JP2016090988A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of executing necessary control for separating a recording material after passing through a transfer part from an image carrier, without excess or deficiency by accurately estimating a curl amount in the direction of winding on a photoreceptive drum when transferring a toner image to the second surface of the recording material, without directly measuring the curl amount.SOLUTION: A CPU 22 determines the curl radius of a curl generated in the direction of winding on a pressure roller 11 in the recording material after passing through a nip part TN on the basis of the detection temperature of a front surface side temperature sensor 19 brought into contact with the recording material after passing through the nip part TN and the detection temperature of a rear surface side temperature sensor 20. The CPU 22 controls a separation high-voltage circuit 25 and a transfer high-voltage circuit 26 on the basis of the determined curl radius, to adjust the discharge performance of a separation discharge needle 6 or the transfer performance of a transfer roller 5.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an image forming apparatus for transferring a toner image from an image carrier to a second surface (back surface) of a recording material whose first surface (front surface) is heated by a heating device.

  2. Description of the Related Art Image forming apparatuses that transfer a toner image from an image carrier to a recording material at a transfer unit and then heat-press the recording material onto which the toner image has been transferred at a nip portion of a fixing device to fix the image on the recording material are widely used. ing. In the image forming apparatus, after the toner image is transferred to the first surface of the recording material and passed through the nip portion of the fixing device, the same recording material is passed to the transfer portion and the toner is also applied to the second surface of the recording material. When transferring an image, so-called double-sided printing may be performed.

  At this time, if a marked curl has occurred in the recording material that has passed through the nip portion of the fixing device, the recording material is transferred to the second surface of the recording material after passing the transfer portion through the same recording material. It may be difficult to wrap around the image carrier from the tip and separate it.

  Therefore, in Patent Document 1, the environmental condition is detected, and the transfer voltage applied to the transfer nip portion is uniformly reduced under a high humidity condition in which the curl is remarkable in the recording material that has passed through the nip portion of the fixing device. Is easily separated from the image carrier.

  Further, in Patent Document 2, the curl amount actually generated in the recording material that has passed through the nip portion of the fixing device is measured, and when the curl amount exceeding the threshold value is generated, the transfer applied to the recording material in the transfer unit. The voltage is lowered to facilitate separation of the recording material from the image carrier.

JP-A-5-333654 JP 11-59967 A

  Since Patent Document 1 relies only on whether or not the high humidity condition is met, the amount of curl generated on the recording material cannot be accurately estimated. Therefore, when the high humidity condition is met, it is necessary to greatly reduce the transfer voltage more than necessary. If the transfer voltage is greatly reduced, the transfer efficiency of the toner image is reduced, and the influence on the image quality cannot be ignored.

  In Patent Document 2, if an attempt is made to accurately measure the curl amount, a large space for holding the curled recording material in a free state is required. Therefore, there is a problem that the image forming apparatus is increased in size.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus that can accurately determine a curl amount in a direction in which a recording material onto which a toner image is transferred onto a second surface is wound around a photosensitive drum.

  Another object of the present invention is to provide an image forming apparatus capable of executing necessary control for separating a recording material that has passed through a transfer portion from an image carrier in accordance with an estimated curl amount.

  The image forming apparatus of the present invention includes a heating device that heats the recording material, a first temperature detection unit that detects the temperature of the first surface of the recording material that has passed through the heating device, and the recording material that has passed through the heating device. A second temperature detection unit that detects the temperature of the second surface of the recording medium, and a control unit that determines the curl state of the recording material based on the detected temperatures of the first temperature detection unit and the second temperature detection unit. Is.

  In the image forming apparatus of the present invention, the curl state of the recording material is determined based on the temperature of the first surface and the second surface of the recording material that has passed through the heating device. The curl amount of the recording material can be determined more accurately than relying only on whether or not. In addition, a space for measuring the curl amount of the recording material is unnecessary.

  Therefore, it is possible to execute necessary control for separating the recording material that has passed through the transfer nip portion from the image carrier in accordance with the curl amount accurately determined without directly measuring the curl amount.

2 is an explanatory diagram of a configuration of an image forming apparatus according to Embodiment 1. FIG. FIG. 3 is an explanatory diagram of a configuration of a fixing device. It is explanatory drawing of arrangement | positioning of a surface side temperature sensor and a back surface side temperature sensor. It is a block diagram of the control system of a transfer voltage and a static elimination voltage. It is explanatory drawing of the thermal contraction rate data of a recording material. It is explanatory drawing of the parameter of the curled recording material. 3 is a flowchart of control according to the first embodiment. It is explanatory drawing of the detection temperature of a surface side temperature sensor and a back surface temperature sensor. It is explanatory drawing of arrangement | positioning of the surface side temperature sensor in Embodiment 2, and a back surface side temperature sensor. FIG. 6 is an explanatory diagram of a time transition of a recording material surface temperature and a recording material back surface temperature in Embodiment 2. It is explanatory drawing of another structure of an image forming apparatus.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

<Embodiment 1>
(Image forming device)
FIG. 1 is an explanatory diagram of a configuration of the image forming apparatus according to the first embodiment. As shown in FIG. 1, the image forming apparatus 100 is a monochrome printer, a copying machine, or a laser printer that transfers a toner image formed on a photosensitive drum 1 onto a recording material.

  The photosensitive drum 1 has a photosensitive layer such as OPC or a-Si formed on the peripheral surface of an aluminum cylinder, and is driven by a motor (not shown) to rotate in the direction of arrow R1. A charging roller 2, an exposure device 3, a developing device 4, and a transfer roller 5 are disposed around the photosensitive drum 1.

