JP6422356B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus such as a copying machine, a laser beam printer, and a facsimile apparatus using an electrophotographic system or an electrostatic recording system.

  Conventionally, in an image forming apparatus using an electrophotographic method or an electrostatic recording method, a toner image is formed on an image carrier such as an electrophotographic photosensitive member, an electrostatic recording dielectric, or an intermediate transfer member by an appropriate image forming process. The This toner image is transferred to a recording material (recording medium, transfer material) such as recording paper (recording paper, transfer paper) by a transfer means. The recording material is output from the main body of the image forming apparatus as an image formed product (print, copy) after the toner image is thermally fixed thereon by the fixing unit.

  In such an image forming apparatus, a transfer roller is often used as a transfer unit that transfers a toner image from an image carrier to a recording material. The transfer of the toner image by the transfer roller is performed as follows. That is, a transfer roller having electrical conductivity and elasticity is brought into contact with the image carrier to form a transfer nip portion that is a transfer portion (transfer position). A recording material is introduced into the transfer nip portion at a predetermined control timing and is nipped and conveyed. Further, while the recording material is nipped and conveyed at the transfer nip portion, a transfer voltage having a polarity opposite to the charging polarity of the toner is applied to the transfer roller. Thereby, the toner image on the image carrier is electrostatically moved to the surface of the recording material (electrostatic transfer).

  As a method for controlling the transfer voltage, the following ATVC (Active Transfer Voltage Control) control for predicting the electrical resistance of the transfer roller and appropriately controlling the transfer voltage is known (Patent Document 1). That is, a predetermined constant current is passed from the transfer roller to the image carrier during the pre-rotation process, which is a preparatory operation before the image forming process, and the electric resistance of the transfer roller is predicted from the voltage value applied at that time, and during actual transfer An appropriate transfer voltage corresponding to the electric resistance is applied to the transfer roller.

  On the other hand, as a fixing means for thermally fixing a toner image transferred onto a recording material, a heat roller type or a heating film type heat fixing device (hereinafter also simply referred to as “fixing device”) is often used. The heat roller type fixing device includes a heating roller (fixing roller) maintained at a predetermined temperature and a pressure roller having an elastic layer and press-contacting the heating roller to form a fixing nip portion. Then, a recording material is introduced into the fixing nip portion and is nipped and conveyed. As a result, the toner image is thermally fixed to the recording material by the heat of the heating roller. A heating film type fixing device includes a heating body, a film that slides on the heating body (hereinafter referred to as “fixing film”), and a pressure member that forms a fixing nip portion with the heating body via the fixing film. And having. Then, a recording material carrying a toner image is introduced into the fixing nip portion and is nipped and conveyed. As a result, the toner image is thermally fixed to the recording material by the heat from the heating body via the fixing film (Patent Document 2).

  By the way, when a recording material with relatively high moisture absorption (hereinafter also simply referred to as “moisture absorbing paper”) is introduced into the fixing nip portion of the fixing device, a recording material with relatively low moisture absorption (hereinafter simply referred to as “dry paper”). In this case, a larger amount of water vapor is generated from the recording material heated rapidly at a higher temperature than in the case of "." When an image is continuously formed in a large amount of moisture-absorbing paper, a large amount of water vapor generated in a short period of time condenses on the recording material conveyance path and becomes conveyance resistance, causing conveyance failure such as corner breakage and wrinkles of the recording material. There is a fear.

  In Patent Document 3, when the current value flowing through the transfer unit is detected while the recording material passes through the transfer nip portion and the current value exceeds a predetermined value, an image is formed on the moisture-absorbing paper. It has been proposed to suppress the generation of water vapor by judging and correcting the temperature of the fixing device.

JP-A-2-123385 JP-A-4-44075 JP 2001-290316 A

  However, in the method of Patent Document 3, the value of the current flowing through the transfer means is affected not only by the moisture absorption state of the transfer material but also by the electrical resistance of the transfer material itself and the transferred toner. It turned out that there are cases where it cannot be judged. Conventionally, when the image forming apparatus has a plurality of print modes, it is difficult to accurately determine that an image is formed on the moisture absorbent paper according to the print modes. Therefore, conventionally, it has been difficult to perform control to reduce the amount of water vapor generated or generated by heating the recording material with a fixing device at an appropriate timing.

  Therefore, an object of the present invention is to generate or occur when the recording material is heated by the heating means at an appropriate timing by more accurately detecting that an image is formed on the recording material having relatively high moisture absorption. Another object of the present invention is to provide an image forming apparatus capable of reducing the amount of water vapor.

  The above object is achieved by the image forming apparatus according to the present invention. In summary, the main configuration of the present invention includes an image carrier that carries a toner image, a transfer member that transfers a toner image from the image carrier to a recording material at a transfer portion when a voltage is applied, and A series of image forming operations with respect to a single recording material or a plurality of recording materials according to one start instruction, including an applying unit that applies a voltage to the transfer member and a heating unit that heats the recording material onto which the toner image has been transferred. In an image forming apparatus that selectively executes in a plurality of print modes with different operation settings, a detecting unit that detects a value correlated with an electric resistance of the transfer member by applying a voltage to the transfer member by the applying unit; Water generated by heating the recording material by the heating unit based on the amount of change in the value detected by the detection unit due to the recording material passing through the transfer portion in the image forming operation. Control means for executing dehumidification control to reduce the amount of air or the amount of generated water vapor, and the control means is a value detected by the detection means before the transfer is started in an image forming operation. When the amount of change in the value detected by the detecting means after the transfer is started in the image forming operation becomes equal to or greater than a predetermined threshold in the direction in which the electrical resistance of the transfer member decreases, In the image forming apparatus, the dehumidification control is executed and the threshold value is changed according to a print mode of the image forming operation.

  According to the present invention, it is more accurately detected that an image is formed on a recording material having relatively high moisture absorption, and water vapor generated or generated by heating the recording material by a heating unit at an appropriate timing. The amount of can be reduced.

1 is a schematic cross-sectional view of an image forming apparatus. FIG. 2 is a schematic cross-sectional view of a fixing device. 3 is a block diagram illustrating a control mode of a main part of the image forming apparatus. FIG. It is a graph which shows an example of the change of the electrical resistance of the transfer roller at the time of continuous printing operation. It is a flowchart figure of moisture absorption paper detection control and dehumidification control. FIG. 6 is a graph showing changes in electrical resistance of a transfer roller for each fixing mode. It is a graph for demonstrating the threshold value in the moisture absorption paper detection control for every fixing mode. It is a graph for demonstrating the number of pages which can detect a moisture absorption paper. It is a graph which shows the change of the electrical resistance of the transfer roller in single-sided printing mode and double-sided printing mode. It is a graph for demonstrating the threshold value in the hygroscopic paper detection control in single-sided printing mode and double-sided printing mode. It is a graph for demonstrating the number of pages which can detect a moisture absorption paper. FIG. 6 is a graph for explaining threshold values in hygroscopic paper detection control for each temperature of a transfer roller at the start of a printing operation. FIG. 6 is a graph for explaining threshold values in hygroscopic paper detection control for each temperature of a transfer roller at the start of a printing operation.

  The image forming apparatus according to the present invention will be described below in more detail with reference to the drawings.

[Example 1]
(1) Overall Configuration and Operation of Image Forming Apparatus FIG. 1 is a schematic cross-sectional view of the image forming apparatus of this embodiment. The image forming apparatus 100 according to this embodiment is a laser beam printer using a transfer type electrophotographic process. This image forming apparatus 100 develops image data (print data) sent from a host computer into dot image information, and an electrophotographic engine unit in the apparatus main body 110 forms an image on a recording material P based on this image information. Is formed. Here, the recording material P may be described as paper generally used as the recording material P, but the recording material P is not limited to paper.

