfer portion N2 while the sheet is passing through the secondary transfer portion N2. The transfer section current detection section 143 of this embodiment is an ammeter installed between the secondary transfer roller 56 and the ground section.

In the first and second embodiments, the case where the charge amount variation in the sheet conveying direction mainly occurs in the fixing device 7 is considered. On the other hand, if the resistance of the sheet is not constant in the conveying direction, variations in the charge amount in the conveying direction of the sheet may occur when the sheet passes through the secondary transfer portion N2. In this embodiment, the static elimination parameter can be appropriately set even in such a case by performing the following configuration and control.

In the present embodiment, static elimination parameter control while the sheet is passing through the static elimination section is performed according to the sequence shown in FIG. After the image forming job is started, the image is transferred to the sheet at the secondary transfer portion N2 (S201). The detected transfer current is saved as a data series Tp(t) (S202). Next, the image is fixed on the sheet by the fixing device 7 (S203). Detected by 141, the fixing detection current is stored as a data series Fp(t) (204). Here, as a method for storing the data series of each detected current, for example, the sheet is divided into 10 parts in the conveying direction, and the detected current in each divided part is averaged.

Next, the control unit 110 reads physical property information of the sheet (S205). Based on this information, the control unit 110 determines whether it is necessary to remove static electricity from the sheet (S206). If the sheet does not require static elimination, such as a sheet with a low resistance value (NO in S206), the sheet is discharged without static elimination (S209).

On the other hand, if it is determined in S206 that the static electricity of the sheet needs to be eliminated (YES in S206), the control unit 110 calculates the static elimination parameter (S207). Specifically, from the detection results Tp(t) and Fp(t), the static elimination parameter calculation formula Dp(t)=α×Tp(t)−β×Fp(t)+γ . formula 1
, the neutralization current Dp(t) is obtained as series data. Then, the control unit 110 changes the output of the static elimination power source 90 of the static eliminator 9 based on the series data (S208).

Here, the coefficients α and β in Calculation Formula 1 are correction coefficients for each detected current, and the coefficient γ is a constant correction term for the static elimination parameter. For example, the coefficient α and the coefficient β may be corrected as the amount of charge that is attenuated from the surface of the sheet while the sheet passes through the secondary transfer portion N2, the fixing device 7, and the static elimination device 9. FIG. In addition, when frictional electrification or the like is constantly generated between a member in contact with the sheet before the static eliminator 9 on the conveying path and the sheet, the electrification of the sheet due to this frictional electrification is eliminated. The correction factor γ can be determined as follows.

FIGS. 12(a) to 12(c) show an example of static elimination parameter calculation especially when the coefficients are α=1, β=1, and γ=0. Here, the sheet is divided into 10 parts in the conveying direction, and data obtained by averaging the detected current in each divided part is schematically shown. FIG. 12A shows a data series Tp(t) of the transfer current detected by the transfer section current detector 143. FIG. FIG. 12(b) is a data series Fp(t) of the fixing detection current detected by the fixing section current detection section 141. FIG. FIG. 12(c) is series data Dp(t) of static elimination current based on these detection results.

Note that in the present embodiment as well, the fixing section potential detection unit 142 detects the surface potential of at least one of the fixing roller 72 and the pressure roller 73 (rotating body) from the sheet while the sheet is passing through the fixing device 7 . may be used. For example, a fixing section potential detection section (surface potential sensor) 142 may be arranged downstream of the rotation of the pressure roller 73 of the fixing device 7, and the neutralization parameter may be controlled based on this detection result. Although both the fixing section current detection section 141 and the fixing section potential detection section 142 are shown in FIG. 10, either one of them may be provided.

In the present embodiment as described above, the method of detecting the charge amount fluctuations of both the secondary transfer portion N2 and the fixing device 7 by each current detection portion or potential detection portion and determining the charge elimination parameter accordingly has been described. . However, the measurement results at each current detection unit or potential detection unit are stored in advance as table values for sheet physical properties and transfer/fixing conditions, and charge amount fluctuations at either the secondary transfer unit N2 or the fixing device 7 are detected. It is also possible to control the static elimination parameter by referring to a table for the transfer current Tp(t) or the fixing unit detection current Fp(t) without performing the above.

