JP2008111949A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus having a fixing device in which the surface temperature of a heating roller is detected using a non-contact type thermistor, the image forming apparatus being capable of safely and surely preventing fixing error due to undershoot without complicating the control mechanism of the fixing device and increasing the cost of the device. <P>SOLUTION: When detected temperature by a center thermistor 37 reaches control temperature 160°C after starting warming-up, such second control is performed that power-on/off of a halogen heater 28 is forcibly repeated by setting a rate (duty ratio) of on-time in one power on/off cycle as a predetermined value at which the temperature of a roller falls if the temperature of the roller is lower than predetermined temperature (for example, 120°C) and printing data included in a printing output signal shows the predetermined number of sheets or more when receiving a printing signal. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an image forming apparatus such as a copying machine, a facsimile, and a printer having a fixing device of a heat roller fixing system having a heating roller and a pressure roller, and in particular, a temperature for maintaining a fixing temperature constant. It is about control.

  In a conventional image forming apparatus using an electrophotographic process, the charging is partially attenuated by irradiating a laser beam from an exposure unit onto an image carrier such as a photosensitive drum uniformly charged by a charging unit. A predetermined electrostatic latent image is formed, and a toner is attached to the electrostatic latent image by a developing unit to form a toner image (visible image). Then, the toner image is transferred onto a recording medium using a transfer unit, An image forming process is performed in which a non-fixed toner is heated and pressed by a fixing device to form a permanent image. Further, as a fixing device, a heat source is incorporated in at least one of a pair of fixing rollers forming a nip to form a fixing roller (heating roller), and a recording medium carrying an unfixed toner image in the nip portion of the roller pair A heat roller fixing system in which toner is fixed by inserting the toner is widely used.

  FIG. 6 shows the configuration of a heat roller fixing type fixing device. The fixing device 101 includes an upper housing 101a and a lower housing 101b. The upper housing 101a stores a heating roller 102a that rotates clockwise in the figure, and the lower housing 101b counterclockwise. A rotating pressure roller 102b is accommodated. A heater 103 is built in the heating roller 102a, and the pressure roller 102b is pressed against the heating roller 102a with a predetermined pressure by a biasing means (not shown). The heating roller 102a and the pressure roller 102b constitute a fixing roller pair 102 that fixes unfixed toner on the paper passing through the fixing nip N.

  The toner image is transferred by a transfer roller (not shown), and the sheet conveyed from the right direction in FIG. 6 is carried into the fixing device 101 from the upstream inlet 105a. A fixing approach guide 106 is provided at the introduction port 105 a, and the leading edge of the sheet is guided to the fixing roller pair 102 along the fixing approach guide 106 and passes through the fixing nip portion N of the fixing roller pair 102. At this time, the toner image on the paper is made a permanent image by being heated and pressurized at a predetermined temperature and pressure. Thereafter, the sheet is conveyed to the outside of the fixing device 101 from the downstream discharge port 105b and discharged to the outside of the image forming apparatus by a discharge roller (not shown). In the fixing device 101 configured as described above, in order to keep the surface temperature of the heating roller 102a constant, the thermistor 104 that contacts the heating roller 102a and detects the surface temperature is provided.

  Conventionally, contact type thermistors have been widely used as temperature detection elements for fixing roller pairs. However, contact type thermistors have high detection accuracy, but they are used in contact with the roller surface. The friction between the roller and the thermistor causes the surface of the roller to be scratched or scraped, and the fixing performance of that portion is lowered, causing problems such as vertical stripes and poor fixing. In addition, there is a problem of false detection due to adhesion of unfixed toner or paper dust, and there is a method of providing a thermistor cleaning means to prevent this, but the warm-up time during continuous paper passing increases due to the cleaning operation, This also leads to an increase in the cost of the device.

  On the other hand, when a contact thermistor is brought into contact with the non-sheet passing area, there is little problem with the durability of the roller. The control conditions need to be set finely according to the size, thickness, etc., and there is a problem in terms of the stability of the roller surface temperature when a paper of an unexpected size is passed. In addition, it is difficult to provide a margin for control with respect to changes in external factors such as the heat distribution characteristics of the heater at the center and end of the roller, a decrease in supply voltage, and the environment outside the machine. There is also a design problem that it is necessary to review the settings again if the models are different or if there is a design change even if they are the same model.