  The electrophotographic engine unit is provided with a photosensitive drum 1 which is a drum type (cylindrical) electrophotographic photosensitive member (photosensitive member) as an image carrier. The photosensitive drum 1 is driven to rotate at a predetermined peripheral speed in a clockwise direction indicated by an arrow R1 in the drawing. The peripheral surface of the rotating photosensitive drum 1 is uniformly charged to a predetermined potential having a predetermined polarity (negative polarity in this embodiment) by a charging roller 2 which is a roller-shaped charging member as a primary charging unit. . In the present embodiment, the charging process is uniformly performed at −700 V, for example. The surface of the photosensitive drum 1 that has been uniformly charged is subjected to scanning exposure with a laser beam L output from a laser section of a laser scanner 3 as an image exposure unit. The laser scanner 3 outputs a laser beam L of a laser unit that is blink-modulated based on image data output from an image controller (not shown) that is an image development unit. Thereby, an electrostatic latent image (electrostatic image) corresponding to the image information is formed on the surface of the photosensitive drum 1. The laser beam L output from the laser scanner 3 is deflected to the exposure position of the photosensitive drum 1 by the mirror 3a. The electrostatic latent image formed on the surface of the photosensitive drum 1 is developed as a toner image by the developing device 4 as a developing unit. In this embodiment, the toner as the developer is a negative toner. In other words, in this embodiment, the toner charging polarity (regular charging polarity) during development is negative (negative polarity). In this embodiment, the electrostatic latent image is subjected to reversal development processing. That is, a toner image is formed by adhering toner charged to the same polarity as the charging polarity of the photosensitive drum 1 to a portion where the absolute value of the potential is reduced by exposure after being uniformly charged.

  On the lower side of the photosensitive drum 1 in the figure, a transfer roller 5 as a roller-like transfer member as a transfer unit is disposed. The transfer roller 5 is in contact with the surface of the photosensitive drum 1 with a predetermined pressing force. A contact portion between the transfer roller 5 and the photosensitive drum 1 is a transfer nip portion (transfer portion, transfer position) T. The transfer roller 5 is configured by forming an elastic layer around a core metal (core material). The elastic layer is formed of a foamed elastic member (sponge rubber) made of a rubber material. The transfer roller 5 is an example of a transfer member that transfers a toner image from an image carrier to a recording material in a transfer portion when a voltage is applied.

  On the other hand, a recording material P such as recording paper is fed from a paper feed cassette 6 as a first paper feed port or a manual feed tray (MP tray) 12 as a second paper feed port. Then, the recording material P is introduced into the transfer nip T at a predetermined control timing and is nipped and conveyed. While the recording material P is nipped and conveyed at the transfer nip T, the transfer roller 5 receives a transfer voltage, which is a direct current voltage controlled to a predetermined voltage value VT, from a transfer power source 5a as an application unit. (Transfer bias) is applied. The polarity of the transfer voltage is a positive (plus) polarity opposite to the normal charging polarity of the toner. Accordingly, the toner image on the photosensitive drum 1 is sequentially electrostatically transferred onto the surface of the recording material P at the transfer nip T. In this embodiment, the voltage value VT of the transfer voltage applied to the transfer roller 5 at the time of transfer of the print operation is determined by ATVC control executed in the pre-rotation process of the print operation (described later). This voltage value VT is determined in accordance with a voltage value Vt0, which will be described later, obtained by ATVC control in the pre-rotation process. The voltage value VT may be Vt0 itself, or may be obtained from Vt0 based on an arithmetic expression obtained in advance, a lookup table, or the like.

  Here, when the paper feeding mode (cassette paper feeding mode) from the paper feeding cassette 6 is selected, the paper feeding half-moon roller 7 rotates and the recordings stacked and stored in the paper feeding cassette 6 are performed. One piece of material P is separated and fed. This recording material P is a path that sequentially passes through a pair of transport rollers 8, a sheet path (cassette paper feed transport path) 9, a pair of transport rollers 10, a leading edge detection sensor S2, and a pre-transfer guide plate (pre-transfer transport path) 11. , Introduced into the transfer nip T. On the other hand, when the paper feed mode (manual paper feed mode) from the manual feed tray 12 is selected, the paper feed roller 13 rotates and the recording material P loaded and set on the manual feed tray 12 is one sheet. Separated and fed. The recording material P is a path that sequentially passes through a sheet path (manual paper feed conveyance path) 14, a conveyance roller pair 10, a leading edge detection sensor S2, and a pre-transfer guide plate (pre-transfer conveyance path) 11, and a transfer nip T To be introduced.

  The leading edge of the recording material P is detected by the leading edge detection sensor S2 disposed on the recording material outlet side of the conveying roller pair 10 to determine the start timing of a series of operations for forming an image on the recording material P. Is done. That is, when the leading edge of the recording material P is detected by the leading edge detection sensor S2, the above-described image controller outputs image data. Based on the image data, the laser beam L output from the laser unit of the laser scanner 3 is blinked, and the photosensitive drum 1 is scanned and exposed. The presence / absence of the recording material P in the paper feed cassette 6 is detected by the presence / absence sensor S1. A constant voltage component 11 a is connected to the pre-transfer guide plate (pre-transfer conveyance path) 11 so that the pre-transfer guide plate 11 is maintained at a predetermined voltage. This constant voltage component 11 a may not be used depending on the electrical characteristics of the image forming apparatus 100.

  The recording material P that has passed through the transfer nip T is sequentially separated from the surface of the photosensitive drum 1. The recording material P is guided to a pre-fixing guide plate (pre-fixing conveyance path) 15 and introduced into a fixing nip portion n of a fixing device 16 as a fixing unit, and undergoes a thermal fixing process of a toner image. The fixing device 16 in this embodiment is a heating film type fixing device. The fixing device 16 will be described in detail later.

  The recording material P exiting the fixing device 16 passes through the conveyance roller pair 17, the sheet path (post-fixing conveyance path) 18, and the conveyance roller pair 19 in this order, and as an image formed product P ′, the image forming apparatus 100. It is output from the main body 110 and stacked on a paper discharge unit (paper discharge tray) 20. Note that whether or not the recording material P on which the toner image is heated and fixed has been discharged to the paper discharge unit 20 is detected by a paper discharge sensor S3.

  The image forming apparatus 100 includes a fan F that sucks air from the outside (outside the apparatus) of the apparatus main body 110 to the inside (inside the apparatus) or exhausts air from the inside of the apparatus main body 110 to the outside. The fan F can be used for any purpose as long as it affects the amount of water vapor in the apparatus main body 110 (including the amount of water droplets adhering to the conveyance path of the recording material P) by the rotational operation. For example, the main purpose is to discharge the air in the apparatus main body 110 heated by the heating element (such as the fixing device 16) in the apparatus main body 110. Further, the main purpose is to inhale air into the apparatus main body 110 in order to cool the heating element (such as the fixing device 16) in the apparatus main body 110.

  Further, the image forming apparatus 100 includes an automatic double-side mechanism as double-sided image forming means for automatically printing on both sides of the recording material P. At the timing when the leading edge of the recording material P in the conveyance direction when the first image is formed passes through the discharge roller pair 19 and the trailing edge of the recording material P passes through the fixing device 16 and is conveyed to the sheet path 18. The conveying direction of the recording material P is switched back by the paper discharge roller pair 19. In the recording material P, the rear end in the transport direction of the recording material P when the image on the first surface is formed becomes the front end in the transport direction of the recording material P when the image on the second surface is formed. It is conveyed in the direction of 51. Thereafter, the recording material P passes through the conveying roller pair 10 and the like and is introduced into the transfer nip portion T, and is discharged to the paper discharge portion 20 after transferring the toner image and fixing the toner image in the same manner as the first surface. The In this embodiment, an automatic double-side mechanism is constituted by the paper discharge roller pair 19, the double-sided conveyance roller 51, and the like.

(2) Fixing Device FIG. 2 is a schematic cross-sectional view of a fixing device 16 as a fixing unit in this embodiment. The fixing device 16 as a fixing unit is an example of a heating unit that heats the recording material onto which the toner image is transferred. In particular, the fixing device 16 of the present embodiment is a tensionless type heating film type image heating device.

  The fixing device 16 includes a ceramic heater (heater) 21 serving as a heating body, a heating body support 22 having a substantially semicircular arc shape in cross section, a cylindrical heat-resistant film (fixing film) 23, and a pressure member. Elastic pressure roller (pressure roller) 24.

  The heater 21 is a horizontally long member having a longitudinal direction in the direction perpendicular to the paper surface of FIG. The heater 21 in this embodiment includes a heater substrate 21a made of ceramic such as alumina, and an energization heating resistor layer 21b such as silver-palladium (Ag-Pd) provided on the surface side of the heater substrate 21a. The heater 21 includes a heater cover portion 21c such as a heat-resistant glass layer that covers the surface of the heater substrate 21a including the energization heating resistor layer 21b, and a temperature detection element 21d such as a thermistor disposed on the back side of the heater substrate 21a. And having. The heater 21 has a low heat capacity as a whole, and quickly increases or decreases the temperature with good responsiveness in response to ON or OFF of energization to the energization heating resistor layer 21b.

