JP2015125165A - Image forming apparatus and fixing apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent cold offset at a low cost, while maintaining energy-saving performance of a fixing apparatus.SOLUTION: Heating means of a fixing member (fixing roller 128) includes a plurality of heating areas (thermal heater 156) formed in a longitudinal direction of a fixing member, which is orthogonal to a conveyance direction of a recording medium. Heating control means 142 controls a temperature of the fixing member independently for each of the heating areas, according to information on a position where an unfixed image is formed on the recording medium. A heating temperature of a non-image area is controlled to be lower than a control temperature corresponding to an image area (T2), and a heating temperature of the heating area corresponding to the non-image area is increased before the start of fixation of the image area by predetermined turns of a pressure member, according to a ratio of the non-image area for each of the heating areas (Ts).

Description

  The present invention relates to an image forming apparatus and a fixing device used in the image forming apparatus.

  In an image forming apparatus such as a copying machine, a printer, or a facsimile, a toner image is formed on an image carrier based on image information. The toner image is transferred onto a recording medium such as paper or an OHP sheet, and the recording medium carrying the toner image is passed through a fixing device.

  As a result, the toner image is fixed on the recording medium by heat and pressure. As the fixing device, a heat roller method and a film method are mainly known. Furthermore, an internal heating system having a heat source (such as a halogen heater) inside a heat roller type fixing roller and an external heating system having a heat source outside the fixing roller are known.

  A heat roller type internal heating type fixing device includes a fixing roller heated by a heat source, and a pressure roller that forms a fixing nip portion with the fixing roller. By passing the recording medium carrying the image through the fixing nip portion, the toner is melted by the heat of the fixing roller and fixed to the recording medium by the pressure of the pressure roller.

  The fixing roller is generally composed of a metal core, and the pressure roller is generally composed of a metal core that is covered with an elastic layer such as rubber. A heat roller type fixing device is widely used from the viewpoint of safety, compatibility with high-speed machines, and the like.

  However, a heat roller type internal heating type fixing device has an extremely large heat capacity of the fixing roller. For this reason, the internal heating method has a problem that it takes several minutes to start up until the surface temperature of the fixing roller reaches a predetermined fixing temperature, and the image output operation cannot be performed quickly.

  In this method, in order to execute the image output operation promptly, for example, it is necessary to perform standby standby heat to maintain the fixing roller at a certain temperature even when the fixing device is not used, and there is a problem that too much energy is consumed. there were.

  In recent years, for example, as described in FIGS. 3, 6, 18, and 22 of Patent Document 1 (Japanese Patent Application Laid-Open No. 2001-343860), in order to reduce or reduce the standby heat and realize energy saving. Various external heating methods have been proposed.

  This is based on the idea that the amount of heat for fixing is sufficient only for the heat stored in the vicinity of the surface of the fixing roller, which is higher than the internal heating method in which the entire fixing roller is heated by heating from the inside. There are advantages of short time and low energy loss.

  The fixing device of FIG. 1 (FIG. 12) has a plurality of thermal heaters arranged at equal intervals in the paper width direction, and heats each thermal heater 156 independently according to image information (for convenience, “digital heating” To save energy).

  “Digital heating” means that a plurality of thermal heaters as heating means in the paper width direction are independently controlled to be turned on / off. Therefore, the concept of “digital heating” includes heating means such as a halogen heater that extends a predetermined length in the paper width direction and has a plurality of heating regions in the paper width direction.

  The external heating type fixing device described in FIGS. 3, 6, 18, and 22 of Patent Document 1 (Japanese Patent Laid-Open No. 2001-343860) uses a thermal heater or a heat roller that contacts the outer peripheral surface of the fixing roller. It has a configuration for heating.

  Conventionally, belt and film systems are often used as other systems capable of shortening the rise time of the fixing device and realizing energy saving. In this method, a method of heating the heat insulating roller from the outside and a technique of selectively heating only the image area based on the image information are proposed.

  In the film system, for example, as described in Patent Document 2 (Japanese Patent Laid-Open No. 6-95540), a pressure roller is disposed opposite to a plate-like heating body that is in contact with a thin cylindrical heat-resistant film. . Then, the film and the recording medium are sandwiched between the plate-like heating body and the pressure roller, and heat energy is applied to the recording medium.

  Since the film is as thin as about 100 μm, the start-up time can be substantially increased only by increasing the temperature of the plate-like heating body having a small heat capacity, so that the start-up time can be shortened and preheating power can be reduced.

  Japanese Patent Laid-Open No. 6-95540 further discloses a configuration in which the control temperature and heating area of the heating body are changed in accordance with the image formed on the recording medium. This reduces energy supply to non-image areas (parts where no image is present in the image forming area), thereby enabling energy saving.

  Similarly, in Patent Document 3 (Japanese Patent Laid-Open No. 2005-181946) of a film system, the temperature of each dot-like heating element of the heating element of the thermal head is measured to supply appropriate heat. Thereby, the influence of the ambient temperature is taken into consideration, and only the toner portion on the paper surface is heated to save energy.

(Problems in selective heating of image area by external heating method)
An external heating type fixing device disclosed in Japanese Patent Laid-Open No. 2001-343860 selectively heats an image area for energy saving. Further, in order to increase the surface temperature of the fixing roller in contact with the non-image area to a predetermined fixing setting temperature so as not to be delayed from the sheet passing timing, the second setting temperature lower than the fixing setting temperature is controlled as a target temperature. The structure to perform is disclosed.

  Hereinafter, problems in the fixing device of Patent Document 1 will be described. In the following description, the number of divisions of the heater, which is a heating means, is 2 for the sake of simplicity, and the image area and the heating area are the same. In general, the number of heater divisions is three or more, and the position of the heater does not necessarily match the image position.

  The digital heating fixing device heats the image area (heating area) to which the toner is adhered at the fixing temperature, while the non-image area (non-heating area) is heated at a lower temperature. Ensure energy saving. For this reason, the temperature deviation of the pressure roller (especially in the main scanning direction) becomes larger than that of a normal fixing device.

  It is assumed that there is no heat source such as a heater inside the pressure roller, and the surface temperature is not controlled by a temperature sensor or the like. In this case, the temperature of the pressure roller corresponding to the heating area of the image area rises with the passage of paper due to the heat from the heating roller. On the other hand, the temperature of the pressure roller corresponding to the continuous non-heated area where the non-image area continues in the main scanning direction does not rise (see FIG. 9).

  Accordingly, as in the example shown in FIG. 10, when a print pattern (print image x) having a region without an image pattern (= continuous non-image region) in a position corresponding to the heater B in the main scanning direction is continuously printed, The temperature of the pressure roller decreases.

  If a print pattern (print image z) having an image pattern is printed immediately after that, a “cold offset” occurs. “Cold offset” means that, in the heat roller fixing method, the vicinity of the interface between the toner and the paper is not sufficiently melted so that a part of the toner image is removed by electrostatic adsorption, and is also referred to as a low temperature offset.

  That is, in order to ensure energy saving at the time of continuous printing, control is performed so that the heater B is not lit, so that the pressure roller temperature (T) after continuous printing is a position corresponding to the heater B (see FIG. 11). In the heating region b), T <T ′. Here, T ′ is the pressure roller temperature at which a cold offset occurs when the temperature of the heating roller is controlled to the target temperature.