  Therefore, for example, a method is used in which a non-contact thermistor that detects the temperature by infrared rays is disposed opposite to the paper region and the radiant heat from the roller surface is detected. According to this method, the temperature stability of the sheet passing area on the roller surface during continuous sheet passing can be secured, and the above-described problems caused by the contact of the thermistor do not occur. A method of controlling the roller surface temperature more precisely by combining a contact thermistor and a non-contact thermistor has also been proposed.

  However, when temperature control is performed using a non-contact type thermistor, since the radiant heat from the roller surface is detected, there arises a problem of responsiveness of the thermistor that the detected temperature changes behind the actual change of the roller surface temperature. Specifically, when the roller surface temperature is lower than the fixing temperature at the start of printing, the heater is fully turned on (ON) and the roller is heated until the temperature detected by the non-contact thermistor reaches a predetermined control temperature. The detected temperature of the non-contact thermistor follows the actual roller surface temperature with a delay.

  That is, as shown in FIG. 7, when the detected temperature reaches the control temperature (160 ° C.), so-called overshoot occurs in which the actual roller surface temperature rises above the target temperature (180 ° C.). At that time, the energization to the heater is cut off (OFF), and the roller surface temperature starts to decrease, but the detection temperature continues to rise for a while and the energization to the heater remains interrupted until it falls to the control temperature with a delay. Become. As a result, a so-called undershoot (in the broken line circle in the figure) occurs in which the actual roller surface temperature is lower than the target temperature.

  When printing is performed in this state, the actual roller surface temperature is lower than the target temperature, and the paper is passed in a state where no heat is supplied by the heater until the temperature detected by the non-contact thermistor is lowered to the control temperature. Therefore, there is a high possibility that fixing failure will occur.

In Patent Document 1, when the roller surface temperature in the non-sheet passing area is lower than a predetermined value, the heater is forcibly turned on regardless of the roller surface temperature in the sheet passing area. A method for preventing undershoot of the roller end (non-sheet passing region) temperature is disclosed. However, in the control of Patent Document 1 in which the heater is forcibly turned on based on the roller surface temperature in the non-sheet-passing area, the roller end temperature caused by the temperature difference between the sheet-passing area and the non-sheet-passing area immediately after the warm-up. Although undershoot could be prevented, it was not possible to effectively prevent undershoot caused by non-contact thermistor response.
Japanese Patent Laid-Open No. 10-301436

  In view of the above problems, the present invention provides a fixing device that detects the surface temperature of a heating roller using a non-contact type thermistor, and does not undershoot without complicating the control mechanism of the fixing device and increasing the cost of the device. An object of the present invention is to provide an image forming apparatus capable of safely and reliably preventing a fixing defect caused by the fixing failure.

  In order to achieve the above object, the present invention has a fixing roller pair comprising a heating roller having heating means disposed therein and a pressure roller that is disposed opposite to the heating roller and forms a nip through which a sheet is inserted. A fixing device that fixes unfixed toner on the paper, a first detection unit that is disposed to face the paper passing area of the heating roller and detects the surface temperature of the heating roller in a non-contact state, and Control means for controlling energization to the heating means according to a detection result, and when the detection result of the first detection means reaches a predetermined control temperature, ON / OFF of energization to the heating means is turned on. In the image forming apparatus that performs the first control to switch, the control means receives a predetermined number or more of continuous prints at the time of warm-up, and the surface temperature of the heating roller when a print start signal is received is predetermined. When it is below, from the time when the detection result of the first detection means first rises above the control temperature to the time when it falls again to the control temperature or until the end of continuous printing, whichever comes first During this period, the second control is repeatedly performed to turn on / off the current according to a predetermined DUTY ratio at which the surface temperature of the heating roller decreases.

  In the present specification, the ratio of the ON time in the ON / OFF 1 cycle in the case where the energization ON / OFF control is performed is referred to as a DUTY ratio.

  According to the present invention, in the image forming apparatus configured as described above, the DUTY ratio used for the second control is based on a temperature difference between the surface temperature of the heating roller and the control temperature when a print start signal is received. It is characterized by being set.