  The heating element support 22 has a heat resistance and electrical insulation, and is a rigid material that can withstand high loads, such as PPS (polyphenylene sulfide), PAI (polyamideimide), PI (polyimide), PEEK (polyetheretherketone). Etc. The heater 21 is fitted in a groove provided along the longitudinal direction of the heating body support 22 at a substantially central portion of the lower surface of the heating body support 22 in the drawing with the surface side facing outward. Fixed and supported.

  The fixing film 23 is a heat-resistant film having a thickness of about 40 μm to 100 μm, for example. The fixing film 23 is a film in which a base film such as polyimide is coated with a release heat resistant resin such as PFA or PTFE. The fixing film 23 is loosely fitted on the heating body support 22 on which the heater 21 is fixed and supported as described above.

  The pressure roller 24 includes a cored bar 24a, a roller layer 24b formed of a material having elasticity and heat resistance such as silicone rubber provided concentrically with the cored bar 24a, and a surface layer 24c. In the pressure roller 24, both ends in the longitudinal direction of the cored bar 24a are the front side (the front side of the paper in FIGS. 1 and 2) and the back side (the back side of the paper in FIGS. 1 and 2) of the image forming apparatus 100, respectively. Between the chassis side plates, the bearings are rotatably supported. Then, the heater 21 is supported on the lower surface in the figure and the heating body support 22 on which the cylindrical film 23 is externally fitted, and the heater 21 portion is arranged on the upper side of the pressure roller 24 in the figure. It is arranged to face the upper surface in the figure. Then, the heating body support 22 is held in a state in which it is pressed against the upper surface of the pressure roller 24 with a predetermined pressing force by a pressing means (not shown). Thus, a fixing nip portion n having a predetermined width is formed with the fixing film 23 sandwiched between the lower surface of the heater 21 in the drawing and the upper surface of the pressure roller 24 in the drawing.

  The pressure roller 24 is rotationally driven at a predetermined peripheral speed in a counterclockwise direction indicated by an arrow R2 in the drawing by a fixing driving mechanism M as a driving unit. When the pressure roller 24 is driven to rotate, a rotational force acts on the fixing film 23 by the frictional force between the pressure roller 24 that is in pressure contact with the fixing nip n and the outer surface of the fixing film 23. The fixing film 23 has its inner surface in close contact with the lower surface of the heater 21 at the fixing nip n and slides in the clockwise direction indicated by the arrow R3 in the drawing in a circumferential direction corresponding to the rotational peripheral speed of the pressure roller 24. The outer periphery of the heating element support 22 is rotated at a speed (pressure roller driving method). The heating element support 22 holds the heater 21 and also serves as a film guide for improving the conveyance stability when the fixing film 23 rotates.

  The pressure roller 24 is driven to rotate, and the fixing film 23 rotates around the outer periphery of the heating body support 22, energizing the heater 21, and the fixing nip n rises to a predetermined temperature by the heat generated by the heater 21. The temperature is adjusted. In this state, the recording material P carrying the unfixed toner image t is introduced into the fixing nip part n, and the surface side carrying the toner image of the recording material P is in close contact with the outer surface of the fixing film 23 in the fixing nip part n. It is nipped and conveyed at the fixing nip n together with the rotating fixing film 23. In the process of nipping and conveying in this manner, the heat of the heater 21 is applied to the recording material P through the fixing film 23, and the unfixed toner image t on the recording material P is fixed to the recording material P by heat and pressure. . When the recording material P passes through the fixing nip n, the recording material P is conveyed with the curvature separated from the outer surface of the rotating fixing film 23.

  In the present embodiment, the temperature control of the heater 21 is performed as follows. The output (detected temperature information) of the temperature detection element 21d provided on the back surface of the heater substrate 21a is taken into the temperature adjustment circuit 25. Then, based on the detected input temperature information, the temperature adjustment circuit 25 controls power supply to the heater 21 by the power supply circuit 26. In other words, the temperature adjustment circuit 25 is configured to increase the temperature of the heater 21 when the temperature of the heater detected by the temperature detection element 21d is lower than a predetermined set temperature, and to decrease the temperature of the heater 21 when the temperature is higher. The temperature is adjusted so that the temperature during fixing is substantially constant. Specifically, the temperature control circuit 25 controls power (such as control of AC voltage phase and wave number) supplied from the power supply circuit 26 to the energization heating resistor layer 21 b of the heater 21.

  In the heating film type fixing device, a heater having a low heat capacity and a high temperature rise such as a ceramic heater can be used, and a thin film having a small heat capacity can be used as the fixing film. Further, by heating only the fixing nip portion, quick start performance is good, and energy-saving heat fixing can be realized. That is, since there is no need to heat during standby, there is an advantage that power consumption can be kept low.

(3) Control Mode FIG. 3 shows a schematic control mode of the main part of the image forming apparatus 100 of the present embodiment. A control unit (engine control unit) 120 serving as a control unit provided in the image forming apparatus 100 includes a CPU 121 that is a central element that performs arithmetic processing, a memory 122 such as a ROM and RAM that are storage elements, and the like. Is done. The RAM stores sensor detection results, calculation results, and the like, and the ROM stores control programs, data tables obtained in advance, and the like. Each control object in the image forming apparatus 100 is connected to the control unit 120. In particular, in relation to the present embodiment, the control unit 120 includes a transfer power source 5a, an ammeter (current detection means) 5b described later, a drive circuit 41 for the fan F, the paper feeding meniscus roller 7 and the paper feeding roller 13. The drive circuit 31 of the paper feed unit 30 is connected.

  In this embodiment, the control unit 120 comprehensively controls each unit of the image forming apparatus 100. Particularly in relation to the present embodiment, the control unit 120 performs ATVC control, moisture absorbent paper detection control described later, dehumidification control described later, and the like.

(4) Basic operations of moisture-absorbing paper detection control and dehumidification control Next, basic operations of moisture-absorbing paper detection control and dehumidification control installed in the image forming apparatus 100 of the present embodiment will be described.

  Here, a series of image formation starting from a start instruction input from a host computer or the like until an image is formed on one or a plurality of recording materials P and discharged (output) from the apparatus main body 110 of the image forming apparatus 100 is performed. The operation is referred to as a “print operation (or print job)”. In addition, a printing operation for continuously forming images on a plurality of recording materials P is particularly referred to as a “continuous printing operation (or continuous printing job)”. The printing operation generally includes an image forming process (printing process), a pre-rotation process, a paper-to-paper (recording material) process when images are formed on a plurality of recording materials P, and a post-rotation process. The image forming process is a period during which the electrostatic latent image is actually formed on the photosensitive drum 1, the toner image is formed, and the toner image is transferred to the recording material P. The pre-rotation process is a period for performing a preparatory operation before the image forming process. The inter-sheet process is a period corresponding to the interval between the preceding recording material P and the succeeding recording material P in the transfer portion T when the image forming process is continuously performed on a plurality of recording materials P. The post-rotation process is a period during which an organizing operation (preparation operation) after the image forming process is performed.

  In the hygroscopic paper detection control of this embodiment, it is determined from the change in the electrical resistance of the transfer roller 5 during the printing operation that an image is continuously formed on the hygroscopic paper. Hereinafter, determining (detecting) that an image is formed on the absorbent paper is also simply referred to as “detecting the absorbent paper”.

  When an image is formed on the moisture-absorbing paper, the moisture-absorbing paper passes through the transfer roller 5 so that moisture adheres to the surface of the transfer roller 5. Further, when the moisture absorbent paper passes through the fixing device 16, the water vapor is gradually accumulated in the apparatus main body 110, and the water vapor adheres to the surface of the transfer roller 5. When moisture or water vapor begins to adhere to the transfer roller 5, the electrical resistance of the transfer roller 5 gradually decreases. In this embodiment, the change in the electric resistance of the transfer roller 5 is monitored during the printing operation to determine whether an image is continuously formed on the moisture absorbent paper. As described above, in the present embodiment, moisture absorption is performed based on the amount of change in the value correlated with the electrical resistance of the transfer roller 5 detected by the detection unit due to the recording material P passing through the transfer nip T in the printing operation. It is detected that an image is formed on the paper.