  On the other hand, even if the printing image z is continuously printed after the printing image y having the heating area continuous in the paper width direction (main scanning direction) and the non-heating area in FIG. 10 is continuously printed, a cold offset occurs. do not do. In the case of a print pattern (print image y) having a continuous image (paper full width image) in the main scanning direction, the temperature of the non-image area corresponding to the paper full width image is controlled to the second target temperature (T2).

  For this reason, the pressure roller temperature (T) after continuous printing is T ′ <T2 <T <T1 in both the heater A part and the heater B part. Note that T1 is a first target temperature of the fixing roller in contact with the full width image of the sheet.

In order to solve the above-mentioned problems, the techniques disclosed in Patent Documents 4 and 5 below are known.
Patent Document 4 (Japanese Patent Laid-Open No. 2012-173462) heats a heating roller using three heater lamps having different heating regions. Image information such as image density is obtained for each page from image data to be printed, and an optimum fixing temperature is provided for each heating region by switching on and off the three heater lamps based on the image information. This improves the power saving effect while improving the fixing performance.

  Japanese Patent Application Laid-Open No. 2012-103672 discloses feedback control of the fixing temperature based on the pressure member temperature so that the post-fixing recording medium temperature becomes constant. This realizes stabilization of fixing quality resulting from the realization of a substantially constant post-fixing recording medium temperature. Reduced excessive energy consumption by changing the fixing member set temperature without making it constant, or for low heat capacity recording media, setting a lower fixing member set temperature than for high heat capacity recording media, etc. Realize.

  The digital heating and fixing apparatus heats an image area (heating area) on which toner is adhered at a fixing temperature, whereas a non-image area (non-heating area) is heated at a lower temperature. While this can ensure energy saving, there is a problem that cold offset occurs.

  That is, when a large number of images are printed in which the non-image area (non-heated area) heated at a low temperature extends in the sub-scanning direction, the temperature deviation of the pressure roller in the main scanning direction increases. In this case, a cold offset may occur when the image pattern is switched after continuous printing.

  In view of the above-described problems, the present invention solves the cold offset problem at low cost while maintaining the energy-saving property of the digital heat fixing device, and the fixing property is uniform even when any image pattern is printed. The purpose is to enable reliable image output.

  In order to solve the above problems, the present invention provides a fixing member between a fixing member that rotates in contact with an unfixed image carried on an image area of a sheet-like recording medium conveyed by a conveying unit, and the fixing member. A pressure member that forms a nip portion; a heating unit that heats the fixing member; and a heating control unit that controls the heating unit, and passing the recording medium through the fixing nip portion. In the fixing device for fixing the unfixed image to the image area, the heating unit has a heating area divided into a plurality of parts in a longitudinal direction of the fixing member orthogonal to a conveyance direction of the recording medium. The heating control means controls the temperature of the fixing member independently for each heating area according to information on the position where the unfixed image is formed on the recording medium, and Image area The heating temperature of the non-image area where no image is formed is controlled to be lower than the control temperature corresponding to the image area, and the fixing of the image area is started according to the ratio of the non-image area for each heating area. In the fixing device, the heating temperature of the heating area corresponding to the non-image area is controlled to rise before the predetermined number of rotations of the pressing member.

  According to the present invention, the heating temperature of the non-image area is controlled to be lower than the control temperature corresponding to the image area, and the predetermined pressure of the pressure member is set more than the start of fixing of the image area according to the ratio of the non-image area for each heating area Since the heating temperature of the heating area corresponding to the non-image area is controlled to rise before the rotation speed, energy saving can be realized, and cold offset, gloss unevenness, fixing unevenness, and the like can be prevented.

1 is a schematic configuration diagram illustrating an image forming apparatus according to an embodiment of the present invention. It is a schematic block diagram which shows an image forming unit. FIG. 2 is a schematic cross-sectional view of a fixing device. FIG. 3 is a perspective view of a main part of the fixing device. It is a block diagram of a control means. (A) is a schematic sectional drawing of the fixing device which concerns on other embodiment, (b) is a top view of a thermal heater. It is a schematic sectional drawing of the fixing device which concerns on other embodiment. (A) (b) is a top view which shows the example of an image formation pattern. (A) (b) is a top view which shows the example of an image formation pattern. 6 is a graph showing a temperature change at an arbitrary reference point on the surface of the fixing roller. It is an example of a print image. It is a graph which shows the example of the temperature transition of a pressure roller (conventional example 1). 3 is a graph showing the relationship between the temperature of a heating roller and the image glossiness. 3 is a graph showing the relationship between the pressure roller temperature and the fixing strength. It is a graph which shows the example of the temperature transition of a pressure roller (conventional example 2). 3 is a flowchart illustrating an operation of the fixing device according to the first exemplary embodiment. 6 is a graph illustrating an example of a temperature transition of a pressure roller of the fixing device according to the first exemplary embodiment. It is a graph which shows the relationship between the temperature of a pressure roller, and a heating required number of sheets. It is a graph which shows the relationship between the number of printed sheets and the temperature of a pressure roller when the paper heat capacities are varied. 6 is a graph showing the relationship between the number of printed sheets and the temperature of a pressure roller when the paper temperature is varied. FIG. 3 is a schematic cross-sectional view of a fixing device in which a temperature detection sensor is disposed on a pressure roller. It is a figure which shows the position of the temperature detection part in a paper. 10 is a flowchart illustrating an operation of a fixing device according to a third exemplary embodiment. It is a figure which shows a control target temperature table.

(Image forming device)
FIG. 1 shows a printer as an image forming apparatus employing one embodiment of the present invention. In FIG. 1, a printer 100 includes four image forming units 1Y, 1M, 1C, and 1K for generating yellow (Y), magenta (M), cyan (C), and black (K) toner images. Yes. Each image forming unit 1 is configured similarly except that it uses yellow toner, magenta toner, cyan toner, and black toner, which are toners of different colors.

  Taking an image forming unit 1Y for generating a yellow toner image as an example, as shown in FIG. 2, the image forming unit 1Y has a photoconductor unit 2Y and a developing unit 7Y. The photosensitive unit 2Y and the developing unit 7Y are configured to be detachable from the apparatus main body of the printer 100 integrally as the image forming unit 1Y. However, when the image forming unit 1Y is detached from the apparatus main body, the developing unit 7Y can be attached to and detached from the photoreceptor unit 2Y.

  An optical writing device 20 is disposed below the image forming unit 1. The optical writing device 20 irradiates the photosensitive drums 3Y, 3M, 3C, and 3K, which are image carriers of the image forming units 1, with laser light L based on the image information.

  As a result, electrostatic latent images for yellow, cyan, magenta, and black are formed on each photosensitive drum 3. In this embodiment, the optical writing device 20 irradiates each photosensitive drum 3 via a plurality of optical lenses and mirrors while deflecting the laser light L emitted from the light source by a polygon mirror 21 that is rotationally driven by a motor. To do. However, instead of this, a device that performs light scanning with an LED array may be employed.

  A first paper feed cassette 31 and a second paper feed cassette 32 are provided below the optical writing device 20 so as to overlap in the vertical direction. In each of the paper feed cassettes 31 and 32, a plurality of sheets of paper Pa as recording media are stored in a bundle of sheets, and the first paper feed roller 31a and second paper Pa are stored in the uppermost paper Pa. The paper feed rollers 32a are in contact with each other.