  The charging roller 2 is a conductive elastic roller having a roller shaft (conductive support, cored bar). The charging roller 2 has a roller axis line arranged in parallel to the drum axis line of the photosensitive drum 1, both ends thereof are rotatably supported via bearing members, and both ends thereof are pressed against the photosensitive drum 1 by a predetermined pressing force. It touches. The charging roller 2 rotates following the photosensitive drum 1. The charging roller 2 is applied with a charging voltage obtained by superimposing an AC voltage on a DC voltage, and charges the peripheral surface of the photosensitive drum 1 to the dark portion potential VD. The peripheral surface of the photosensitive drum 1 is charged by the charging roller 2 to a dark portion potential VD = −500 to −800V.

  The exposure device 3 scans the peripheral surface of the photosensitive drum 1 with a rotating mirror with a laser beam that is ON-OFF modulated with an image signal obtained by developing image data on a scanning line, and forms an electrostatic image of the image. The exposure apparatus 3 performs image exposure based on image information using a laser scanner of a semiconductor laser (wavelength 780 nm). The exposure removes the charge in the exposed area, and the dark area potential VD is lowered to the bright area potential VL to form an electrostatic image.

  The developing device 4 carries a one-component developer of magnetic toner on a developing sleeve 4s that rotates around a non-rotating magnet roller 4m. The developing sleeve 4s is processed to a predetermined surface roughness by blasting or coating carbon on the surface of the aluminum roller. The developing device 4 is a one-component reversal jumping development method using a one-component magnetic negative toner. By applying a developing voltage in which an AC voltage is superimposed on a DC voltage to the developing sleeve 4s, the toner is transferred from the developing sleeve 4s to the photosensitive drum 1, and the electrostatic image is developed into a toner image. By applying a developing bias (AC + DC charging method) in which a predetermined DC voltage and a predetermined AC voltage are superimposed from the high voltage power supply unit to the developing sleeve 4 s and transferring the toner to the electrostatic image on the photosensitive drum 1, the toner image Is developed (visualized).

  The toner image developed on the photosensitive drum 1 moves to the transfer portion T2 as the photosensitive drum 1 rotates. On the other hand, the recording material 16 stored in the recording material cassette 17 is taken out by the feeding roller 15, separated one by one by the separation roller 8, and waits at the registration roller 10.

  The registration roller 10 feeds the recording material to the transfer unit T2 in synchronization with the toner image on the photosensitive drum 1. The recording material on which the toner image is transferred through the transfer portion T2 is conveyed to the fixing device 14 along a conveyance guide (not shown) with the first unfixed toner image carried on the surface. The recording material 16 carrying the toner image is heated and pressed by the fixing device 14 to fix the image on the surface, and then discharged to the upper tray 101 of the image forming apparatus 100 by the discharge roller 13.

(Duplex printing mode)
In the double-sided printing mode in which images are formed on both sides of the recording material, the recording material on which the image is fixed on the first surface by the fixing device 14 is nipped at the rear end by the discharge roller 13 and held above the upper surface tray 101. The transport is stopped in the state. The recording material is fed to the reverse conveyance path 102 by rotating the discharge roller 13 in the reverse direction, and conveyed by the conveyance rollers 103 and 104. The recording roller is reversed again with the rear end at the front end turned to the photosensitive drum 1 side. Wait at 10.

  A toner image of the image on the second surface is formed on the photosensitive drum 1, and the recording material is fed to the transfer portion T <b> 2 by the registration roller 10, and the toner image is also transferred to the second surface. The recording material that has passed through the transfer portion T2 is conveyed to the fixing device 14 and the image on the second surface is also fixed, and then discharged to the upper surface tray 101 of the image forming apparatus 100.

(Drum cleaning device)
The drum cleaning device 7 rubs the photosensitive drum 1 with a cleaning blade to scrape off and collect the transfer residual toner on the surface of the photosensitive drum 1. Toner remaining on the surface of the photosensitive drum 1 without being transferred to the recording material 16 is removed by a cleaning blade. The toner collected by the drum cleaning device 7 is transported to a recovery toner box disposed in the housing of the image forming apparatus 100 by a recovery toner transport screw (not shown).

  The cleaning blade is a casting type. The casting type is a type of cleaning blade that is molded into a desired shape by pouring a rubber material or the like into the mold. Common materials for the cleaning blade are, for example, polyurethane, styrene-butadiene copolymer, chloroprene, butadiene rubber. It is a material having moderate elasticity and hardness, such as an elastomer such as ethylene-propylene-diene rubber, chlorosulfonated polyethylene rubber, fluorine rubber, silicon rubber, acrylic rubber, nitrile rubber, chloroprene rubber.

  In particular, polyurethane that does not damage the photosensitive drum 1 due to friction and has high wear resistance is preferable. Furthermore, considering that plastic deformation strain is small, a two-component thermosetting polyurethane material may be used. As the curing agent, general urethane curing agents such as 1,4-butanediol, 1,6-hexanediol, hydroquinone diethylol ether, bisphenol A, trimethylolpropane, and trimethylolethane can be used. The thickness of the strip-shaped rubber of the cleaning blade is usually 1.0 mm to 4 mm, preferably 1.5 mm to 2.0 mm.

(Fixing device)
FIG. 2 is an explanatory diagram of the configuration of the fixing device. As shown in FIG. 2, the fixing device 14 forms a nip portion TN by pressing a fixing belt 12 of a cylindrical rotating body containing a heater 18 as a heating member and a pressure roller 11 of an elastic rotating body. . The heater 18 is supported by a non-rotating beam member 18 </ b> H provided through the fixing belt 12, and heats the toner carried on the recording material 16 via the fixing belt 12.