  In this embodiment, a constant current of 18.0 μA is passed as a predetermined value from the transfer roller 5 to the photosensitive drum 1 during the pre-rotation process and the sheet-to-paper process, and the voltage value Vtn (n ≧ 0) applied at that time is obtained ( ATVC control). Then, the electric resistance of the transfer roller 5 is predicted from the voltage value Vtn (n ≧ 0). That is, the control unit 120 detects the current that flows by applying a voltage from the transfer power supply 5a to the transfer roller 5, and controls the output voltage value of the transfer power supply 5a so that the current value approaches a predetermined value. Then, the voltage value at that time is obtained. In this embodiment, the control unit 120 and the ammeter 5b constitute detection means for detecting a value correlated with the electrical resistance of the transfer member by applying a voltage to the transfer member by the applying means. In this embodiment, the detection means detects a voltage value when a voltage controlled by a constant current by the application means is applied to the transfer member.

  In this embodiment, the voltage value Vtn (n ≧ 0) is defined as follows. Assume that the voltage value obtained during the pre-rotation process in one printing operation is Vt0, and the voltage value obtained in the inter-sheet process immediately after the nth sheet during the printing operation is Vtn (n ≧ 1). As described above, the inter-sheet process means a timing at which the recording material P is not in the transfer nip T between the preceding recording material P and the subsequent recording material P during the continuous printing operation. Further, the voltage value Vtn tends to decrease as the electric resistance of the transfer roller 5 decreases.

  FIG. 4 shows changes in the electrical resistance of the transfer roller 5 when continuous printing operation is performed using moisture-absorbing paper and dry paper, respectively, in a room temperature and humidity environment. The horizontal axis represents the number of pages, and the vertical axis represents a voltage value Vtn (n ≧ 0) for predicting the electric resistance of the transfer roller 5.

  FIG. 4 shows that the voltage value Vtn (n ≧ 0) decreases as the number of pages in the continuous printing operation progresses. This means that the transfer roller 5 absorbs moisture due to the printing operation, a new conductive path is formed in the transfer roller 5, and the electrical resistance of the transfer roller 5 is reduced. In the printing operation, moisture adheres to the surface of the transfer roller 5 as the moisture absorbent paper passes through the transfer roller 5, and water vapor generated as the moisture absorbent paper passes through the fixing device 16 adheres to the surface of the transfer roller 5. As a result, the transfer roller 5 absorbs moisture. Further, FIG. 4 shows that the amount of decrease in the voltage value Vtn (n ≧ 0) is larger in the printing operation on the moisture-absorbing paper than in the printing operation on the dry paper. This means that a larger amount of water vapor is generated in the printing operation on the moisture-absorbing paper, and the electric resistance of the transfer roller 5 is greatly reduced due to the influence.

  In the present embodiment, the control unit 120 executes arithmetic processing, and the voltage value Vt0 obtained in the pre-rotation process in the printing operation and the voltage value Vtn (n ≧ 1) obtained for each inter-sheet process during the printing operation. And compare. Then, the control unit 120 calculates the value of ΔVtn calculated as the difference between the voltage value Vt0 and the voltage value Vtn (n ≧ 1) (that is, the amount of change of the voltage value Vtn (n ≧ 1) from the voltage value Vt0). When the predetermined threshold value X is exceeded, it is determined that an image is formed on the moisture absorbent paper. If the difference ΔVtn is a positive value, it means that the voltage value Vtn (n ≧ 1) is smaller than the voltage value Vt0, and the electric resistance of the transfer roller 5 is changed in a decreasing direction. Here, it is assumed that the threshold value X of the difference ΔVtn is set to Δ80V in advance. Then, it is determined that an image is formed on the moisture-absorbing paper at the timing of the Ath sheet that first detects that the value of the difference ΔVtn is 80 V or more during the actual printing operation. In this embodiment, the threshold value X is set so that the difference ΔVtn does not exceed the number of images that are continuously formed on a predetermined dry paper.

  In this embodiment, the control unit 120 controls the amount of water vapor generated by heating the recording material P by the fixing device 16 based on the change amount (that is, the difference ΔVtn) of the voltage value Vtn (n ≧ 0). Execute dehumidification control to reduce In particular, in this embodiment, when the control unit 120 determines that an image is formed on the moisture absorbent paper based on the difference ΔVtn, the temperature setting of the fixing device 16 is immediately corrected to be lower while continuing the printing operation. To do. That is, the amount of water vapor generated from the recording material P is suppressed by lowering the heating temperature by the fixing device 16.

  As described above, in this embodiment, the control unit 120 is detected by the detection unit after the transfer is started in the printing operation from the value detected by the detection unit before the transfer is started in the printing operation. Find the amount of change in value. Then, the control unit 120 performs dehumidification control when the amount of change becomes equal to or greater than a predetermined threshold in the direction in which the electric resistance of the transfer roller 5 decreases. The difference ΔVtn is an example of a change amount of a value correlated with the electric resistance of the transfer roller 5 detected by the detection unit due to the recording material P passing through the transfer nip portion T in the printing operation.

  Here, the dehumidification control is not limited to the correction of the temperature setting of the fixing device 16 as described above. As dehumidification control to reduce the amount of water vapor generated, the feeding control of the recording material P is corrected so as to widen the interval (inter-paper) between the recording materials conveyed to the transfer nip T for transfer. May be. The amount of water vapor generated can be reduced by at least one of the correction of the set temperature of the fixing device 16 and the correction of the feeding control of the recording material P. Further, the dehumidification control is not limited to the control for reducing the amount of generated water vapor, but may be control for reducing the amount of water vapor generated by heating the recording material P by the fixing device 16. For example, the rotation control of the fan F may be corrected in order to increase the discharge efficiency of water vapor in the apparatus main body 110. That is, the amount of the generated water vapor is reduced by inhaling air from the outside of the apparatus main body 110 or changing the rotational speed of the fan that exhausts air from the inside of the apparatus main body 110 to the outside. it can. Changing the rotational speed includes changing from a stopped state to a rotational state, and changing the first rotational speed to a second rotational speed that is greater than the first rotational speed. As described above, the dehumidification control may be any control that controls the amount of water vapor in the apparatus main body 110 (more specifically, the conveyance path of the recording material P) so as not to exceed a predetermined level.

  In addition, it is preferable to continue correction | amendment of the various controls as the above-mentioned dehumidification control until a printing operation is complete | finished. The operation may be continued until the operation of the image forming apparatus 100 stops. Moreover, the correction | amendment of the various controls as the above-mentioned dehumidification control can be performed combining two or more arbitrarily.

  FIG. 5 is a flowchart schematically showing the flow of moisture absorbent paper detection control and dehumidification control in the present embodiment.

  When the printing operation is started (step 1), the control unit 120 executes ATVC control for predicting the electric resistance of the transfer roller 5 after a predetermined time has elapsed since the drive system was driven, and the voltage value Vt0 is obtained (step 2). Next, when the leading end of the conveyed recording material P is detected, the control unit 120 starts the formation of a toner image and also conveys the recording material P to the transfer nip T, so that a predetermined voltage value VT is reached. Then, electrostatic transfer is performed (step 3). This voltage value VT is a transfer voltage determined from Vtn (n ≧ 0). Next, the control unit 120 determines whether there is an image forming process for the subsequent recording material P (step 4). If the control unit 120 determines in step 4 that there is no subsequent recording material P, the control unit 120 ends the printing operation as it is.

  On the other hand, if the control unit 120 determines in step 4 that there is the subsequent recording material P, the control unit 120 performs ATVC control again after the trailing end of the preceding recording material P passes through the transfer nip T, and the voltage value Vtn (n ≧ 1) is obtained (step 5). Next, the control unit 120 executes arithmetic processing, and calculates a difference ΔVtn obtained by subtracting the voltage value Vtn (n ≧ 1) obtained in the previous step 5 from the voltage value Vt0 obtained in step 2 (step 6). ). Next, the control unit 120 determines whether or not the value of the difference ΔVtn calculated in step 6 is greater than or equal to the threshold value X (step 7). If the controller 120 determines that the value of the difference ΔVtn is greater than or equal to the threshold value X in step 7, the controller 120 corrects the temperature setting of the fixing device 16 to be lower (step 8). At this time, as described above, for example, correction of the content of the rotation control of the fan F may be performed together or instead. If the controller 120 determines in step 7 that the value of the difference ΔVtn is less than the threshold value X, the controller 120 continues the printing operation without performing the above correction.

  Thereafter, the control unit 120 returns the processing to Step 3 and Step 4 again, and repeats the processing from Step 5 to Step 8 in order until it is determined in Step 4 that there is no subsequent recording material P. Finally, it is determined in step 4 that there is no subsequent recording material P, and the printing operation is terminated.

  In the present embodiment, the voltage value Vtn (n ≧ 1) is obtained by executing ATVC control for every sheet interval in the printing operation. However, every predetermined number of sheets (for example, every other sheet interval). The ATVC control may be executed to obtain the voltage value Vtn (n ≧ 1).