  When the first paper feed roller 31 a is driven to rotate counterclockwise in FIG. 1 by a driving unit (not shown), the uppermost paper Pa in the first paper feed cassette 31 is fed toward the paper feed path 33. Is done. Further, when the second paper feed roller 32 a is driven to rotate counterclockwise in FIG. 1 by a drive means (not shown), the uppermost paper Pa in the second paper feed cassette 32 is directed toward the paper feed path 33. Be fed.

  A plurality of transport roller pairs 34 are arranged in the paper feed path 33, and the paper Pa fed to the paper feed path 33 is sandwiched between the rollers of the transport roller pair 34 and moves downward in the paper feed path 33. It is transported upward from. A timing roller pair 35 is disposed at the end of the paper feed path 33.

  The timing roller pair 35 temporarily stops its rotation as soon as the paper Pa sent from the conveyance roller pair 34 is nipped, and sends the paper Pa toward a secondary transfer nip described later at a predetermined timing.

  Above each image forming unit 1, a transfer device 40 that is driven to run counterclockwise while stretching the intermediate transfer belt 41 is disposed. In addition to the intermediate transfer belt 41, the transfer device 40 includes a belt cleaning unit 42, a first bracket 43, a second bracket 44, and four primary transfer rollers 45Y, 45M, 45C, and 45K.

  The transfer device 40 includes a secondary transfer backup roller 46, a driving roller 47, an auxiliary roller 48, a tension roller 49, and the like. The intermediate transfer belt 41 is driven to run in the counterclockwise direction by the rotation of the drive roller 47 while being stretched around each roller.

  Each primary transfer roller 45 forms a primary transfer nip by sandwiching the intermediate transfer belt 41 that is driven to travel with each photosensitive drum 3. For example, a positive transfer bias having a polarity opposite to that of the toner is applied to the inner peripheral surface of the intermediate transfer belt 41.

  As the intermediate transfer belt 41 passes through the primary transfer nip for each color as it travels, a yellow toner image, a magenta toner image, a cyan toner image, and a black toner on each photosensitive drum 3 are formed on the outer peripheral surface thereof. Primary transfer is performed so that the images are superimposed. As a result, a four-color superimposed toner image is formed on the intermediate transfer belt 41.

  The secondary transfer backup roller 46 sandwiches the intermediate transfer belt 41 with the secondary transfer roller 50 disposed on the outer peripheral surface side of the intermediate transfer belt 41 to form a secondary transfer nip.

  The timing roller pair 35 feeds the paper Pa toward the secondary transfer nip at a timing synchronized with the four-color superimposed toner image on the intermediate transfer belt 41.

  The four-color superimposed toner image on the intermediate transfer belt 41 is formed between a secondary transfer roller 50 to which a secondary transfer bias is applied and a secondary transfer backup roller 46, and a secondary transfer electric field and nip pressure. Due to the influence, the secondary transfer is collectively performed on the paper Pa in the secondary transfer nip. The four-color superimposed toner image becomes a full-color toner image in combination with the white color of the paper Pa.

  The residual toner remaining on the intermediate transfer belt 41 without being transferred to the paper Pa even after passing through the secondary transfer nip is cleaned by the belt cleaning unit 42. The belt cleaning unit 42 abuts the cleaning blade 42 a on the outer peripheral surface of the intermediate transfer belt 41, thereby scraping off and removing the transfer residual toner on the intermediate transfer belt 41.

  A fixing device 60 is disposed above the secondary transfer nip. The fixing device 60 includes a pressure roller 130 including a heat source such as a halogen lamp and a fixing belt unit 62. The fixing belt unit 62 includes a fixing belt 64, a fixing roller 128 as a fixing member heated by a heating unit, a tension roller 65, a driving roller 66, and the like.

  The fixing belt 64 travels counterclockwise while being stretched by the rollers 65, 66, and 128, and the fixing belt 64 is heated from the back side by the fixing roller 128 in the course of the traveling movement. A pressure roller 130 that is driven to rotate in the clockwise direction is in pressure contact with the fixing roller 128 of the heated fixing belt 64 from the outer peripheral surface side. As a result, a fixing nip portion where the pressure roller 130 and the fixing belt 64 are pressed against each other is formed.

  A temperature sensor (not shown) is disposed on the outer peripheral surface side of the fixing belt 64 so as to face the surface of the fixing belt 64 with a predetermined interval. This temperature sensor is a surface of the fixing belt 64 immediately before entering the fixing nip portion. The temperature is detected, and the detection result is sent to a fixing power supply circuit (not shown).

  Based on the detection result, the fixing power source circuit controls on / off of the power supply to a heat source (not shown) included in the heat source 63a and the fixing roller 128, whereby the surface temperature of the fixing belt 64 is about 140 ° C. Maintained.

  The paper Pa that has passed through the secondary transfer nip is separated from the intermediate transfer belt 41 and then sent to the fixing device 60, and is transported upward from below while being sandwiched by the fixing nip portion in the fixing device 60. Heat is applied by the fixing belt 64 and the fixing roller 128.

  The paper Pa is further pressed to fix the full-color toner image on the paper Pa. The paper Pa on which the fixing process has been performed in this manner is discharged out of the apparatus by a pair of paper discharge rollers 67 and sequentially stacked on a stack unit 68 formed on the upper surface of the apparatus main body.

  Above the transfer device 40, four toner cartridges 19Y, 19M, 19C, and 19K are disposed as toner replenishing means for storing yellow toner, magenta toner, cyan toner, and black toner.

  Each color toner in each toner cartridge 19 is appropriately supplied according to the toner color used in each developing unit 7 in each image forming unit 1. Each toner cartridge 19 is configured to be detachable from the apparatus main body independently of each image forming unit 1.

  A thermistor 71 as an in-machine temperature detecting means for detecting the temperature inside the apparatus main body of the printer 100 is disposed at a portion between the intermediate transfer belt 41 and the fixing device 60. In the vicinity of the toner cartridge 19, a temperature sensor 72 as a toner temperature detecting means is provided.

  The detection results of the thermistor 71 and the temperature sensor 72 are sent to the control means 80 described later. Note that the position of the temperature sensor 72 may be in the vicinity of the toner supply path.

  Returning to FIG. 2 again, the photosensitive unit 2Y includes a photosensitive drum 3Y which is a latent image carrier, a drum cleaning device 4Y, a static eliminator (not shown), a charging device 5Y, and the like. The charging device 5Y uniformly charges the surface of the photosensitive drum 3Y that is driven to rotate clockwise by a driving unit (not shown) (for example, −690 V).

  In the present embodiment, the photosensitive drum 3Y is uniformly charged by bringing the charging roller 6Y, which is rotated counterclockwise while being applied with a charging bias by a power source (not shown), close to the photosensitive drum 3Y. The charging device 5Y is used.

  Instead of the charging roller 6Y, a member that contacts a charging brush or a member that uniformly charges the photosensitive drum 3Y by a charger method such as a scorotron charger may be used.

  The surface of the photosensitive drum 3Y uniformly charged by the charging device 5Y is exposed and scanned by the laser light L emitted from the optical writing device 20, and a yellow electrostatic latent image is formed.

  The surface potential of the photoreceptor 3Y is lowered only in the area exposed by the laser beam L (for example, −50 V), and an electrostatic latent image is formed. The electrostatic latent image is conveyed to the developing area facing the developing roller 12Y of the developing unit 7Y as the photoconductor 3Y rotates.