  The heater 18 is a ceramic heater composed of a resistor and alumina, and a temperature sensor for temperature adjustment is arranged at the center of the opposite surface that contacts the fixing belt 12. The fixing belt 12 is a polyimide film having a thickness of 40 to 100 μm and having a surface layer coated with PFA or PTFE. The pressure roller 11 has a surface of a sponge rubber layer obtained by foaming EPDM rubber, silicon rubber or fluorine rubber as a base layer on a rotating metal shaft body, and has heat resistance and releasability such as silicon rubber, fluorine rubber or fluorine resin. It is covered with a surface layer of resin.

  When receiving the print signal, the fixing device 14 starts supplying power to the heater 18. A temperature control circuit (not shown) controls power supply so that the temperature sensor for temperature adjustment (18s: FIG. 4) maintains a predetermined target temperature. The nip portion TN is heated to a predetermined temperature range in a state where the input power of the heater 18 is ON-OFF controlled so that the back surface of the heater 18 has a predetermined set temperature.

(Transfer roller)
As shown in FIG. 1, the transfer roller 5 abuts on the photosensitive drum 1 to form a toner image transfer portion T2 for the recording material. By applying a voltage having a polarity opposite to the charging polarity of the toner to the transfer roller 5, the toner image on the photosensitive drum 1 is transferred to the recording material 16 that passes through the transfer portion T2. The recording material 16 to which the toner image has been transferred is neutralized by the separation static elimination needle 6 and separated from the photosensitive drum 1 by the action of the curvature of the photosensitive drum 1.

  The toner image carried on the photosensitive drum 1 is applied by applying a predetermined DC voltage having a polarity opposite to the charging polarity of the toner from the transfer high voltage circuit 26 to the transfer roller 5 at the timing when the recording material 16 is conveyed to the transfer portion T2. Is transferred to the recording material 16. Here, since the charging polarity of the toner is negative, a positive DC voltage (1 to 5 kV) corresponding to the opposite polarity is applied to the transfer roller 5.

  The transfer roller 5 is a sponge roller having a single layer structure in which a conductive foam layer is disposed on the outer periphery of a metal shaft. The metal shaft body is not particularly limited, and a metal core made of a metal solid body, a metal cylinder body hollowed out inside, or the like is used. The metal material is not particularly limited, such as iron or aluminum. The conductive foam layer is an ion conductive material provided with a sponge rubber obtained by foaming a mixed rubber of NBR rubber (acrylonitrile butadiene rubber) and hydrin rubber.

  The conductive foam layer preferably has a hardness range in which the Asker C hardness measured with a sponge hardness meter is 20 to 35 [° / 500 g load]. The electric resistance value of the conductive foam layer is preferably 104 to 108Ω, particularly preferably 105 to 107Ω. The conductive foam layer can employ ethylene-propylene-diene rubber (EPDM), acryl-nitrile-butadiene rubber (NBR), or the like as a matrix component of the ionic conductive agent. Hydrin rubber can be mixed with the ionic conductive agent, and it contains a resistance modifier (electronic conductive conductive agent), foaming agent, foaming aid, softener, plasticizer, filler, vulcanizing agent, and vulcanization accelerator. Is possible.

  The electron conductive conductive agent can be mixed with metal oxides such as carbon black, graphite, a solid solution of zinc oxide and aluminum oxide, a solid solution of tin oxide and antimony oxide, and a solid solution of indium oxide and tin oxide. The electronic conductive agent is used alone or in combination of two or more. As the foaming agent, dinitropentamethylenetetramine (DPT), azodicarbonamide (ADCA), 4,4-oxybisbenzenesulfonyl hydragit (OBSH) or the like is used alone or in combination.

  As described above, the image forming apparatus 100 reversely conveys the first surface and the second surface of the recording material heated by the fixing device 14 which is an example of the heating device, and conveys the recording material to the transfer roller 5. A path 102 is provided. The recording material having the toner image transferred from the photosensitive drum 1 to the first surface by the transfer roller 5 and heated by the fixing device 14 is conveyed again to the transfer roller 5 through the reverse conveyance path and is transferred from the photosensitive drum 1 to the second surface. Transfer the toner image.

  A fixing belt (fixing film) 12, which is an example of a heating rotator, heats the surface of the recording material on which the toner image is transferred. A pressure roller 11, which is an example of a pressure rotator, forms a nip portion TN between the fixing belt 12 and presses the recording material. A photosensitive drum 1, which is an example of an image carrier, carries a toner image for transfer onto a recording material and rotates.

  The transfer roller 5 and the transfer high-voltage circuit 26, which are examples of the transfer device, charge the recording material in contact with the photosensitive drum 1 to a polarity opposite to the charging polarity of the toner image, and transfer the toner image on the photosensitive drum 1 to the recording material. . The transfer roller 5 rotates in contact with the photosensitive drum 1. A transfer high voltage circuit 26, which is an example of a transfer power supply, applies a voltage having a polarity opposite to the charging polarity of the toner image to the transfer roller 5.

  A separation static elimination needle 6 and a separation high-voltage circuit 25 which are examples of a charge removal device remove charges from the recording material that has passed through the transfer roller 5, and facilitate separation of the recording material from the photosensitive drum 1. A separation / neutralizing needle 6, which is an example of a corona discharge electrode, is disposed downstream of the transfer roller 5 in the recording material conveyance direction. The separation high voltage circuit 25, which is an example of a static elimination power source, removes electric charges from the recording material by applying a voltage to the separation static elimination needle 6.

(Recording material curl)
When the recording material on which the toner image is transferred is heated and pressed by the fixing device 14 to fix the image on the recording material, the recording material heated and pressed by the fixing device 14 may be curled. When a large curl is generated when an image is fixed on the first surface of the recording material, the photosensitive drum 1 is transferred from the front end of the recording material by the curl generated when the toner image is transferred to the second surface of the recording material and then separated. There is a possibility that the recording material jams. Therefore, when curling occurs in the recording material, it is desirable to lower the transfer voltage or increase the static elimination voltage applied to the static elimination needle.