(5) Effect verification of moisture-absorbing paper detection control and dehumidification control Next, an experiment was conducted to verify the effects of moisture-absorbing paper detection control and dehumidification control. The experimental contents were a continuous printing operation of 500 sheets on moisture-absorbing paper having a moisture content of about 10% in a normal temperature and humidity environment, and condensation on the sheet path (post-fixing conveyance path) 18 as a representative conveyance path of the recording material P. The amount of water droplets to be measured was measured. The moisture-absorbing paper having a water content of about 10% is a paper that has been left in a high-temperature and high-humidity environment for 48 hours or more. The transfer roller 5 is a roller having an electric resistance value (hereinafter the same) of 50 MΩ when 2 kV is applied in a room temperature / humidity environment of 23 ° C./50%, and the threshold value X of the difference ΔVtn is set to Δ80V. . Note that this experiment was performed by setting a later-described fixing mode as a standard mode and performing a continuous printing operation in the single-sided printing mode. The amount of water droplets condensed on the sheet path 18 was calculated from the change in the weight of the sheet path 18 before and after the printing operation. In the control of this embodiment, it is determined that an image is formed on the moisture-absorbing paper at the timing of the 50th sheet during continuous printing, and the temperature setting of the fixing device 16 is corrected to be lower from the 50th sheet onward. The rotation control of the fan F was changed from the stopped state to the rotating state. In this embodiment, the fan F is changed from the stopped state to the rotating state. However, as described above, the rotation of the fan F may be switched from the low-speed rotation to the high-speed rotation.

  As a result, it was confirmed that the amount of water droplets was reduced to 1/5 or less compared to the comparative example that did not use the control of the present embodiment, and conveyance failures such as corner breaks and wrinkles of the recording material P could be suppressed.

  As described above, according to this embodiment, it is determined whether or not an image is formed on the moisture-absorbing paper from the change in the electrical resistance of the transfer roller 5 during the printing operation. The electrical resistance of the transfer roller 5 is measured at a timing when the recording material P is not in the transfer nip T before and during image formation. Therefore, it is possible to detect whether an image is formed on the moisture-absorbing paper without being affected by the type of the recording material P and the toner amount of the image. Further, the detection result is fed back to the temperature control of the fixing device 16 and the rotation speed of the fan F, so that the recording material P is bent or wrinkled when a large amount of images are continuously formed on the moisture absorbent paper. It is possible to suppress the conveyance failure such as. As described above, according to this embodiment, it is more accurately detected that an image is formed on the recording material P having relatively high moisture absorption, and is generated or generated by heating the recording material P by the fixing device 16. The amount of steam generated can be reduced.

(6) Hygroscopic Paper Detection Control According to Print Mode Next, the hygroscopic paper detection control according to the print mode (image formation mode) will be described. The image forming apparatus 100 according to the present exemplary embodiment can selectively execute a print operation in a plurality of print modes having different operation settings. In this embodiment, the threshold value X of the difference ΔVtn is changed according to a fixing mode (more specifically, a fixing temperature and a length between sheets) as a print mode.

  The image forming apparatus 100 according to the present exemplary embodiment selectively selects a standard mode corresponding to plain paper, a mode having a high fixing temperature corresponding to thick paper, a mode having a low fixing temperature corresponding to thin paper, or an inter-paper extension mode as a fixing mode. Can be used to perform a printing operation. In the mode where the fixing temperature is low, curling of the recording material P when thin paper is used as the recording material P can be reduced. In the inter-paper extension mode, the temperature rise in the machine can be suppressed. The fixing temperature in the mode where the fixing temperature is high is higher than the fixing temperature in the standard mode, and the fixing temperature in the mode where the fixing temperature is low is lower than the fixing temperature in the standard mode. In addition, the interval between sheets in the sheet extension mode is longer than that in the standard mode, the mode in which the fixing temperature is high, and the mode in which the fixing mode is low. The fixing temperature in the inter-paper extension mode is the same as the fixing temperature in the standard mode.

  FIG. 6 shows changes in the electrical resistance of the transfer roller 5 when a continuous printing operation is performed using moisture-absorbing paper in a normal temperature and humidity environment in each of the fixing modes. The horizontal axis represents the number of pages, and the vertical axis represents a voltage value Vtn (n ≧ 0) for predicting the electric resistance of the transfer roller 5. The transfer roller 5 has an electric resistance value of 50 MΩ, and the voltage value Vt0 corresponding to the initial electric resistance of the transfer roller 5 is 900V.

  FIG. 6 shows that the voltage value Vtn (n ≧ 0) decreases as the number of pages in the continuous print operation progresses. This means that the transfer roller 5 absorbs moisture due to the printing operation, a new conductive path is formed in the transfer roller 5, and the electrical resistance of the transfer roller 5 is reduced. In the printing operation, moisture adheres to the surface of the transfer roller 5 as the moisture absorbent paper passes through the transfer roller 5, and water vapor generated as the moisture absorbent paper passes through the fixing device 16 adheres to the surface of the transfer roller 5. As a result, the transfer roller 5 absorbs moisture.

  Further, it can be seen from FIG. 6 that the amount of decrease in the voltage value Vtn (n ≧ 0) is larger in the mode having the higher fixing temperature than in the mode having the lower fixing temperature. For example, when compared with the electric resistance change amount Vt500 when 500 sheets are continuously printed, the voltage decreases by Δ150 V in the low fixing temperature mode and by Δ300 V in the high fixing temperature mode. Note that the electric resistance change amount Vt500 in the standard mode is Δ200 V between the mode in which the fixing temperature is high and the mode in which the fixing temperature is low. This means that the higher the fixing temperature, the greater the amount of heat given to the paper, and the greater the amount of water vapor generated, the greater the electrical resistance of the transfer roller 5 due to the influence.

  On the other hand, it can be seen that the amount of decrease in the voltage value Vtn (n ≧ 0) is smaller in the inter-sheet extension mode than in the standard mode. For example, when compared with the change amount Vt500 of the electric resistance when 500 sheets are continuously printed, the voltage decreases by Δ200V in the standard mode and by Δ120V in the inter-sheet extension mode. This is because the longer the distance between the sheets, the greater the amount of water vapor that diffuses in the fixing device 16 diffuses outside the apparatus, and as a result, the water vapor remaining in the apparatus decreases, and the electrical resistance of the transfer roller 5 is less likely to decrease. Means that.

  FIG. 7 shows a transfer roller when 500 sheets of printing operation are continuously performed on moisture-absorbing paper having a water content of about 10% and dry paper having a water content of about 5% in each of the fixing modes. 5 shows a change amount ΔVt500 of the electrical resistance. It should be noted that the electrical resistance of the transfer roller 5 has been sufficiently lowered at the time when the printing operation for 500 continuous sheets has been completed, so that the amount of change in the electrical resistance of the transfer roller 5 exceeds ΔVt500 even if the number of prints is further increased. There is no.

  In this embodiment, the threshold value X of the difference ΔVtn is optimized according to the fixing mode. In this embodiment, the standard mode threshold value X is set to Δ60V. On the other hand, in the mode in which the fixing temperature is high, since the change amount ΔVt500 of the electric resistance during the printing operation is large, the value of the threshold value X is set large. On the other hand, in the mode where the fixing temperature is low and the inter-paper extension mode, since the change amount ΔVt500 of the electric resistance during the printing operation is small, the value of the threshold value X is set small. Specifically, the threshold value X in the high fixing temperature mode is Δ80V, the threshold value X in the low fixing temperature mode is Δ40V, and the threshold value X in the inter-paper extension mode is Δ30V.

  FIG. 8 shows the amount of change ΔVtn in the electrical resistance of the transfer roller 5 when a continuous printing operation is performed using moisture-absorbing paper having a water content of about 10% and dry paper having a water content of about 5% in a normal temperature and humidity environment. Shows the transition. With reference to the figure, the threshold value of the difference ΔVtn is set as described above, so that it can be determined whether the image is formed on the moisture-absorbing paper in the continuous printing operation regardless of the fixing mode. This will be described in comparison with a case where the threshold value of the difference ΔVtn is a constant value.