  According to the present invention, in the image forming apparatus having the above-described configuration, a second detection unit that detects a surface temperature of the heating roller in a contact state is disposed in a non-sheet passing region of the heating roller. The surface temperature of the heating roller when the printing start signal is received using the means is detected.

  According to the first configuration of the present invention, the energization is turned on / off at a predetermined DUTY ratio such that the surface temperature of the heating roller decreases even after the detection result of the first detection means has increased to the predetermined control temperature. By executing the second control which is forcibly repeated, the roller surface temperature gradually falls as compared with the conventional control in which the power supply to the heating means is completely turned off. Therefore, it is possible to effectively prevent the occurrence of fixing failure due to the roller temperature undershoot immediately after warming up. In addition, when the detection result of the first detection unit is lowered to the control temperature again, or when the continuous printing is finished, the second control for forcibly energizing is required by ending the second control. Thus, the safety of the apparatus can be secured.

  According to the second configuration of the present invention, in the image forming apparatus having the first configuration, the second control is performed based on the temperature difference between the surface temperature of the heating roller and the control temperature when the print start signal is received. By setting the DUTY ratio to be used, it is possible to set an optimal DUTY ratio considering both the effect of preventing undershoot and the safety against temperature rise.

  According to the third configuration of the present invention, in the image forming apparatus having the first or second configuration, printing is started using the contact-type second detection unit arranged in the non-sheet passing area of the heating roller. By detecting the surface temperature of the heating roller when receiving the signal with high accuracy, stable temperature control can be performed regardless of the operating state of the apparatus and the use environment.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic configuration diagram of an image forming apparatus provided with a paper feed mechanism of the present invention. In the image forming apparatus (here, a color printer) 100, when performing a copying operation, the photosensitive drum 1 that rotates counterclockwise in the drawing is uniformly charged by the charging unit 2 in the apparatus main body. The exposure unit 3 irradiates the photosensitive drum 1 with a laser beam based on the original image data input to an image input unit (not shown) from a personal computer or the like, and the electrostatic latent image on the photosensitive drum 1. Is formed.

  The photosensitive drum 1 is formed, for example, by laminating a photosensitive layer on an aluminum drum, and the surface is charged by a charging unit 2. Then, an electrostatic latent image in which charging is attenuated is formed on the surface that has received the laser beam from the exposure unit 3. As the photosensitive material for forming the photosensitive layer, an amorphous silicon photoreceptor or a positively charged organic photoreceptor (positive OPC photoreceptor) is used. When positive OPC is used as the photosensitive layer, the generation of ozone or the like is small and charging is stable. In particular, the positive OPC having a single-layer structure has little change in photosensitive characteristics even when the film thickness changes after long-term use. Since the image quality is stable, it is preferably used in a system having a long life.

  Reference numeral 4 denotes a rotary developing unit that supplies toner onto the photosensitive drum 1. The developing unit 4 includes cartridge-type yellow, magenta, cyan, and black developing devices 4a, 4b, 4c, and 4d in which a developing device and a toner container are integrated, and the developing devices 4a to 4d are photoreceptors. By sequentially rotating to a position facing the drum 1, positive polarity toner is attached to the electrostatic latent image on the photosensitive drum 1 to form toner images of each color.

  Reference numeral 5 denotes an intermediate transfer belt onto which a toner image is transferred. The intermediate transfer belt is passed over intermediate transfer rollers 6a and 6b, a belt driving roller 8 and a driven roller 9, and is in contact with the photosensitive drum 1 by a driving means (not shown). Rotate clockwise. A sheet made of dielectric resin is used for the intermediate transfer belt 5, and a belt in which both ends thereof are overlapped and joined to form an endless shape, or a belt without a seam (seamless) is used.

  When the user inputs image formation start, a yellow toner image is formed on the photosensitive drum 1 at a predetermined timing. The yellow toner image on the photosensitive drum 1 is transferred onto the intermediate transfer belt 5 by the intermediate transfer rollers 6a and 6b to which a negative transfer bias is applied (primary transfer). Thereafter, the toner remaining on the surface of the photosensitive drum 1 is removed by the cleaning roller 7a and the cleaning blade 7b, and the developing unit 4 rotates by a predetermined amount (here, 90 °), and this time, the magenta toner image is the same as described above. Are formed on the photosensitive drum 1 and transferred onto the intermediate transfer belt 5.