  The developing unit 7Y includes a first developer circulation transport passage 9Y provided with a first transport screw 8Y, a toner supply port (not shown), a dispersion magnet 101Y (see Japanese Patent Laid-Open No. 2000-89550), and the like. Further, the developing unit 7Y includes a second developer circulation transport path 14Y provided with a second transport screw 11Y, a toner concentration sensor 10Y including a magnetic permeability sensor, a developing roller 12Y, a doctor blade 13Y, and the like.

  The developer circulation transport passages 9Y and 14Y are connected to each other through communication ports at both ends (front and rear ends in FIG. 2), and contain Y developer (not shown) composed of negatively charged Y toner and a magnetic carrier. ing. The first transport screw 8Y is rotated by a driving unit (not shown), and transports the Y developer in the first transport path 9Y from the back side to the front side in FIG.

  The Y developer conveyed to the front side moves to the second conveyance path 14Y through the communication port on the front side. The second transport screw 11Y rotates in the same manner, and transports the Y developer that has entered the second transport path 14Y from the near side to the far side in FIG.

  The Y developer during conveyance is detected by the toner concentration sensor 10Y fixed to the bottom of the second conveyance path 14Y. Above the second conveyance path 14Y, a developing roller 12Y is arranged in parallel with the second conveyance screw 11Y.

  The developing roller 12Y includes a non-magnetic developing sleeve 15Y that rotates counterclockwise in FIG. 2 and a non-rotating magnet roller 16Y that is included in the developing sleeve 15Y. A part of the Y developer conveyed in the second conveyance path 14Y is pumped up to the surface of the developing sleeve 15Y by the magnetic force of the magnet roller 16Y.

  The developing sleeve 15Y is provided with a doctor blade 13Y that is opposed to a minute gap, and the layer thickness (pumping amount) of the pumped Y developer is regulated when passing through the tip of the doctor blade 13Y. Is done. The Y developer that has passed through the tip of the doctor blade 13Y is transported to a developing area facing the photoreceptor 3Y.

  Due to the potential difference between the developing potential (for example, −550 V) applied to the developing sleeve 15Y and the surface potential (for example, −50 V) of the exposed portion of the photoreceptor 3Y, only the Y toner in the Y developer conveyed to the developing area is obtained. It adheres only to the exposed portion of the photoreceptor 3Y.

  Thereby, a Y toner image is formed on the photoreceptor 3Y. The Y developer that has consumed Y toner by this development is returned to the second transport path 14Y. And it is conveyed to the back side of FIG. 2 by the 2nd conveyance screw 11Y, and is moved to the 1st conveyance path 9Y by the communication port of the back side.

  The Y developer returned to the first transport path 9Y is newly supplied with toner at a toner supply port (not shown). A dispersion magnet 101Y for dispersing toner is disposed downstream of the toner replenishing port, and the developer drawn from the conveyance path 9Y by the dispersion magnet 101Y forms a wall (ear). Yes.

  The replenishment toner is blocked by the developer walls (ears) and dispersed in the developer. The Y developer to which the replenishment toner is sufficiently dispersed and supplied is again conveyed to the front side of FIG. 2 by the first conveying screw 8Y, and is supplied to the developing sleeve 15Y via the second conveying screw 11Y as described above. The

  The Y toner image formed on the photoreceptor 3Y is intermediately transferred onto the intermediate transfer belt 41 in FIG. Waste toner remaining on the surface of the photoreceptor 3Y after the intermediate transfer is removed by the drum cleaning device 4Y. The photoreceptor 3Y from which the waste toner has been removed is neutralized by the neutralization device 17Y, and charged by the charging device 5Y in order to perform the next image formation.

The above is the outline of the image forming apparatus as one embodiment of the present invention. Next, an embodiment of the fixing device 60 will be described in more detail.
(Fixing device)
As shown in FIGS. 3A and 3B, the fixing device 60 according to an embodiment of the present invention is an external heating method. In the figure, the fixing belt 64 and the driving roller 66 in FIG. 1 are not shown. The image forming apparatus includes a fixing roller 128, a pressure roller 130 as a pressure member that forms a fixing nip portion SN between the fixing roller 128, and a thermal heater 156 as a heating unit. The thermal heater 156 and the power source 140 constitute an external heating means.

  The thermal heater 156 has a heating region composed of a plurality of (seven in this embodiment) heaters arranged in the width direction of the paper Pa. Each heater is connected in parallel to the power source 140 so that the fixing roller can be heated independently.

  A thermistor 134 as temperature detecting means for detecting the surface temperature of the fixing roller 128 is disposed downstream of the fixing nip SN of the fixing roller 128 and upstream of the thermal heater 156. A thermistor 136 is attached to the thermal heater 156 as temperature detecting means for detecting the temperature.

  Then, detection signals from the thermistor 134 and the thermistor 136 are input to the heating control means 142. The heating control unit 142 controls the power source 140 based on the input signal, and the thermal heater 156 is controlled based on the detection information of the thermistors 134 and 136.

(Heating control means)
The heating control unit 142 includes a microcomputer including a CPU, ROM, RAM, I / O interface, and the like. FIG. 4 is a block diagram of control means including the heating control means 142. The control means 80 captures image information, detection information of the thermistors 134 and 136, and detection temperature information from the thermistor 71 and the temperature sensor 72, respectively. The image information includes information on the position of an unfixed image formed on the paper Pa as a recording medium. And the flowchart of FIG. 15, FIG. 22 mentioned later is performed by the heating control means 142. FIG.

  The control unit 80 takes in speed information of a motor (not shown) that rotationally drives the developing roller 12, and controls the drive system of the printer 100 to switch between continuous operation and intermittent operation. The control means 80 is provided with a ROM 81, and the speed information and the travel distance of the developing roller 12 are stored in the ROM 81 every predetermined time (for example, 10 seconds).

(Fixing roller)
The fixing roller 128 has an aluminum cored bar 128a having an outer diameter of 40 mm and a thickness of 1 mm, and a heat insulating layer 128b coated on the surface of the cored bar 128a. The heat insulating layer 128b is made of silicon rubber and has a thickness of 3 mm. The heat insulating layer 128b may be formed of foamed silicon rubber with less heat escape in order to further enhance its heat insulating function.

  On the heat insulating layer 128b of the fixing roller 128, a good heat conductive layer 128c made of nickel is formed. The good heat conductive layer 128c is not limited to nickel, but may be any iron-based alloy such as stainless steel, metal-based aluminum or copper, graphite sheet, or the like, as long as the thermal conductivity is at least higher than that of the heat insulating layer 128b. By forming the good heat conductive layer 128 c on the fixing roller 128, local temperature unevenness of the surface temperature of the fixing roller 128 due to uneven heat generation of the thermal heater 156 is reduced.

  In addition, the temperature of the region slightly larger than the region heated by the thermal heater 156 is raised by the function of the good heat conductive layer 128c, so that there is an advantage that a slight deviation from the image can be compensated. In other words, there is an advantage that the degree of freedom in design is large in setting the size and interval of each heater constituting the thermal heater 156.