  However, if the absolute value of the transfer voltage is uniformly reduced, the transfer efficiency of the toner image may be reduced, and the image quality may be affected. If the absolute value of the static elimination voltage applied to the static elimination needle is uniformly increased, unnecessary discharge may occur and the unfixed image on the recording material may be disturbed. For this reason, it is desirable to accurately control the curling state generated on the recording material and to control the necessary and sufficient transfer voltage and static elimination voltage. However, even if the recording material is the same and the environmental humidity is the same, the curl generated on the recording material varies.

  Therefore, in the first embodiment, the surface temperature is measured on each of the first surface of the recording material on the heater 18 side of the fixing device 14 and the second surface of the recording material on the pressure roller 11 side. The curl state generated in the recording material is estimated based on the surface temperature of the first surface and the second surface temperature. Only when the estimated curl state is significant, control is performed to shift the transfer voltage and the static elimination voltage from normal values.

(Control system for transfer voltage and static elimination voltage)
FIG. 3 is an explanatory view of the arrangement of the front surface side temperature sensor and the back surface side temperature sensor. FIG. 4 is a block diagram of a control system for the transfer voltage and the static elimination voltage.

  As shown in FIG. 3, the front surface temperature sensor 19 is disposed on the heater 18 side downstream of the nip portion TN of the fixing device 14 and detects the temperature of the first surface of the recording material. The back surface temperature sensor 20 is disposed on the pressure roller 11 side downstream of the nip portion TN of the fixing device 14 and detects the temperature of the second surface of the recording material.

  The measuring device 23 measures the front side temperature of the recording material with the front surface side temperature sensor 19 and measures the back side temperature of the recording material with the back surface temperature sensor 20. The measuring instrument 23 measures the results detected by the front surface temperature sensor 19 and the back surface temperature sensor 20.

  In the memory 21, the measurement results measured by the measuring instrument 23 are recorded in time series. The memory 21 preliminarily stores thermal shrinkage rate data of the recording material, a curl radius threshold value at which a jam of the recording material occurs, and a calculation formula for the curl radius R.

  The CPU 22 calculates a curl radius and determines whether the calculated curl radius is equal to or less than a threshold value. The CPU 22 calculates a curl radius from the front side temperature and the back side temperature of the recording material recorded in the memory 21 and the stored thermal contraction rate data.

  The CPU 22 determines whether or not the calculated curl radius is equal to or smaller than a threshold value. Only when the calculated curl radius is equal to or smaller than the threshold value, a signal is sent from the CPU 22 to the high voltage controller 24 to reduce the output of the transfer high voltage circuit 26 or the output of the separation high voltage circuit 25. Do either or both.

  The high voltage controller 24 changes the outputs of the transfer high voltage circuit 26 and the separation high voltage circuit 25 according to the determination result of the CPU 22. A voltage is applied to the transfer roller 5 from the transfer high-voltage circuit 26. A voltage is applied to the separation static elimination needle 6 from the separation high voltage circuit 25.

  FIG. 5 is an explanatory diagram of the heat shrinkage rate data of the recording material. FIG. 6 is an explanatory diagram of parameters of the curled recording material. As shown in FIG. 4, the calculation formula for calculating the curl radius R is stored in the memory 21 and is read out and used by the CPU 22. The thermal contraction rate data of the recording material for calculating the curl radius R is stored in the memory 21 and is read out and used by the CPU 22. The formula for calculating the curl radius is introduced as follows.

  As shown in FIG. 5, the recording material 16 curls toward the pressure roller when fixing the image on the first surface in the nip portion TN. As a result, the recording material is wound around the photosensitive drum during the transfer of the toner image on the second surface. Assume that separation becomes difficult.

At this time, the linear expansion coefficient (thermal contraction coefficient) of the recording material is α, the length of the recording material in the transport direction before passing through the nip portion TN is L, and the temperature change amount of the recording material (from room temperature to the detected temperature) When the temperature difference is ΔT, the thermal shrinkage amount ΔL of the recording material is expressed by the following equation.
ΔL = L · α · ΔT
α · ΔT = ΔL / L

  As shown in FIG. 6, the relationship between the moisture content of the recording material and the thermal contraction rate data based on the temperature change amount (temperature difference from room temperature to the detected temperature) is obtained by experiments and stored in a table in the memory 21. . The thermal shrinkage coefficient α is obtained as the slope of the broken line by substituting numerical values from the recording material heat shrinkage rate data by the recording material moisture content and the temperature change amount ΔT of the recording material.

  As shown in FIG. 5, as a result of the recording material 16 having a thickness t and a conveyance direction length L passing through the nip TN of the fixing device and exposed to a temperature change of ΔT, the conveyance direction lengths of both surfaces are Lp. , Lf, and the entire recording material 16 is curled into an arc having a radius R and an angle θ.

  The thermal contraction coefficient on the front side of the recording material is α1, and the thermal contraction coefficient on the back side is α2. The recording material temperature before passing through the nip TN of the fixing device 14 is T0, and the recording material peak temperatures after passing through the nip TN of the fixing device 14 are T1 and T2. The thickness of the recording material is t, the length in the conveyance direction of the recording material on the pressure roller 11 side after passing through the nip portion TN is Lp, and the length of the recording material in the conveyance direction on the heater 18 side is Lf. The angle formed by the arc drawn by the difference between the length Lp and the length Lf is defined as θ.