  Thereafter, cyan and black toner images are transferred onto the intermediate transfer belt 5 from the photosensitive drum 1 by the same method as described above. These four color images are formed with a predetermined positional relationship predetermined for forming a predetermined full-color image. A transfer roller 15 is positioned below the intermediate transfer belt 5, and a belt cleaning blade 16 removes residual toner on the surface of the intermediate transfer belt 5.

  The sheet P is fed from the paper feed mechanism (paper feed cassette) 10 to the pickup roller 11, the paper feed roller pair 12, the transport rollers 13a and 13b, and the registration roller pair toward the intermediate transfer belt 5 on which the toner image is formed as described above. 14, a full-color toner image sequentially formed on the surface of the intermediate transfer belt 5 is transferred onto the paper P at a time by the transfer roller 15 to which a negative transfer bias is applied (secondary transfer). ). Then, the sheet P on which the toner image is transferred is conveyed to a fixing device 18 having a fixing roller pair 17 including a heating roller 17a and a pressure roller 17b, and the toner image is fixed. The paper P that has passed through the fixing device 18 is discharged to a discharge tray 21 via a paper conveyance path 19 and a discharge roller pair 20.

  FIG. 2 is a side cross-sectional view showing a schematic configuration of the fixing device in FIG. 1, and an arrow A indicates a sheet conveyance direction. As shown in FIG. 2, the fixing device 18 includes a heating roller 17a, a pressure roller 17b, and a housing 25 that rotatably supports these rollers. The housing 25 includes a heating roller housing 25a and a pressure roller housing 25b. The housing 25a covers the heating roller 17a, and the housing 25b covers the pressure roller 17b. An introduction port 26 for introducing the paper P into the fixing device 18 is provided on the upstream side of the housing 25 in the paper conveyance direction, and the introduction port 26 is introduced on the downstream side of the housing 25 in the paper conveyance direction. A discharge port 27 for discharging the paper is provided.

  As the heating roller 17a, for example, a so-called hard roller is used in which a cylindrical cored bar made of a metal such as aluminum or iron having excellent thermal conductivity is simply coated with a fluororesin coating or a tube. Further, a halogen heater 28 as a heat source is provided inside the cored bar of the heating roller 17a. As the pressure roller 17b, an elastic layer such as silicon rubber is formed on a cylindrical base material made of synthetic resin, metal or other material, and the surface of the elastic layer is excellent in releasability such as fluororesin. Those covered with resin are used.

  The heating roller 17a and the pressure roller 17b are attached to the housings 25a and 25b by bearings (not shown) provided at both ends so as to be orthogonal to the conveyance direction (arrow A direction) of the paper P. Each is supported rotatably. The pressure roller 17b is brought into pressure contact with the heating roller 17a with a predetermined pressure and is driven to rotate by the heating roller 17a. The sheet P conveyed to the pressure contact portion (fixing nip portion) between the heating roller 17a and the pressure roller 17b is heated and pressed to melt and fix the powdered unfixed toner. .

  Next, temperature detection means for detecting the surface temperature of the heating roller 17a will be described. As the temperature detection means, a center thermistor 37 that detects the surface temperature of the substantially central region (sheet passing region) in the width direction and an edge thermistor 38 that detects the surface temperature of the end region in the width direction (non-sheet passing region) are arranged. It is installed. Since the center thermistor 37 is disposed on the paper passing area of the heating roller 17a, if the center thermistor 37 is disposed in contact with the heating roller 17a, the surface of the heating roller 17a is scratched or scraped by friction with the center thermistor 37, and the fixing of the portion is performed. There is a risk that the performance may be reduced, causing vertical stripes, poor fixing, and the like. For this reason, a non-contact thermistor such as an infrared temperature-sensitive thermistor that is disposed at a predetermined interval from the surface of the heating roller 17a is used.

  On the other hand, since the edge thermistor 38 is disposed in the non-sheet passing region of the heating roller 17a, a contact type thermistor that can contact the surface of the heating roller 17a and detect the surface temperature with high accuracy is used.