  Further, when the fixing roller 128 is brought into direct contact with the paper Pa without using the fixing belt 64, a release layer 128d may be formed on the surface of the good heat conductive layer 128c. The release layer 128d is formed with a thickness of about 5 to 30 μm using, for example, a fluorine-based resin such as PFA or PTFE. As a result, the durability of the fixing roller 128 can be improved and the releasability can be ensured.

(Pressure roller)
The pressure roller 130 includes an iron cored bar 130a having an outer diameter of 40 mm and a thickness of 2 mm, and an elastic layer 130b coated on the surface of the cored bar 130a. The elastic layer 130b is made of silicon rubber and has a thickness of 5 mm. It is desirable to form a fluororesin layer having a thickness of about 40 μm on the surface of the elastic layer 130b in order to improve the releasability.

  The pressure roller 130 is pressed against the fixing roller 128 by an urging means (not shown). The thermal heater 156 is pressed against the surface of the fixing roller 128 by an urging means (not shown).

(Modification 1 of fixing device)
In the first embodiment described above, the thermal heater 156 is configured to be a contact heating method in which the surface of the fixing roller 128 is brought into contact with the heating. However, the external heating means is configured by, for example, a coil and an inverter, and a non-contact heating method based on the IH method. It is good.

  In the IH system, a heating region and a heating amount are controlled by arranging a large number of heating coils in the longitudinal direction of the fixing roller. Alternatively, a heating region and a heating amount are controlled by arranging a large number of members for canceling the magnetic flux, such as a demagnetizing secondary coil, in the longitudinal direction of the fixing roller.

  The non-contact heating method may be a method in which the external heating means is constituted by a halogen heater and a condensing mirror and the heat of the halogen heater is supplied to the surface of the fixing roller by the condensing mirror instead of the IH method. Also in this method, since the surface temperature of the fixing roller can be controlled based on the image information, the same fixing failure prevention function as described above can be obtained. Further, a method of irradiating the surface of the fixing roller with a heating laser via an optical system may be employed.

(Modification 2 of fixing device)
The fixing device according to the embodiment of the present invention is not limited to the one using the external heating type fixing roller 128 of FIGS. 3A and 3B, but an internal heating type cylindrical fixing belt 138 as shown in FIGS. 5 and 6. It is also possible to use the fixing devices 60A and 60B using the.

  In the fixing devices 60A and 60B using the internal heating method of FIGS. 5 and 6, the thermal heater 156 is a thermal head or a ceramic heater in which a resistance heating element is formed on a plate-like substrate. The thermal heater 156 is disposed inside the fixing belt 138, and the temperature of the fixing belt 138 is raised by the heat to heat and fix the unfixed image conveyed to the fixing nip SN.

  As shown in FIG. 5B, the thermal heater 156 has a heating area that is divided into a plurality of directions perpendicular to the paper conveyance direction, and, similar to FIGS. 3A and 3B, the thermal heater 156 as each heating means. The temperature can be controlled independently.

  The fixing belt 138 includes a SUS base 138a having an outer diameter of 40 mm and a thickness of 40 μm, and an elastic layer 138b coated on the surface of the base 138a. The elastic layer 138b is made of silicon rubber and has a thickness of 100 μm. On the surface of the fixing belt 138, a release layer 138 c having a thickness of 5 to 50 μm is formed with a fluorine-based resin such as PFA or PTFE in order to enhance durability and secure release properties. The base 138a of the fixing belt may be made of resin such as polyimide instead of SUS.

  As shown in FIG. 5A, a support member 161 extending in the axial direction of the fixing belt 138 is disposed inside the fixing belt 138. Most of the inner peripheral side of the fixing belt 138 is slidably supported by the support member 161.

  A pressing member 160 is disposed inside the fixing belt 138 at the fixing nip portion SN. Further, a thermal heater 156 is disposed on the upstream side of the pressing member 160. Then, both end portions in the longitudinal direction of the pressing member 160 and the thermal heater 156 are connected to outer members (not shown) disposed on both sides of the fixing belt 138 to support the fixing belt 138.

  As shown in FIG. 6, the plate-like thermal heater 156 may be arranged at the fixing nip SN so as to function as a pressing member. In this case, the pressing member 160 in FIG. 5A can be omitted, and the number of parts can be reduced.

(Basic operation of fixing device)
The fixing device 60 is configured as described above. As will be described below, the fixing device 60 realizes energy saving by controlling each heater in accordance with image information, as well as cold offset, gloss unevenness, and fixing when an image pattern is switched after continuous printing. Prevent unevenness.

  The heating control unit 142 changes the heating rate of the thermal heater 156 based on image information for forming an image on the paper Pa. An example of this control operation is shown below.

  FIGS. 7A and 7B show an image forming pattern in which an image area a, a non-image area b, and an image area c exist on the paper Pa in order from the leading end side in the transport direction. Fixing is required in the image area a and the image area c, but in the non-image area b other than the image areas a and c, there is no need for fixing because there is no toner to be fixed.

  When image information of the pattern is input to the heating control unit 142 from an image processing device (not shown), the heating control unit 142 controls the fixing roller 128 to a predetermined temperature. That is, the fixing roller 128 is controlled so that the temperature of the part of the fixing roller 128 corresponding to the non-image area b is lower than the temperature of the part of the fixing roller 128 corresponding to the image areas a and a ′. Here, “corresponding” to an image region or a non-image region means a position where the fixing roller 128 is in close contact.

  That is, the power PW1 that provides the fixing temperature is supplied to the entire area of the thermal heater 156 at the site corresponding to the image area a, and the power PW2 that is smaller than the PW1 is supplied at the site corresponding to the non-image area b. Then, at the portion corresponding to the paper rear end image area a ', the electric power PW1 for obtaining the fixing temperature is supplied again.

  At this time, the actual supply power is input so as to preliminarily heat the portion before the image area enters the fixing nip portion as indicated by the shaded portion in the figure. If the thermal heater 156 is heated after the image area enters the fixing nip portion, the rise of the temperature of the thermal heater 156 is delayed, causing problems such as cold offset.

  In consideration of the heat generation length of the thermal heater 156 in the circumferential direction and the heating time of the heat generator itself, a predetermined preheating region is set to prevent cold offset and the like. In addition, it is desirable to make the preliminary heating area the minimum necessary from the viewpoint of energy saving.

  FIG. 8A shows an image forming pattern in which an image region c and a non-image region d exist on the paper Pa in the longitudinal direction of the fixing member perpendicular to the transport direction. Similarly, in this case, the heating control unit 142 applies pressure so that the temperature of the part of the fixing roller 128 corresponding to the non-image area d is lower than the temperature of the part of the fixing roller 128 corresponding to the image area c. The roller 130 is controlled.

  That is, the power PW1 that provides the fixing temperature corresponding to the control target temperature Ts is supplied to the thermal heater 156 at the part corresponding to the image area c, and the supplied power PW2 that is smaller than the power PW1 at the part corresponding to the non-image area d. Reduce. Also in this case, a preheating region indicated by a hatched portion is necessary as described above.

  As a control method, the power supply by the power supply 140 may be completely stopped (off, PW2 = 0) at the portions corresponding to the non-image areas b and d. However, if the temperature of the thermal heater 156 is excessively lowered, the rising of the fixing nip SN to the fixing temperature may not be in time when the next image area arrives due to environmental conditions or the like.

  For this reason, as shown in FIG. 9, the fixing member temperature is kept at the second target temperature (T2) that is lower than the first target temperature (T1) corresponding to the image area but is higher than the room temperature by a predetermined temperature. It is desirable to control it.