As shown in FIG. 5, the post-shrinkage length Lf on the heater side is expressed by the following equation (1).
Lf = L + ΔL = L + L · α1 · ΔT = L (1 + α1 (T2−T0)) = (R + t) θ (1)

Further, the length Lp after contraction of the recording material on the pressure roller side is expressed by the following equation (2).
Lp = L + ΔL = L + L · α2 · ΔT = L (1 + α2 (T1−T0)) = Rθ (2)

From the above equations (1) and (2), the angle θ can be calculated by the following equation (3).
θ = L / t [α1 (T2−T0) −α2 (T1−T0)] (3)

By substituting the above equation (2) into the above equation (3), the curl radius R can be calculated by the following equation (4).
R = t (1 + α2 (T1−T0)) / [α1 (T2−T0) −α2 (T1−T0)] (4)

  Here, since the surface side of the recording material is on the side of the heater 18 serving as a heat source, the moisture content of the recording material becomes small. Therefore, 5.5% data with a small amount of moisture is substituted into the thermal contraction coefficient α1 on the recording material surface side of the above equation from the thermal contraction rate data of FIG. Further, since the back side of the recording material is not the heat source side, the moisture content of the recording material increases. Therefore, 9.5% data with a large amount of moisture is substituted into the thermal contraction coefficient α2 on the back side of the recording material in the above equation from the thermal contraction rate data of FIG. Thus, the curl radius R can be calculated.

(Control of Embodiment 1)
FIG. 7 is a flowchart of control according to the first embodiment. As shown in FIG. 7 with reference to FIG. 2, when the user presses the copy start button, image formation is started and the recording material 16 passes through the nip portion TN of the fixing device 14.

  When the recording material heated by the heater 18 reaches the front surface temperature sensor 19 and the back surface temperature sensor 20 and detects a change in the temperature of the front surface and the back surface of the recording material, the CPU 22 causes the measuring device 23 to detect the front surface temperature of the recording material. And the temperature measurement of the back side temperature is started (S1).

  The CPU 22 refers to the recording material heat shrinkage rate data based on the recording material moisture content and the recording material temperature change shown in FIG. 6 stored in advance in the memory 21 (S2).

  The CPU 22 calculates the curl radius R from the measured surface temperature and back surface temperature of the recording material and the recording material thermal shrinkage rate data stored in the memory 21 by the above equation (4) (S3).

  The CPU 22 refers to the threshold value of the curl radius R stored in the memory 21 (S4), and determines whether the calculated curl radius R is less than the threshold value (S5).

  If the CPU 22 determines that the curl is significant below the threshold (YES in S5), it sends a signal to the high voltage controller 24 to cause the transfer high voltage circuit 26 to decrease the output or the separation high voltage circuit 25 to increase the output (S6). .

  When the CPU 22 determines that the curl is slight beyond the threshold (NO in S5), the output of the transfer high voltage circuit 26 or the separation high voltage circuit 25 is not changed (S7).

  The CPU 22 stops the image forming apparatus 100 when the image forming operation ends.

  As described above, the surface-side temperature sensor 19 which is an example of the first temperature detecting unit detects the temperature of the surface of the recording material that has passed through the nip portion TN and is in contact with the fixing belt 12. A back surface temperature sensor 20, which is an example of a second temperature detector, detects the temperature of the surface of the heated recording material that has passed through the nip portion TN and that is in contact with the pressure roller 11.

  The CPU 22, which is an example of a control unit, controls the separation high voltage circuit 25 based on the outputs of the front surface side temperature sensor 19 and the back surface temperature sensor 20. The CPU 22 determines the curl state of the recording material output from the fixing device 14 which is an example of the heating device, and adjusts the charge removal function based on the determination result. As the determined curl radius is smaller, jamming is more likely to occur, and the static elimination performance is improved. That is, when the curl radius of the curl generated in the direction of winding around the pressure roller 11 in the process of passing through the nip portion TN is the first radius, the curl radius is greater than the second radius larger than the first radius. Also, the charge eliminating function of the recording material in the separation charge eliminating needle 6 is enhanced. When enhancing the charge removal performance of the recording material in the separation charge eliminating needle 6, the absolute value of the voltage applied to the separation charge eliminating needle 6 is increased.

  At the same time, the CPU 22 adjusts the transfer function by controlling the transfer high-voltage circuit 26 based on the outputs of the front surface temperature sensor 19 and the back surface temperature sensor 20. The CPU 22 reduces the transfer function because the smaller the determined curl radius, the easier the jam occurs. That is, when the curl radius of the curl generated in the direction of winding around the pressure roller 11 in the process of passing through the nip portion TN is the first radius, the curl radius is greater than the second radius larger than the first radius. Also, the transfer function of the transfer roller 5 is lowered. That is, the absolute value of the voltage applied to the transfer roller 5 is reduced, and the charging performance of the recording material in the transfer roller 5 is lowered.

(Temperature sensor placement)
FIG. 8 is an explanatory diagram of detection temperatures of the front surface temperature sensor and the back surface temperature sensor. As shown in FIG. 3, the pressure roller 11 side surface of the recording material 16 that has passed through the nip portion TN comes into contact with the two flags of the back surface side temperature sensor 20 to detect the temperature. The back surface temperature sensor 20 is a thermocouple type temperature sensor that measures the difference in electromotive force between the two flags according to the temperature by joining the tips of the two metal flags in contact with the surface of the recording material. The surface side temperature sensor 19 operates similarly.

As shown in FIG. 3A, in the first embodiment, when the recording material reaches the temperature measurement position, the conveyance direction distance A from the nip portion TN to the front surface side temperature sensor 19 and the back surface from the nip portion TN. The conveyance direction distance B to the side temperature sensor 20 is equal.
A = B

As shown in FIG. 3B, in the first embodiment, the distance a from the center line of the recording material 16 passing through the center of the nip portion TN to the front surface temperature sensor 19 and the back surface temperature sensor 20 from the center line. Is equal to the distance b.
a = b

In the first embodiment, the front surface temperature sensor 19 and the rear surface temperature sensor 20 are installed on the center line in the recording material conveyance width direction. Flags of the temperature sensor are arranged so that the front and back are at the same distance from the center line of the recording material along the recording material conveyance direction.
a = b = 0

  As shown in FIG. 8, after the recording material passes through the nip portion TN, the front surface temperature sensor 19 and the back surface temperature sensor 20 detect the temperature of the recording material. FIG. 8 is a diagram of the time transition of the recording material surface temperature and the recording material back surface temperature in the first embodiment. The horizontal axis represents time transition and the vertical axis represents temperature.