<Other embodiments>
In each of the above-described embodiments, the current value is used as the static elimination parameter, but the voltage value may be used as the static elimination parameter, and the static elimination power supply 90 of FIG. 4 may be controlled by constant voltage instead of by constant current control. In this case, by treating the static elimination voltage as a static elimination parameter based on the characteristics of the static elimination charge amount to the sheet with respect to the static elimination voltage, it is possible to similarly perform an appropriate static elimination operation.

7: Fixing device (fixing section)
9: static elimination device (static elimination unit)
72: Fixing roller (rotating body)
73 Pressure roller (rotating body)
REFERENCE SIGNS LIST 100: Image forming apparatus 110: Control section 141: Fixing section current detection section 142: Fixing section potential detection section 143: Transfer section current detection section N2: Secondary transfer section (transfer part)

Claims (8)

a transfer unit that transfers the toner image onto the sheet;
a fixing unit that heats and presses the sheet onto which the toner image has been transferred by the transfer unit to fix the toner image onto the sheet;
a static elimination unit that applies a static elimination voltage to the sheet on which the toner image is fixed by the fixing unit to eliminate static electricity from the sheet;
a control unit that controls the static elimination voltage applied to the static elimination unit,
The image forming apparatus, wherein the control section can change the static elimination voltage while one sheet is passing through the static elimination section. The control unit changes the static elimination voltage based on a charge amount variation table in which a relationship between a fixing condition in the fixing unit and a charge amount variation generated on the sheet by the fixing operation of the fixing unit is determined in advance. The image forming apparatus according to claim 1, characterized by: The charge amount fluctuation table is used to determine the relationship between the fixing conditions in the fixing section and the charge amount fluctuations generated on the sheet during the fixing operation by the fixing section, the transfer conditions in the transfer section, and the transfer operation by the transfer section. 3. The image forming apparatus according to claim 2, wherein the image forming apparatus according to claim 2, wherein the image forming apparatus is determined based on a relationship with a charge amount variation that occurs on the sheet. The fixing unit has a pair of rotating bodies for nipping and conveying the sheet,
a fixing unit current detection unit that detects current flowing from the sheet to at least one of the pair of rotating members while the sheet is passing through the fixing unit;
The image forming apparatus according to claim 1, wherein the control section changes the static elimination voltage according to the detection result of the fixing section current detection section. a transfer section current detection section that detects a current flowing through the transfer section while the sheet is passing through the transfer section;
5. The image forming apparatus according to claim 4, wherein the control section changes the neutralization voltage according to the detection result of the fixing section current detection section and the detection result of the transfer section current detection section. . The fixing unit has a pair of rotating bodies for nipping and conveying the sheet,
a fixing section potential detecting section for detecting a surface potential of at least one of the pair of rotating bodies while the sheet passes through the fixing section;
The image forming apparatus according to claim 1, wherein the control section changes the neutralization voltage according to the detection result of the fixing section potential detection section. a transfer section current detection section that detects a current flowing through the transfer section while the sheet is passing through the transfer section;
7. The image forming apparatus according to claim 6, wherein the control section changes the neutralization voltage according to the detection result of the fixing section potential detection section and the detection result of the transfer section current detection section. . a transfer unit that transfers the toner image onto the sheet;
a fixing unit having a pair of rotating bodies for nipping and conveying a sheet, and heating and pressurizing the sheet to which the toner image has been transferred by the transfer unit to fix the toner image on the sheet;
a static elimination unit that applies a static elimination voltage to the sheet on which the toner image is fixed by the fixing unit to eliminate static electricity from the sheet;
a control unit that controls the static elimination voltage applied to the static elimination unit,
The control unit controls a first area from when at least one of the pair of rotating bodies comes into contact with the leading edge of the sheet to when it makes one revolution while the sheet is passing through the fixing section, and When the area in the conveying direction of the sheet is divided into the remaining second area, the static elimination voltage can be changed between the first area and the second area while the sheet is passing through the static elimination section. An image forming apparatus characterized by:

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