  In this way, power is supplied even at the portion corresponding to the non-image area b, but the power supply is reduced. That is, since the supply power in the region P ′ is smaller than the supply power in the region P in FIG. 9, energy saving can be achieved.

  The configuration and basic operation of the image forming apparatus and the fixing device according to the embodiment of the present invention have been described above. Next, Examples 1 to 6 for preventing cold offset, gloss unevenness, and fixing unevenness by the fixing device of the present invention will be described.

Example 1
First, problems in conventional control of the fixing device will be described with reference to FIGS. As a control method from the viewpoint of energy saving, it is conceivable to completely stop (turn off) the power supply at a portion corresponding to the non-image areas c and d.

  However, when the temperature is excessively lowered in the non-image areas c and d and the next image area enters the fixing nip portion by switching the image pattern in the main scanning direction of the print image, the temperature rises to the fixing temperature. May not be in time.

  For this reason, as shown in FIG. 9, the fixing member temperature is kept at the second target temperature (T2) that is lower than the first target temperature (T1) corresponding to the image area but is higher than the room temperature by a predetermined temperature. It is desirable to control it.

  In this case, attention is paid to the image patterns shown in FIGS. FIG. 8A shows an image pattern having a portion where no image is completely present in the sub-scanning direction (belt rotation direction) (hereinafter, such an image is referred to as an “image having a continuous non-image region”). FIG. 8B shows an image pattern in which there is an image in the sub-scanning direction but the image area ratio in the sub-scanning direction is small.

  Consider a case where an image having an image pattern (an image having a large image area ratio in the sub-scanning direction) is printed after the two types of image patterns are continuously passed. Note that the start of the image area can be either within the print job or between print jobs.

  In the following, for the sake of simplification, as shown in FIG. 10, the area is two areas of heater A and heater B, and the heater position and the presence or absence of an image match (the image area matches the heating area, The case where the non-image area coincides with the non-heated area) will be described. A hatched portion indicates an image region, and a white portion indicates a non-image region.

  Consider a case where a print image z is printed after printing a plurality of print images x (for example, 100 sheets). When printing such a print image, in the conventional control, while the print image x is being printed, the heater A portion continues to be heated at T1 because the image exists, whereas the heater B portion has no image.

  Therefore, the power supply is completely turned off, or the temperature is controlled to the second target temperature (T2) lower than T1 in consideration of the rise of the temperature of the fixing member (heating roller) (T2 <T1). . As a result, when the image area comes to the fixing nip portion SN, it can be heated to T1 in a short time in time for the paper feed timing, which is advantageous in energy saving.

  In the former case, that is, when the power supply to the heating area of the heater B is completely turned off and heating is performed immediately before the 101st sheet so that the temperature rise in the heating area is in time, as shown in FIG. The pressure roller temperature converges to a temperature higher than T (cold).

  Here, T (cold) is a cold offset temperature or a fixing lower limit temperature. On the other hand, the pressure roller temperature of the heater B is converged to the sheet temperature. Note that Ts in the figure indicates a control target temperature before the start of printing.

  Here, as shown in FIG. 12, the image gloss tends to increase as the heating roller temperature increases. Further, as shown in FIG. 13, the fixing strength tends to increase as the pressure roller temperature increases.

  The fixing strength rank on the vertical axis in FIG. 13 is an index indicating the amount of toner adhesion when the toner fixed on the paper with a cloth or the like is rubbed. The higher the fixing strength rank, the higher the fixing strength (toner Less adhesion).

  When the heating roller temperature is controlled to T1 as shown in FIG. 11, as described in FIG. 10, the heater A portion is fixed, whereas the heater B portion can cause a cold offset (fixing strength rank 1). There is sex.

  Even when a cold offset does not occur, the variation in the heating roller temperature of the heater B portion becomes large, or the difference in the heating roller temperature between the heater A portion and the heater B portion becomes large. A big difference occurs. As a result, the printed material may be perceived as uneven gloss and feel uncomfortable.

  In the latter case, that is, when the heating region temperature of the heater B is controlled to the second target temperature (T2), as shown in FIG. 14, the pressure roller temperature of the heater A is T (cold). Converge to a higher temperature.

  On the other hand, the pressure roller temperature of the heater B may converge to a temperature lower than T (cold), which causes a cold offset (fixing strength rank 1). Further, even when the temperature converges to a temperature higher than T (cold), the non-image area (heater B portion) that does not need to be heated is always heated, so that energy saving is impaired.

  In response to the problem, the heating temperature of the heating area corresponding to the non-image area is controlled before a predetermined number of rotations of the pressure member before the start of the image area according to the ratio of the non-image area for each heating area. Thus, it is possible to achieve both image quality (fixing strength, gloss unevenness, etc.) and energy saving.

  Specifically, before the start of the image area, the non-image area before the start of the image area and the heating area of the print image after the start of the image area are set so that the pressure roller temperature becomes T (cold) or higher. Heat.

  Here, the start timing of the image area may be matched with the timing of change (non-image → image) within the print image plane (in one page of the print image), or change for each print image page (non-image → image). ). Further, the control may be performed in accordance with a change in the ratio of the average non-image area of a plurality of print image pages (high ratio → low ratio).

(Contrast with conventional technology)
In Patent Document 4 (Japanese Patent Application Laid-Open No. 2012-173462), the pressure roller temperature is not controlled. Therefore, even if the temperature of the heating region of the heating roller can be controlled as intended, the fixing depends on the image pattern. An offset or the like may occur.

  On the other hand, in the embodiment of the present invention, the heating temperature of the non-image area is controlled and the temperature of the pressure roller is controlled according to the switching of the image pattern in the main scanning direction of the print image. As a result, while maintaining low cost and energy saving, a good image output with no unevenness in fixability can be performed regardless of the image pattern printed using the digital heat fixing device.

  In Patent Document 5 (Japanese Patent Laid-Open No. 2012-103672), the temperature of the pressure roller fluctuates because the temperature of the heating roller is changed in order to keep the post-fixing sheet temperature constant. The fixing offset occurs depending on the pressure roller temperature, and the paper glossiness depends on the temperature of the heating roller in contact with the toner. For this reason, even when the paper temperature is constant, fixing offset or gloss unevenness may occur.

  On the other hand, in the embodiment of the present invention, in addition to controlling the temperature of the heating roller using a normal temperature sensor, the heating temperature of the non-image area is controlled, and the temperature of the pressure roller is also controlled. Thereby, while maintaining low cost and energy saving, any image pattern can be printed using the digital heating and fixing device, and good image output without fixing offset or gloss unevenness can be performed.

  Further, as in Patent Document 6 (Japanese Patent Laid-Open No. 2003-280449), the heating means (auxiliary heaters 114 to 116) of the pressure roller precedes in time before the image portion that becomes the heating region reaches the fixing roller. ) Has also been proposed. However, the use of an auxiliary heater increases the cost and also reduces energy savings.

  In the embodiment of the present invention, the heating means of the fixing roller is divided into a plurality of parts in the main scanning direction (thermal heater 156), and only the image area that needs to be heated out of the image area and the non-image area is heated. Cost saving and energy saving can be improved.