  From FIG. 8, when the threshold value of the difference ΔVtn is uniformly set to Δ80V regardless of the fixing mode, the mode with the high fixing temperature is the 30th sheet, the standard mode is the 50th sheet, the mode with the low fixing temperature is the 80th sheet, In the inter-paper extension mode, the 120th sheet is determined as moisture-absorbing paper. On the other hand, in this embodiment, the threshold value of the difference ΔVtn is changed according to the fixing mode. Therefore, it can be determined that the paper is moisture-absorbing paper near the 30th sheet in any of the high fixing temperature mode, the low fixing temperature mode, and the inter-paper extension mode. That is, in this embodiment, moisture-absorbing paper can be detected with a smaller number of pages than when the threshold value of the difference ΔVtn is set to a constant value regardless of the fixing mode. If moisture-absorbing paper can be detected early with a small number of pages, it will also reduce condensation on the conveyance path of the recording material P and reduce the level and frequency of conveyance defects such as corner breaks and wrinkles of the recording material P. Can do.

  As described above, according to the present embodiment, the threshold for determining whether or not the paper is moisture-absorbing paper is optimized according to a plurality of fixing modes having different fixing temperatures and different paper intervals. As a result, moisture-absorbing paper can be detected with a smaller number of pages, and the level and frequency of occurrence of conveyance failures such as corner breaks and wrinkles of the recording material P can be further reduced. That is, according to the present embodiment, it is more accurately detected that the image is formed on the recording material P having relatively high moisture absorption according to the fixing mode, and the dehumidification control is executed at an appropriate timing. Can do.

[Example 2]
Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the first embodiment. Therefore, elements having the same or corresponding functions or configurations as those of the image forming apparatus according to the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted.

  The present embodiment is characterized in that the threshold value X of the difference ΔVtn in the hygroscopic paper detection control is made different between the single-sided print mode and the double-sided print mode as the print mode.

  FIG. 9 shows changes in the electrical resistance of the transfer roller 5 when a continuous printing operation is performed in either a single-sided printing mode or a double-sided printing mode using moisture-absorbing paper and dry paper in a room temperature and humidity environment. Show. The horizontal axis represents the number of pages, and the vertical axis represents a voltage value Vtn (n ≧ 0) for predicting the electric resistance of the transfer roller 5. The fixing mode is a standard mode corresponding to plain paper, the transfer roller 5 having an electric resistance value of 50 MΩ is used, and the voltage value Vt0 corresponding to the initial electric resistance of the transfer roller 5 is 900V.

  FIG. 9 shows that the amount of decrease in the voltage value Vtn (n ≧ 0) is larger in the double-sided printing mode than in the single-sided printing mode. For example, when compared with the electric resistance change amount Vt500 when 500 sheets are continuously printed, the voltage decreases by Δ200 V in the single-sided printing mode and by Δ600 V in the double-sided printing mode. This is because in the double-sided printing mode, the paper after the first side image is formed and passed through the fixing device 16 passes through the transfer nip T again to form the second side image. This is because the temperature increases. The impedance of the transfer roller 5 is dominated by an elastic layer made of sponge rubber or the like, and rubber generally has a characteristic that the resistance decreases as the temperature rises. That is, in the double-sided printing mode, not only the influence of water vapor by the moisture-absorbing paper but also the influence of the heat of the paper passing through the transfer nip T for image formation on the second side overlaps, and the electrical resistance of the transfer roller 5 is greatly reduced. .

  FIG. 10 shows a case where 500 sheets of printing operation are continuously performed in either single-sided printing mode or double-sided printing mode on moisture-absorbing paper having a water content of about 10% and dry paper having a water content of about 5% in a normal temperature and humidity environment. The change amount ΔVt500 of the electric resistance of the transfer roller 5 is shown. It should be noted that the electrical resistance of the transfer roller 5 has been sufficiently lowered at the time when the printing operation for 500 continuous sheets has been completed, so that the amount of change in the electrical resistance of the transfer roller 5 exceeds ΔVt500 even if the number of prints is further increased. There is no.

  FIG. 10 shows that the value of ΔVt500 in the duplex printing mode exceeds Δ60V even for dry paper. This Δ60V is the threshold value X of ΔVtn in the standard mode set in consideration of only the single-sided print mode in the first embodiment. If the moisture-absorbing paper detection control is operated while the threshold value X is set to Δ60 V in the double-sided print mode, there is a possibility that the paper is erroneously detected as moisture-absorbing paper even if it is dry paper.

  In this embodiment, the threshold value X of the difference ΔVtn is changed between the single-sided print mode and the double-sided print mode in order to suppress the erroneous detection and to effectively operate the hygroscopic paper detection control even in the double-sided print mode. In the double-sided printing mode, since the change amount ΔVt500 of the electric resistance during the printing operation is large, the value of the threshold value X is set larger than that in the single-sided printing mode. Specifically, the threshold value Xt in the single-sided printing mode is Δ60V, and the threshold value Xt in the double-sided printing mode is Δ180V.

  FIG. 11 shows a case where a continuous printing operation is performed in either a single-sided printing mode or a double-sided printing mode using a moisture-absorbing paper having a water content of about 10% and a dry paper having a water content of about 5% in a normal temperature and humidity environment. The transition of the change amount ΔVtn of the electric resistance of the transfer roller 5 is shown. With reference to the figure, by setting the threshold value of the difference ΔVtn as described above, the single-sided printing mode and the double-sided printing can be determined as to what number of sheets in the continuous printing operation it can be determined that an image is formed on the moisture absorbent paper. This will be described in comparison with the case where the threshold value is the same for each mode.

  From FIG. 11, when the threshold value of the difference ΔVtn is set to Δ60V in both the single-sided printing mode and the double-sided printing mode, it is possible to correctly determine the moisture-absorbing paper and the dry paper near the 30th sheet in the single-sided printing mode. However, in this case, in the double-sided printing mode, there is a possibility that it is determined as moisture-absorbing paper if it exceeds about 70 sheets, regardless of dry paper. On the other hand, in this embodiment, the threshold value of the difference ΔVtn is optimized in accordance with the single-sided printing mode or the double-sided printing mode. For this reason, in this embodiment, in both the single-sided printing mode and the double-sided printing mode, it is possible to correctly determine the vicinity of about the 30th sheet without erroneously detecting moisture-absorbing paper and dry paper.

  As described above, according to the present embodiment, it is possible to correctly detect moisture-absorbing paper even in the double-sided printing mode, and it is possible to more reliably suppress conveyance failures such as corner breaks and wrinkles of the recording material P. In other words, according to the present embodiment, it is more accurately detected that an image is formed on the recording material P having relatively high moisture absorption depending on whether the single-sided printing mode or the double-sided printing mode, and at an appropriate timing. Dehumidification control can be executed.

[Example 3]
Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the first embodiment. Therefore, elements having the same or corresponding functions or configurations as those of the image forming apparatus according to the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted.

  In this embodiment, the threshold value X of the difference ΔVtn in the moisture absorbent paper detection control is corrected according to the temperature of the transfer roller 5 at the start of the print operation (more specifically, before transfer is started in the print operation). It is characterized by.

  12 and 13 show the relationship between the temperature of the transfer roller 5 at the start of the printing operation and the change in electrical resistance of the transfer roller 5 due to the subsequent printing operation. The horizontal axis is the temperature of the transfer roller 5 at the start of the printing operation, and the vertical axis is a printing operation of 500 sheets of moisture-absorbing paper having a water content of about 10% and dry paper having a water content of about 5% in a normal temperature and humidity environment. A change amount ΔVt500 of the electric resistance of the transfer roller 5 when it is performed is shown. 12 shows the case of the single-sided printing mode, and FIG. 13 shows the case of the double-sided printing mode.

  12 and 13, a region H where the temperature of the transfer roller 5 is sufficiently warmed is, for example, an elapsed time of 10 minutes after the end of the printing operation after 200 or more continuous printing operations are performed. The case of within can be illustrated. In FIGS. 12 and 13, the region C where the temperature of the transfer roller 5 is sufficiently cooled (room temperature) can be exemplified, for example, when 30 minutes or more have elapsed since the end of the last printing operation.

  12 and 13, it can be seen that the higher the temperature of the transfer roller 5 at the start of the printing operation, the smaller the change ΔVt500 in the electrical resistance of the transfer roller 5 due to the subsequent printing operation. This is because if the temperature of the transfer roller 5 is high at the start of the printing operation, the temperature of the transfer roller 5 is difficult to increase in the subsequent printing operation, and the amount of change in the electrical resistance of the transfer roller 5 is small.