  When the recording material discharged from the fixing device 14 reaches the front surface temperature sensor 19 and the back surface temperature sensor 20, the detected temperature rises, the peak value of the front surface temperature is 115.3 ° C., and the peak value of the back surface temperature. Is 96.3 ° C. The temperature difference between the front surface temperature and the back surface temperature at that time is Δ = 19.0 ° C. Thereafter, as time elapses, the temperature difference between the front surface and the back surface of the recording material becomes Δ = 0 ° C.

  That is, as shown in FIG. 3A, the conveyance direction distance A from the nip portion TN to the recording material surface temperature measurement position and the conveyance direction distance B to the recording material back surface temperature measurement position are not the same. The temperature difference between the recording material printing surface and the non-printing surface over time cannot be measured accurately.

  Further, at the recording material width direction end, heat dissipation and moisture evaporation are different from the recording material central portion. For this reason, it is better to install the flag on the center line (a = b = 0) in the width direction of the recording material or to set the flag so that the position in the width direction from the center line is the same distance (a = b). The temperature difference on the back side of the recording material can be measured more accurately.

(Comparative Example 1)
In Comparative Example 1, as shown in Patent Document 1, a countermeasure is taken to reduce drum winding jam that occurs at the time of transferring the toner image on the second surface due to the curling of the recording material that occurs at the time of fixing the image on the first surface. Run. A temperature / humidity sensor is arranged in the housing of the image forming apparatus 100 to detect temperature and humidity. When the temperature / humidity sensor matches a predetermined high temperature / humidity environment, the absolute value of the transfer voltage at the time of transferring the toner image on the second surface is calculated. make low. The ambient temperature and humidity of the recording material cassette 17 are detected, and the absolute value of the transfer voltage at the time of transferring the toner image on the second surface is reduced as the amount of moisture in the air increases.

  In Comparative Example 1, even if the curl radius generated when fixing the image on the first surface is large or small, the temperature and humidity are detected by the temperature / humidity sensor and the secondary transfer roller is determined to be a high-temperature / high-humidity environment. The transfer voltage applied to 5 is lowered. When it is determined that the environment is high temperature and high humidity, the transfer voltage applied to the secondary transfer roller 5 is lowered even when the curl radius generated when fixing the image on the first surface is large and the drum winding jam does not occur when transferring the toner image. .

  In Comparative Example 2, when temperature and humidity are detected by a temperature / humidity sensor and a high temperature / humidity environment is determined, a target temperature for temperature adjustment of the fixing device 14 is lowered to generate a recording that occurs at the time of image fixing on the first surface. The curl of the material is reduced.

  That is, in Comparative Example 1, curl is predicted only by detecting high temperature and high humidity. In Comparative Example 2, the fixing temperature is lowered instead of lowering the transfer voltage when detecting high temperature and high humidity.

  As shown in Table 1, in Comparative Example 1, since the transfer voltage applied to the transfer roller 5 is lowered, the toner image on the second surface is generated regardless of whether the curl radius generated during image fixing on the first surface is large or small. Jam around the photosensitive drum 1 during transfer can be prevented.

  However, in Comparative Example 1, the prediction accuracy was low because of curl prediction only by high temperature and high humidity detection. This is because the curl radius changes depending on the temperature of the pressure roller 11 of the fixing device 14. Therefore, even when the curl radius is large and it is not necessary to consider the winding jam, the transfer voltage is lowered more than necessary, resulting in a transfer failure. In Comparative Example 1, curl is predicted only by high-temperature and high-humidity detection. Therefore, in a high-temperature and high-humidity environment, the transfer voltage is always lowered, and the problem of transfer failure is likely to occur.

  That is, when curling is predicted only by high-temperature and high-humidity detection, the transfer voltage is always lowered in a high-temperature and high-humidity environment.

  Similarly, since the comparative example 2 also determines curl only by detecting high temperature and high humidity, the prediction accuracy is low. Since the transfer voltage was not lowered, no transfer failure occurred, but a fixing failure occurred as a result of lowering the fixing temperature more than necessary.

  When the temperature of the pressure roller 11 is high, the curl radius is originally large, and the fixing temperature is lowered even if the curl radius does not need to be further increased.

  On the other hand, in the first embodiment, as shown in FIG. 8, the curl radius generated at the time of image fixing on the first surface can be accurately determined from the detected temperatures of the front surface temperature sensor 19 and the back surface temperature sensor 20. For this reason, when the curl radius is large, such as when the temperature of the pressure roller 11 is high, and it is difficult to jam at the time of transferring the toner image on the second surface, the transfer voltage is not lowered even in a high temperature and high humidity environment. No defect occurs. Even in a high-temperature and high-humidity environment, if the curl radius is large, the fixing temperature is not lowered, so that fixing failure does not occur.

(Comparative Example 3)
In Comparative Example 3, as shown in Patent Document 2, in order to cope with the curl change at the pressure roller temperature of the fixing device, a sensor for detecting the curl height and shape is provided in the recording material conveyance path to measure the curl. It is carried out.

  However, in Comparative Example 3, a space for allowing the recording material to curl freely is required in the recording material conveyance path. In the case of the image forming apparatus 100, since there is no such space and the recording material is restrained in the thickness direction by the conveyance guide member anywhere in the conveyance path, the accurate curl height of the recording material at the time of image fixing on the first surface is reduced. Cannot be detected. This is because a conveyance guide member is installed in the conveyance path of the recording material so that even a curled recording material can be conveyed. Furthermore, the accurate curl radius of the recording material cannot be detected even when the recording material is constrained by a plurality of conveying rollers in the conveying path.