(Example 2)
FIG. 15 shows a flowchart of the operation of the fixing device according to the second embodiment of the present invention. When a print job is received at the beginning of this flowchart (S0), it is determined whether or not there is a continuous non-image area from a plurality of print image data (S1). If there is no continuous non-image area, the temperature of the non-image area is set to T2 (S2).

  On the other hand, if there is a continuous non-image area, it is determined whether there is a start of the image area thereafter (S3). If the image area does not start, the power supply 140 is turned off and the continuous non-image area is not heated throughout the job (S4).

  On the other hand, if there is an image area start, the temperature of the pressure roller at the time of image switching is calculated (predicted) from the number of printed sheets having a continuous non-image area and the paper size (S5). Since the current temperature of the pressure roller can be detected by, for example, the temperature sensor 157 in FIG. 20, the image switching is performed by calculating the amount of heat taken by the sheet from the number of printed sheets and the sheet size on the basis of the temperature. It is possible to calculate (predict) the temperature of the pressure roller at the time.

  Next, the required heating number Δs (or required heating time = u × Δs / V) of the non-image area before the start of the image area is calculated from the pressure roller temperature (S6). V is the sheet conveying speed, and u is the sheet length in the conveying direction.

  At that time, the required heating number Δs may be corrected by the type and temperature of the paper. After the above determination and calculation are completed, printing is started (S8). If there are a plurality of different continuous non-image areas in the print image data, the flow of S1 to S7 is performed for each continuous non-image area.

  FIG. 16 shows the transition of the temperature of the pressure roller corresponding to the position of the heater B in FIG. 10 when the operation of the first embodiment is performed. Two temperature transitions are shown, the reached number of printed sheets s1 while the temperature is decreasing and the reached number of printed sheets s2 after the temperature has converged to the sheet temperature.

  The pressure roller temperature in the continuous non-image area (heater B portion) changes as shown by the solid line in FIG. On the other hand, if the power in the heating area is maximized from Δs1 and Δs2 earlier than the reached number of printed sheets s1 and s2, respectively, the pressure roller temperature rises as shown by the oblique broken line. Therefore, at the time of the reached number of printed sheets s1, s2, the temperature can be raised to a temperature equivalent to the control target temperature Ts before the start of printing, which is set sufficiently higher than T (cold).

  The power of the heating area at this time is determined by the maximum value that does not damage the heating roller, the pressure roller, and the printing paper. The pressure roller temperature is determined by the product (power amount) of power and time in the heating region, and the shorter the time, the smaller the heat energy loss. For this reason, it is more advantageous for energy saving to maximize the power in the heating region.

Here, in order to derive the relationship between the required number of sheets Δs and the pressure roller temperature T, the following is expressed.
T: Pressure roller temperature at the number of printed sheets s Ts: Control target temperature before starting printing (> T (cold))
M: Mass of the pressure roller C: Specific heat of the pressure roller W: Power in the heating area V: Paper transport speed u: Paper size (length in the transport direction)

  The power applied to the heating area is used only to recover the amount of heat taken away by the paper. Also, the pressure roller temperature is assumed to increase linearly with the electric power applied to the heating area.

From the law of conservation of heat, the equation W × u × Δs / V = M × C × (Ts−T) holds.
Therefore, the required number of sheets Δs is inversely proportional to the pressure roller temperature T as shown in FIG.
Therefore, the relationship between the temperature T of the pressure roller and the required number of sheets Δs is as shown in FIG.

  The pressure roller temperature transition changes depending on the heat capacity of the paper to be passed, the paper temperature at the time of paper feeding, and the like. For this reason, the required number of sheets Δs for heating in the non-image area before the image switching is not only the number and size of the same image pattern output before the image pattern switching, but is also corrected by the paper type and the paper temperature. The subsequent paper temperature is closer to the control target temperature Ts.

  Finally, the method of predicting and correcting the pressure roller temperature transition may be based on a plurality of actually measured pressure roller temperature data obtained under various conditions obtained through experiments, a heat conduction equation, etc. You may perform prediction and correction | amendment based on the simulation result using.

  As an example, FIG. 18 shows pressure roller temperature data when the heat capacities of the sheets are varied. When the paper temperature is constant, the pressure roller temperature tends to converge to the paper temperature earlier as the paper heat capacity increases. Therefore, Δs = Δs1 increases when the number of printed sheets is small and the paper temperature does not converge (Δs1 ′> Δs1), and Δs = Δs2 when the paper temperature converges does not change (Δs2 ′ = Δs2).

  Further, FIG. 19 shows pressurization temperature data when the paper temperature is varied. When the heat capacity of the paper is constant, regardless of the number of printed sheets, the pressure roller temperature becomes larger as the paper temperature is lower (Δs1 ′> Δs1, Δs2 ′> Δs2).

  By performing the above method, it was confirmed that when the pressure roller temperature at the time of image pattern switching is the control target temperature Ts, there is no occurrence of cold offset and gloss unevenness in the printed image after the image pattern switching.

  In the above description, for convenience of description, the heating timing of the heating unit is assumed to be during the printing operation before the start of the image area, but the second embodiment is not limited to this. Specifically, it will be described later in Examples 4 and 5.

(Example 3)
In the method of the second embodiment (see FIG. 15), the heating required number Δs of the non-image area before the image switching is calculated from the number of printed sheets having the continuous non-image area and the paper size. However, depending on the paper type and paper temperature conditions, the pressure roller temperature cannot be controlled to the control target temperature Ts, and a cold offset or uneven gloss may occur in the printed image.

  In other words, as described above, the pressure roller temperature transition depends not only on the number of printed sheets and paper size having a continuous non-image area, but also on the heat capacity of the paper to be passed and the paper temperature (or environmental temperature) at the time of paper feeding. Change. For this reason, when the paper type and paper temperature conditions change greatly, it is desirable to correct the required heating number Δs of the non-image area before the image switching by the paper type and paper temperature.

  Furthermore, even if correction is performed based on the paper type and the paper temperature, it may be difficult to correct with high accuracy according to various other conditions of the paper, the state of the fixing device, and the paper passing history.

  To address this problem, as shown in FIGS. 20 and 21, a temperature sensor 157 for detecting the pressure roller temperature corresponding to the left and right paper margins is installed in the pressure roller 130, and the paper passage in the fixing nip portion SN is performed. Measure the temperature of the paper margin on the left or right side of the paper. Then, the correction accuracy can be improved by feeding back the temperature data to the power supplied from the power source 140 to the thermal heater 156.

  The reason why the left and right paper margins (temperature detection unit in FIG. 21) are detected is to measure the temperature of the area where the image is not always present and is not heated by the heater, as in the continuous non-image area. In FIG. 21, the temperature detection unit is on the left side in the paper conveyance direction, but the right or left and right sides may be temperature detection units.

  FIG. 22 shows a flowchart of the operation of the fixing device according to the third embodiment. When printing is started at the beginning of this flowchart (S8), the pressure roller temperature of the left and right paper margins (= pressure roller temperature of the non-image area) is detected (S9).

  Next, it is compared whether or not this temperature (= actual temperature) is lower than the predicted temperature (S10). When the temperature is low, heating is performed until the actual temperature becomes equal to the predicted temperature (S11). If it is not low, printing is continued without heating.

  When the number of printed sheets = s−Δs (image switching) (S12), the heater is heated with the maximum power (S13). Then, the print image is switched when the number of prints = s, all the print image data is printed, and the printing is completed (S15).