  Further, FIG. 12 shows that when the temperature of the transfer roller 5 at the start of the printing operation is high, if the continuous printing operation is performed, the electrical resistance change ΔVt500 may be negative (in the single-sided printing mode). When using dry paper). This means that the electric resistance of the transfer roller 5 has rather increased during 500 continuous printing operations. This is presumably because the normal temperature paper repeatedly passes through the transfer nip portion T in the single-sided print mode, so that the transfer roller 5 gradually loses heat to the paper, and the electrical resistance of the transfer roller 5 increases. In addition, when dry paper is used, it is considered that one of the reasons is that the transfer roller 5 hardly absorbs moisture and the electric resistance of the transfer roller 5 is not easily lowered.

  As described above, the amount of change in the electrical resistance of the transfer roller 5 due to the subsequent printing operation varies greatly depending on the temperature of the transfer roller 5 at the start of the printing operation. Therefore, in this embodiment, in order to correctly detect the moisture absorbent paper, the threshold value X for detecting the moisture absorbent paper is corrected according to the temperature of the transfer roller 5 at the start of the printing operation.

  12 and 13 show the threshold value Xt defined in Example 2 as a comparative example. This threshold value Xt assumes that the transfer roller 5 is sufficiently cooled (normal temperature) at the start of the printing operation, and correctly detects moisture-absorbing paper when the temperature of the transfer roller 5 is high at the start of the printing operation. There are cases where it is not possible. That is, in the temperature range higher than the temperature at which the graph when the moisture-absorbing paper is used intersects the threshold value Xt line, it is determined that the paper is dry paper, although it is moisture-absorbing paper. On the other hand, as shown in FIGS. 12 and 13, the threshold value Xu of this embodiment is corrected according to the temperature of the transfer roller 5 at the start of the printing operation, and therefore the transfer roller 5 at the start of the printing operation. Absorbent paper can be detected correctly regardless of temperature.

  Here, in this embodiment, it is determined (predicted) whether the transfer roller 5 is cooled (normal temperature) or warmed before the printing operation, and the moisture absorbent paper is detected according to the determination result (predicted temperature). The threshold value Xu is determined. In this embodiment, the temperature is set so as to decrease as the temperature of the transfer roller 5 at the start of the printing operation increases. The temperature of the transfer roller 5 at the start of the printing operation can be determined as a segment for each predetermined predicted temperature range such as standard, high temperature, and low temperature, and a threshold value Xu is set in association with each segment. Can do. Note that the threshold value Xu may be set for each finer temperature range, such as continuously changing the threshold value Xu with respect to the temperature of the transfer roller 5 at the start of the printing operation.

  Whether the transfer roller 5 is cooled or warmed before the printing operation can be determined based on the previous printing operation or a history of a plurality of previous printing operations. For example, the history information can be predicted based on the number of images output in the printing operation in the previous or a plurality of previous double-sided printing modes or single-sided printing modes and the elapsed time after the printing operation is completed. When there is a print operation history in the double-sided print mode, the temperature of the transfer roller 5 at the start of the print operation tends to be higher than in the case of only the print operation history in the single-sided print mode. Further, as the number of output images (the number of recording materials P used in the image forming operation) in the previous or multiple previous printing operations increases, the temperature of the transfer roller 5 at the start of the printing operation tends to increase. Further, the shorter the elapsed time from the end of the printing operation, the higher the temperature of the transfer roller 5 at the start of the printing operation. Therefore, the relationship with the temperature of the transfer roller 5 before the start of the printing operation based on a combination of these pieces of information can be examined in advance and stored in the memory 122. That is, the temperature of the transfer roller 5 is the number of recording materials used in the image forming operation performed before the image forming operation for changing the threshold, the print mode of the image forming operation, and the image forming operation. It can be determined based on at least one of the elapsed time since. In this case, the control unit 120 uses the number of recording materials P used (the number of image outputs) at the start (such as time) and the end (such as time) of printing operations sequentially stored in the memory 122 as history detection means. ), The history information can be detected from the print mode information and the like.

  Note that the temperature of the transfer roller 5 is not limited to being predicted by the method described above. For example, a method of using the measured value of the electrical resistance of the transfer roller 5 or directly monitoring the temperature with an infrared sensor or the like may be employed.

  As described above, according to this embodiment, even when the temperature state of the transfer roller 5 at the start of the printing operation is different, the moisture absorbent paper can be detected with high accuracy. Therefore, the frequency of occurrence of conveyance failures such as corner breaks and wrinkles of the recording material P can be reduced according to the history of printing operations. That is, according to the present embodiment, it is more accurately detected that an image is formed on the recording material P having relatively high moisture absorption, regardless of the history of the printing operation, and dehumidification control is executed at an appropriate timing. can do.

  In this embodiment, the threshold value for detecting moisture-absorbing paper is changed between the single-sided print mode and the double-sided print mode as print modes, and the threshold value in each mode is the temperature of the transfer roller 5 at the start of the printing operation. It was corrected (changed) according to. Similarly, the threshold value may be changed according to the fixing mode as the print mode, and the threshold value in each mode may be corrected (changed) according to the temperature of the transfer roller 5 at the start of the printing operation. Further, the principle of this embodiment can be applied to change the threshold for detecting moisture-absorbing paper even when the threshold is not changed according to the print mode.

[Others]
As mentioned above, although this invention was demonstrated according to the specific Example, this invention is not limited to the above-mentioned Example.

  For example, in the above-described embodiment, the detection result of the moisture absorbent paper is fed back to the correction of the temperature setting of the fixing device and the correction of the fan control, but is not limited thereto. For example, an improvement in image performance of the moisture absorbent paper can be expected by feeding back the detection result of the moisture absorbent paper to the correction of the voltage value applied to the transfer unit or the fixing unit. For example, the absolute value of the voltage applied to at least one of the transfer member and the fixing device can be made higher when moisture-absorbing paper is detected than when it is not. Further, by feeding back the detection result of the moisture absorbent paper to the recording material conveyance conditions such as the motor driving speed and the recording material conveyance speed, improvement of the moisture absorption paper conveyance performance can be expected. For example, when moisture-absorbing paper is detected, the recording material conveyance speed can be made slower than when it is not. That is, the control unit can perform control to change any image forming process condition so that the image quality of the output image is closer to that when dry paper is used based on the detection result of the moisture absorbent paper. . In this case as well, the moisture-absorbing paper can be detected more accurately by changing the threshold for detecting the moisture-absorbing paper according to the print mode or the temperature of the transfer member at the start of the printing operation, as in the above-described embodiment. Thus, the process conditions can be changed at an appropriate timing.

  In the above-described embodiment, the transfer member whose electric resistance change amount is detected in order to detect moisture-absorbing paper is a transfer roller that abuts on a photosensitive drum as an image carrier and forms a transfer portion. However, the present invention is not limited to this. The transfer member may be, for example, a transfer member that forms a secondary transfer portion in contact with an intermediate transfer member in an intermediate transfer type image forming apparatus. The intermediate transfer type image forming apparatus is an intermediate transfer body in which a toner image formed on, for example, a photosensitive drum as a first image carrier is formed by, for example, an endless belt as a second image carrier. Primary transfer is performed on the intermediate transfer belt. The toner image primarily transferred to the intermediate transfer belt is secondarily transferred to a recording material such as recording paper by the action of a secondary transfer member which is a transfer member. Then, the recording material on which the toner image is secondarily transferred undergoes a fixing process in the same manner as in the above-described embodiment, and is then discharged out of the main body of the image forming apparatus. As the secondary transfer member, for example, a secondary transfer roller that is in contact with the intermediate transfer belt and forms a secondary transfer portion is used, similarly to the transfer roller in the above-described embodiment. Even in such an image forming apparatus, if moisture absorbent paper is used in the printing operation, the electrical resistance of the secondary transfer roller changes, and therefore, moisture absorbent paper can be detected by detecting this.

  In addition, the transfer member is not limited to a roller-like member, and is, for example, a plate shape (blade shape), a sheet shape, or a brush shape that is disposed so as to contact and rub the moving image carrier. In addition, it may be in any form such as a block shape.

  In the above-described embodiment, the current when the voltage is applied to the transfer member is detected by the current detection means, and the voltage value for obtaining a predetermined current value is obtained, thereby obtaining information on the electrical resistance of the transfer member. Although acquired, it is not limited to this. Since information regarding the electrical resistance of the transfer member only needs to be obtained, a voltage value when a voltage is applied to the transfer member may be detected by a voltage detection unit to obtain a current value for obtaining a predetermined voltage value. For example, when the transfer voltage is controlled at a constant current, a voltage controlled at a constant voltage in the pre-rotation process is applied to the transfer member, the current value at that time is detected, and a desired value at the time of transfer is determined according to the current value. Current value can be obtained. In this case, the detection means can detect information correlated with the electrical resistance of the transfer member by detecting the current value when the voltage controlled by the application means is applied to the transfer member.