(Effect of Embodiment 1)
In the first embodiment, the CPU 22 substitutes the detected temperature of the front surface temperature sensor 19 and the detected temperature of the back surface temperature sensor 20 to determine the direction of curl and the curl radius generated in the recording material that has passed through the nip portion TN. It has an estimation formula to be estimated. Therefore, the curl direction and the curl radius can be estimated only by the detected temperatures of the front surface temperature sensor 19 and the back surface temperature sensor 20. It is possible to estimate an average curl radius that is not affected by variations among recording materials. There is no need to actually measure the curl radius, and no space or sensor is required to measure the curl radius.

  In the first embodiment, when the curl radius calculated by the estimation formula is larger than a predetermined threshold, the CPU 22 keeps the absolute value of the voltage applied to the separation static elimination needle 6 constant regardless of the calculated curl radius. Keep on. For this reason, it is not necessary to unnecessarily increase the voltage applied to the separation static elimination needle 6 in a state where there is no separation problem.

  In the first embodiment, when the curl radius calculated by the estimation formula is larger than a predetermined threshold, the CPU 22 keeps the absolute value of the voltage applied to the transfer roller 5 constant regardless of the calculated curl radius. keep. For this reason, it is not necessary to unnecessarily reduce the voltage applied to the transfer roller 5 in a state where there is no problem in separation.

  In the first embodiment, the distance from the nip TN to the front surface temperature sensor 19 along the recording material conveyance path is equal to the distance from the nip TN to the back surface temperature sensor 20. Therefore, it is possible to estimate the curl radius that accurately reflects the temperature history of both surfaces of the recording material in the nip portion TN.

  In the first embodiment, the distance to the front surface temperature sensor 19 and the back surface temperature sensor 20 with respect to the center line along the transport direction of the recording material passing through the center of the nip portion TN in the width direction perpendicular to the transport direction of the recording material. Is equal to the distance. For this reason, it is possible to estimate the curl radius that accurately reflects the temperature history of both sides of the recording material, avoiding the influence of the temperature distribution in the direction along the nip portion TN.

  In Embodiment 1, the temperature of the first surface and the second surface of the recording material after fixing is measured, the measured temperature difference between the first surface and the second surface, and the heat shrinkage rate data stored in advance. By calculating the curl radius from the curve, it is possible to accurately grasp the state of occurrence of the curl. For this reason, even when the pressure roller 11 of the fixing device 14 changes in temperature and the curl radius changes, it is possible to implement the optimum countermeasure against jamming around the photosensitive drum during the second surface transfer.

  In the first embodiment, since the curl radius can be accurately grasped by performing the above-described control in the above-described configuration, when the curl radius is large and does not cause a jam under a condition where the temperature of the pressure roller 11 is high. Does not lower the transfer voltage even in a high temperature and high humidity environment. For this reason, it is possible to eliminate the adverse effect of transfer failure caused by lowering the transfer voltage. Since the curl radius can be accurately grasped, it is possible to take an optimum countermeasure against the jamming around the photosensitive drum when transferring the toner image on the second surface.

<Embodiment 2>
FIG. 9 is an explanatory diagram of the arrangement of the front surface side temperature sensor and the back surface temperature sensor in the second embodiment. FIG. 10 is an explanatory diagram of the time transition of the recording material surface temperature and the recording material back surface temperature in the second embodiment.

  As shown in FIG. 9, in the second embodiment, the conveyance direction distance A from the nip portion TN to the recording material surface temperature measurement position is different from the conveyance direction distance B from the fixing nip to the recording material back surface temperature measurement position. Thus, the front surface side temperature sensor 19 and the back surface temperature sensor 20 are provided. Since the other configuration and control are the same as those in the first embodiment, in FIG. 9, the same reference numerals as those in FIG.

  As shown in FIG. 9, in the second embodiment, the conveyance direction distance B is the same as that in the first embodiment, but the conveyance direction distance A is larger than that in the first embodiment. For this reason, as shown in FIG. 10, the detection timing of the back surface temperature of the recording material is the same as in the first embodiment, but the detection timing of the front surface temperature of the recording material is less than the detection timing of the back surface temperature. 4 seconds later. FIG. 10 shows the time transition of the surface side temperature and the back side temperature in both Embodiments 1 and 2 in comparison. The horizontal axis represents time transition, and the vertical axis represents temperature.

  In the second embodiment, the conveyance direction distance A from the nip portion TN to the surface temperature measurement position is different from the conveyance direction distance B from the nip portion TN to the back surface temperature measurement position. For this reason, a difference occurs between the time until the front surface temperature reaches the peak value and the time until the back surface temperature reaches the peak value.

  Furthermore, since the front surface temperature measurement position is farther from the nip portion TN than the rear surface temperature measurement position, the peak of the front surface temperature is lower by 5 ° C. than in the first embodiment due to heat dissipation during that time. Therefore, the temperature difference between the front surface temperature and the back surface temperature of the recording material cannot be accurately measured, and there is a possibility that the difference between the calculated curl radius and the actual curl radius becomes large.

  Therefore, in the second embodiment, as shown in FIG. 9, the temperature drop of the peak value due to heat radiation when the front surface temperature measurement position is farther from the nip portion TN than the back surface temperature measurement position is corrected.

  For example, when the recording material surface temperature peak value is 115.3 ° C. and is 50 mm downstream in the conveyance direction, the surface of the recording material is 0.22 sec at a conveyance speed of 230 mm / sec. The side temperature peak value decreases by 5 ° C. to 110.3 ° C.