  When there are a plurality of different continuous non-image areas in the print image data, the flow of S9 to S14 is performed for each continuous non-image area.

  By carrying out the above method, it is possible to maintain the pressure roller temperature at the time of image pattern switching at the control target temperature Ts or higher, and confirm that there is no occurrence of cold offset and gloss unevenness in the printed image after the image pattern switching. did it.

Example 4
The heating timing of the non-image area by the fixing device according to the fourth embodiment is such that the heating area is heated during paper printing over the necessary number of prints Δs before the start of the image area. As a result, during the image printing before the start of the image area, the heating roller corresponding to the image area of the next image is preheated, so that a good image output without fixing offset and gloss unevenness can be achieved without causing downtime. It can be performed.

(Example 5)
The heating timing of the non-image area by the fixing device according to the fifth exemplary embodiment is as follows. Immediately before the start of the image area, the printing (passing) of the paper is stopped and the heating area and the pressure roller are rotated for a predetermined time. Heat.

  Since there is no amount of heat taken away by the sheet, the heating efficiency of the pressure roller is increased. Therefore, less power is applied to the heating means, and the heating time (corresponding to the required number of sheets Δs) can be shortened.

(Example 6)
The fixability of toner on paper varies greatly depending on the image information. For this reason, the control target temperature Ts differs depending on the image information. Here, the image information includes resolution (600 dpi, 1200 dpi, etc.), dither (character portion, photo portion), print mode (monochrome, full color), image area ratio (toner adhesion amount), and the like.

FIG. 23 shows an example in which three control target temperatures Ts are set for such image information. That is, as the control target temperature Ts:
・ Normal fixing temperature (Normal),
A first low fixing temperature (Level 1) lower than a normal fixing temperature by a predetermined temperature (eg, 3 degrees),
A second low fixing temperature (Level 2) that is lower than the first low fixing temperature by a predetermined temperature (for example, 5 degrees).
(Temperature is 8 degrees lower than normal fixing temperature)

  The above three temperatures are set. The image information is stored in the ROM 81 of FIG. 4 as a temperature database (control target temperature table).

  For the image after the image switching, the set control target temperature Ts is corrected in advance from the paper type, machine environment, etc., so that further energy saving can be performed while maintaining the image quality. it can. When the control target temperature Ts differs in the circumferential direction of the fixing roller 128, the highest control target temperature Ts may be set for each main scanning direction.

  Specifically, the table-corrected control target temperature Ts can be used for calculating Δs in S6 of FIG. Note that the correction method using the image information of the control target temperature Ts is not limited to the table correction.

1Y, 1M, 1C, 1K: image forming units 2Y, 2M, 2C, 2K: photoconductor units 3Y, 3M, 3C, 3K: photoconductor drums 4Y, 4M, 4C, 4K: drum cleaning devices 5Y, 5M, 5C, 5K: charging devices 6Y, 6M, 6C, 6K: charging rollers 7Y, 7M, 7C, 7K: developing unit 8Y: first conveying screw 9Y: first developer circulation conveying path 10Y: toner concentration sensor 11Y: second Conveying screw 12Y: developing roller 13Y: doctor blade 14Y: second developer circulation conveying path 15Y: developing sleeve 16Y: magnet roller 17Y: static eliminator 19Y, 19M, 19C, 19K: toner cartridge 20: optical writing device 21 : Polygon mirror 28: Fixing roller 31: First paper feed cassette 31 a: First paper feed roller 32: Second paper feed cassette 32 a: Second Paper roller 33: Paper feed path 34: Transport roller pair 35: Timing roller pair 40: Transfer device 41: Intermediate transfer belt 42: Belt cleaning unit 42a: Cleaning blade 43: First bracket 44: Second bracket 45: Primary transfer Roller 45C: Primary transfer roller 45K: Primary transfer roller 45M: Primary transfer roller 45Y: Primary transfer roller 46: Secondary transfer backup roller 47: Drive roller 48: Auxiliary roller 49: Tension roller 50: Secondary transfer roller 60, 60A, 60B: fixing device 62: fixing belt unit 63a: heat source 64: fixing belt 65: tension roller 66: drive roller 67: paper discharge roller pair 68: stack unit 71: thermistor 72: temperature sensor 80: control means 81 : ROM
DESCRIPTION OF SYMBOLS 100: Printer 101Y: Dispersion mug 128: Fixing roller 128a: Core metal 128b: Heat insulation layer 128c: Good heat conductive layer 130: Pressure roller 130a: Core metal 130b: Elastic layer 134: Thermistor 136: Thermistor 138: Fixing belt 138a: Base 138b: Elastic layer 138c: Release layer 140: Power supply 142: Heating control means 156: Thermal heater 157: Temperature sensor 160: Pressing member 161: Support member A, B: Heater L: Laser light PW1, PW2: Supply power Pa : Paper SN: Fixing nip T: Pressure roller temperature Ts: Control target temperature

JP-A-6-95540 JP 2005-181946 A JP 2001-343860 A JP 2012-173462 A JP 2012-103672 A JP 2003-280449 A

Claims (7)

A fixing member that rotates in contact with an unfixed image carried on an image area of a sheet-like recording medium conveyed by a conveying unit, a pressure member that forms a fixing nip portion with the fixing member, and A heating unit that heats the fixing member; and a heating control unit that controls the heating unit. By passing the recording medium through the fixing nip portion, the unfixed image is formed on the image area of the recording medium. A fixing device for fixing;
The heating unit has a heating region divided into a plurality of longitudinal directions of the fixing member perpendicular to the conveyance direction of the recording medium, and the heating control unit forms the unfixed image on the recording medium. According to the position information, the temperature of the fixing member is controlled independently for each heating area, and the heating temperature of the non-image area of the recording medium where the image area is not formed corresponds to the image area. Corresponding to the non-image area before the start of fixing of the image area, according to the ratio of the non-image area for each heating area, and lower than the control temperature. A fixing device, wherein the heating temperature of the heating region is controlled to increase.   The heating control unit corresponds to the non-image area according to an average ratio of the non-image area calculated from the number and size of images having the non-image area output before the start of fixing of the image area. The fixing device according to claim 1, wherein the heating temperature of the heating area is controlled to be increased. Temperature detecting means for detecting the temperature of the pressure member,
The heating control means increases the heating temperature of the heating area corresponding to the non-image area according to the temperature of the pressure member corresponding to the non-image area at the time of printing obtained by the temperature detection means. The fixing device according to claim 1, wherein the temperature is controlled.   Since the heating control means controls the heating temperature of the heating area corresponding to the non-image area to be increased, during the image printing before the predetermined rotation speed of the pressure member from the start of fixing of the image area, The fixing device according to claim 1, wherein the heating area corresponding to the non-image area is heated.   In order for the heating control means to control the heating temperature of the heating area corresponding to the non-image area to be raised, the sheet is passed before the predetermined number of rotations of the pressure member before the start of fixing of the image area. 4. The heating area corresponding to the non-image area is heated while rotating the fixing member and the pressure member for a certain period of time. Fixing device.   The heating control unit performs temperature increase control while correcting the heating temperature of the heating area corresponding to the non-image area according to image information of a print image output after the start of fixing of the image area. The fixing device according to any one of claims 1 to 5.   An image forming apparatus comprising the fixing device according to claim 1.