  In the above-described embodiment, the fixing unit that fixes the unfixed toner image on the recording material is exemplified as the heating unit. However, the heating unit is not limited to this. For example, in addition to a fixing device that fixes an unfixed toner image on a recording material as a heating unit, the image forming apparatus temporarily fixes the toner image in order to improve the smoothness (glossiness) of the image. There may be a case where a gloss imparting device (image heating device) for reheating the recording material is provided. In such an image forming apparatus, as in the case of the fixing device in the above-described embodiment, the detection result of the moisture absorbent paper can be fed back to the correction of the temperature setting of the gloss applying device.

5 Transfer roller 16 Fixing device 18 Sheet path (post-fixing conveyance path)
F Fan P Recording material

Claims (21)

An image carrier for carrying a toner image;
A transfer member for transferring a toner image from the image carrier to a recording material in a transfer portion by applying a voltage;
Applying means for applying a voltage to the transfer member;
Heating means for heating the recording material to which the toner image has been transferred,
In an image forming apparatus that selectively executes a series of image forming operations on a single or a plurality of recording materials according to one start instruction in a plurality of print modes having different operation settings.
Detecting means for detecting a value correlated with an electric resistance of the transfer member by applying a voltage to the transfer member by the applying means;
Based on the amount of change in the value detected by the detection means when the recording material passes through the transfer portion in the image forming operation, the amount of water vapor generated by heating the recording material by the heating means or the generated water vapor Control means for executing dehumidification control to reduce the amount of
The control means changes a value detected by the detecting means after the transfer is started in the image forming operation from a value detected by the detecting means before the transfer is started in the image forming operation. The dehumidification control is executed when the amount becomes a predetermined threshold value or more in a direction in which the electrical resistance of the transfer member decreases, and the threshold value is changed according to the print mode of the image forming operation. Image forming apparatus.   The image forming apparatus according to claim 1, wherein the control unit changes the threshold according to a plurality of print modes having different heating temperatures of the heating unit.   The control means has a second heating temperature printing mode in which the heating temperature by the heating means is lower than the first heating temperature than the threshold value in the printing mode in which the heating temperature by the heating means is the first heating temperature. The image forming apparatus according to claim 2, wherein the threshold value is reduced.   2. The control unit according to claim 1, wherein the control unit changes the threshold value according to a plurality of print modes having different intervals between recording materials that are continuously conveyed to the transfer unit for the transfer. Image forming apparatus.   The control means includes the threshold value in the print mode having a second interval in which the interval between the recording materials is longer than the first interval than the threshold value in the print mode in which the interval between the recording materials is the first interval. The image forming apparatus according to claim 4, wherein the image forming apparatus is made smaller.   2. The control unit according to claim 1, wherein the control unit changes the threshold value between a single-sided print mode in which an image is formed on one side of a recording material and a double-sided print mode in which an image is formed on both sides of a recording material. Image forming apparatus.   The image forming apparatus according to claim 6, wherein the control unit makes the threshold value in the single-sided printing mode smaller than the threshold value in the double-sided printing mode.   The image according to claim 1, wherein the control unit further changes the threshold according to a temperature of the transfer member before the transfer is started in an image forming operation. Forming equipment.   The control means makes the threshold value when the temperature of the transfer member is a second temperature higher than the first temperature smaller than the threshold value when the temperature of the transfer member is a first temperature. The image forming apparatus according to claim 8.   The control means determines the temperature of the transfer member, the number of recording materials used in the image forming operation performed before the image forming operation for changing the threshold, the print mode of the image forming operation, and the image forming operation. The image forming apparatus according to claim 8, wherein the determination is made based on at least one of elapsed time since the end.   The detection unit detects a value correlated with an electric resistance of the transfer member by applying a voltage to the transfer member by the applying unit when there is no recording material in the transfer unit. The image forming apparatus according to any one of 1 to 10.   The control unit starts the transfer to the first recording material from a value detected by the detection unit before the transfer to the first recording material in the image forming operation to a plurality of recording materials is started. The amount of change in the value detected by the detecting means while the area between the recording material and the recording material conveyed continuously to the transfer portion after passing through the transfer portion is compared with the threshold value. The image forming apparatus according to claim 11.   The detecting means is a voltage value when the voltage controlled by the applying means is applied to the transfer member, or a current value when the voltage controlled by the applying means is applied to the transfer member. The image forming apparatus according to claim 1, wherein the image forming apparatus is detected.   The amount of the generated water vapor is reduced by at least one of lowering the heating temperature by the heating means and widening the interval between the recording materials continuously conveyed to the transfer section for the transfer. The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus.   The amount of the generated water vapor is reduced by inhaling air from the outside of the apparatus main body of the image forming apparatus or by changing the rotation speed of a fan that discharges air from the inside of the apparatus main body to the outside. The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus. An image carrier for carrying a toner image;
A transfer member for transferring a toner image from the image carrier to a recording material in a transfer portion by applying a voltage;
Applying means for applying a voltage to the transfer member;
Heating means for heating the recording material to which the toner image has been transferred,
In an image forming apparatus that executes a series of image forming operations on a single or a plurality of recording materials according to one start instruction,
Detecting means for detecting a value correlated with an electric resistance of the transfer member by applying a voltage to the transfer member by the applying means;
Based on the amount of change in the value detected by the detection means when the recording material passes through the transfer portion in the image forming operation, the amount of water vapor generated by heating the recording material by the heating means or the generated water vapor Control means for executing dehumidification control to reduce the amount of
The control means changes a value detected by the detecting means after the transfer is started in the image forming operation from a value detected by the detecting means before the transfer is started in the image forming operation. The dehumidification control is executed when the amount exceeds a predetermined threshold in a direction in which the electrical resistance of the transfer member decreases, and the threshold of the transfer member before the transfer is started in the image forming operation is performed. An image forming apparatus that changes according to temperature.   The control means makes the threshold value when the temperature of the transfer member is a second temperature higher than the first temperature smaller than the threshold value when the temperature of the transfer member is a first temperature. The image forming apparatus according to claim 16.   The control means determines the temperature of the transfer member, the number of recording materials used in the image forming operation performed before the image forming operation for changing the threshold, the print mode of the image forming operation, and the image forming operation. The image forming apparatus according to claim 16, wherein the determination is made based on at least one of elapsed time since the end. An image carrier for carrying a toner image;
A transfer member for transferring a toner image from the image carrier to a recording material in a transfer portion by applying a voltage;
Application means for applying a voltage to the transfer member,
In an image forming apparatus that selectively executes a series of image forming operations on a single or a plurality of recording materials according to one start instruction in a plurality of print modes having different operation settings.
Detecting means for detecting a value correlated with an electric resistance of the transfer member by applying a voltage to the transfer member by the applying means;
Control means for changing an image forming process condition based on a change amount of a value detected by the detecting means when the recording material passes through the transfer portion in an image forming operation;
Have
The control means changes a value detected by the detecting means after the transfer is started in the image forming operation from a value detected by the detecting means before the transfer is started in the image forming operation. When the amount becomes a predetermined threshold value or more in a direction in which the electric resistance of the transfer member decreases, the image forming process condition is changed from the process condition before the change amount becomes the threshold value or more. Is changed according to the print mode of the image forming operation. An image carrier for carrying a toner image;
A transfer member for transferring a toner image from the image carrier to a recording material in a transfer portion by applying a voltage;
Applying means for applying a voltage to the transfer member;
Heating means for heating the recording material to which the toner image has been transferred,
In an image forming apparatus that executes a series of image forming operations on a single or a plurality of recording materials according to one start instruction,
Detecting means for detecting a value correlated with an electric resistance of the transfer member by applying a voltage to the transfer member by the applying means;
Control means for changing an image forming process condition based on a change amount of a value detected by the detecting means when the recording material passes through the transfer portion in an image forming operation;
Have
The control means changes a value detected by the detecting means after the transfer is started in the image forming operation from a value detected by the detecting means before the transfer is started in the image forming operation. When the amount becomes a predetermined threshold value or more in a direction in which the electric resistance of the transfer member decreases, the image forming process condition is changed from the process condition before the change amount becomes the threshold value or more. Is changed in accordance with the temperature of the transfer member before the transfer is started in the image forming operation.   The image forming process condition is a voltage applied to the transfer unit, a voltage applied to a fixing unit for fixing a toner image on a recording material, or a recording material conveyance condition. Image forming apparatus.

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