  As illustrated in FIG. 4, the CPU 22 stores a temperature difference 5 ° C. due to a difference in distance from the nip portion TN in the memory 21 in advance. Then, when the curl radius is calculated by the CPU 22, the accurate curl radius can be calculated by the above estimation formula by adding the temperature decrease of the peak value.

<Other embodiments>
FIG. 11 is an explanatory diagram of another configuration of the image forming apparatus. The present invention is a part of the configuration of the embodiment as long as the temperature of the first surface and the second surface of the recording material is detected downstream of the nip portion TN and at least one of the transfer device and the charge removal device is controlled. Alternatively, another embodiment in which all of them are replaced with the alternative configuration can be implemented. Therefore, the dimensions, materials, shapes, relative arrangements, dimensions, angles, etc. of the components described in the first and second embodiments are limited to the scope of the present invention unless otherwise specified. Not limited.

  In the first embodiment, image formation on the first surface and the second surface is performed using the same transfer unit and the same fixing device in the same image forming apparatus. However, as shown in FIG. 11, for the recording material having the toner image transferred to the first surface by the image forming unit PA and heated by the fixing device 14A, the second surface is formed by the image forming unit PB. Alternatively, the toner image may be transferred to the fixing device 14B to heat the image. This is because the processing speed is higher than that in the first embodiment using the double-sided conveyance path.

  The means for charging the photosensitive drum 1 is not limited to the charging roller 2. Any means for uniformly charging the photosensitive drum 1 can be used regardless of the means. The means for exposing the photosensitive drum 1 may not be a semiconductor laser. Exposure by projection of a surface image using an LED array or an optical system may be used.

  The means for developing the photosensitive drum 1 is not limited to the one-component non-contact developing method and the two-component non-contact developing method in which the photosensitive drum 1 and the developing sleeve 4s are opposed to each other in a non-contact manner. A one-component contact development method in which development is performed in a contact state, or a two-component contact development method in which a developer in which a magnetic carrier is mixed with toner is conveyed by magnetic force so that a magnetic spike is brought into contact with the photosensitive drum 1 may be employed.

  The fixing device employs an on-demand fixing method using a fixing belt, but is not limited thereto. Any roller / roller method, roller / belt method, belt / belt method, or electromagnetic induction heating method may be used as long as the toner image is fixed and the image can be firmly fixed on the recording material 16.

  The transfer unit employs a contact transfer roller system in which the transfer roller 5 is pressed against the photosensitive drum 1, but is not limited thereto. A system in which a transfer blade is pressed against an intermediate transfer belt, which is an example of an image carrier, may be used.

DESCRIPTION OF SYMBOLS 1 Photosensitive drum, 2 Charging roller, 3 Exposure apparatus 4 Developing apparatus, 5 Transfer roller, 6 Separation static elimination needle 7 Drum cleaning apparatus, 8 Separation roller 10 Registration roller, 11 Pressure roller, 12 Fixing belt 13 Discharge roller, 14 Fixing apparatus , 15 Feed roller 16 Recording material, 17 Recording material cassette, 18 Heater 19 Front surface temperature sensor, 20 Back surface temperature sensor, 21 Memory 22 CPU, 23 Measuring instrument, 24 High voltage controller 25 Separation high voltage circuit, 26 Transfer high voltage circuit

Claims (6)

A heating device for heating the recording material;
A first temperature detector that detects the temperature of the first surface of the recording material that has passed through the heating device;
A second temperature detector that detects the temperature of the second surface of the recording material that has passed through the heating device;
An image forming apparatus comprising: a control unit configured to determine a curled state of the recording material based on the temperature detected by the first temperature detection unit and the second temperature detection unit. A transfer device for charging the recording material to a polarity opposite to the charging polarity of the toner image and transferring the toner image formed on the image carrier to the recording material;
A charge eliminating device that removes charges from the recording material that has passed through the transfer device,
2. The image according to claim 1, wherein the control unit controls a transfer function of the transfer device with respect to the heated recording material or a charge removal function of the charge removal device in accordance with the determined curl state of the recording material. Forming equipment. A reversing conveyance path for reversing the first surface and the second surface of the recording material heated by the heating device and conveying the recording material to the transfer device;
The toner image is transferred from the image carrier to the first surface by the transfer device, and the recording material heated by the heating device is conveyed again to the transfer device by the reverse conveyance path and is transferred from the image carrier to the first surface. The image forming apparatus according to claim 2, wherein the toner image is transferred to the second surface. A first image carrier that carries and rotates a toner image;
A first transfer device for transferring the toner image carried on the first image carrier to the first surface of the recording material;
A first heating device for heating the recording material on which the toner image is transferred to the first surface by the first transfer device;
A second image carrier that carries and rotates a toner image;
A toner image transferred to the first surface by the first transfer device and carried on the second image carrier on the second surface of the recording material heated by the first heating device. A second transfer device for transferring
A second heating device that heats the recording material having the toner image transferred to the second surface by the second transfer device;
The heating device is the first heating device, the image carrier is the second image carrier, and the transfer device is the second transfer device. The image forming apparatus according to 2 or 3. The heating device heats the recording material at a nip formed by press-contacting the heating rotator and the pressure rotator,
5. The distance from the nip portion to the first temperature detection portion along the recording material conveyance path is equal to the distance from the nip portion to the second temperature detection portion. 6. The image forming apparatus according to claim 1. The heating device heats the recording material at a nip formed by press-contacting the heating rotator and the pressure rotator,
The distance to the first temperature detection unit and the distance to the second temperature detection unit with respect to the center line along the conveyance direction of the recording material passing through the center of the nip portion in the width direction perpendicular to the conveyance direction of the recording material are The image forming apparatus according to claim 1, wherein the image forming apparatuses are equal to each other.

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