  FIG. 3 is a block diagram showing a control path of the image forming apparatus of the present invention. Portions common to FIGS. 1 and 2 are denoted by the same reference numerals and description thereof is omitted. The image forming apparatus 100 includes an image input unit 30, an AD conversion unit 31, an image forming unit 32, a control unit 33, a storage unit 34, an operation panel 35, a center thermistor 37, an edge thermistor 38, a paper feed mechanism 10, and a fixing device 18. It is the composition which includes.

  When the image forming apparatus 100 is a copying machine, the image input unit 30 is a scanning optical system equipped with a scanner lamp that illuminates an original during copying and a mirror that changes the optical path of reflected light from the original, and reflected light from the original. The image forming apparatus 100 is a printer as shown in FIG. 1. The image forming apparatus 100 includes a condenser lens that collects and forms an image and a CCD that converts the imaged image light into an electrical signal. In this case, the reception unit receives image data transmitted from a personal computer or the like. The image signal input from the image input unit 30 is converted into a digital signal by the AD conversion unit 31 and then sent to an image memory 60 in the storage unit 34 described later.

  The image forming unit 32 includes the photosensitive drum 1, the charging unit 2, the exposure unit 3, the developing unit 4, the intermediate transfer belt 5, the transfer roller 15, and the like. For example, when forming a full-color image, the image forming unit 32 is converted by the AD conversion unit 31. An electrostatic latent image is formed on the photosensitive drum 1 based on the digital signal, and is developed into a toner image of each color by the developing unit 4. This is sequentially transferred to a predetermined position on the intermediate transfer belt 5 to form a full-color image, and then transferred onto the sheet conveyed from the sheet feeding mechanism 10 by the transfer roller 15 at a time. Details of the image forming process have already been described with reference to FIG.

  The storage unit 34 includes an image memory 60, a RAM 61, and a ROM 62. The image memory 60 stores an image signal read by the image input unit 30 and digitally converted by the AD conversion unit 31, and is stored in the control unit 33. Send it out. The RAM 61 and the ROM 62 store the processing program and processing contents of the control unit 33.

  The RAM 61 (or ROM 62) stores the ON / OFF cycle of the current used for temperature control of the halogen heater 28 (see FIG. 2) in the heating roller 17a corresponding to the roller surface temperature at the start of warm-up. Has been. This ON / OFF cycle is read out and sent to the control unit 33 when second control described later is executed.

  The operation panel 35 includes an operation unit composed of a plurality of operation keys and a display unit (none of which is shown) for displaying setting conditions, apparatus statuses, and the like. In addition to inputting a start command, when the image forming apparatus 100 has a facsimile function, it is also used for various settings such as registering a facsimile transmission destination in the storage unit 34 and further reading and rewriting the registered transmission destination. The

  The control unit 33 generally controls the image input unit 30, the image forming unit 32, the paper feed mechanism 10, the fixing device 18, a motor and a clutch (not shown) for driving them according to the set program, and the image. The image signal input from the input unit 30 is converted into image data by scaling processing or gradation processing as necessary. The exposure unit 3 irradiates a laser beam based on the processed image data to form a latent image on the photosensitive drum 1. Furthermore, the control unit 33 also has a function of controlling the surface temperature of the heating roller 17a based on the detection result of the surface temperature of the heating roller 17a detected by the center thermistor 37 and the edge thermistor 38.

  FIG. 4 is a graph showing changes in the surface temperature of the heating roller and changes in the detected temperature of the center thermistor when the fixing temperature control of the present invention is executed. The control operation of the surface temperature of the heating roller 17a (hereinafter simply referred to as roller temperature) when the image forming apparatus warms up will be described with reference to FIGS. When a print output signal is transmitted to the control unit 33, a drive signal is transmitted from the control unit 33 to the fixing device 18, and at the same time, the energization of the halogen heater 28 is turned on to raise the roller temperature to the target temperature (here, 180 ° C.). Let And until the detected temperature of the center thermistor 37 reaches control temperature (here 160 degreeC), it is the same as that of the past (refer FIG. 7).

  Next, the temperature detected by the center thermistor 37 is based on the condition (trigger) that the roller temperature is lower than a predetermined temperature (for example, 120 ° C.) at the start of printing and that the print data included in the print output signal is a predetermined number or more. When the temperature reaches 160 ° C., which is the control temperature, that is, from the time when the energization to the halogen heater 28 is turned off in order to execute the normal ON / OFF control (hereinafter referred to as the first control) in FIG. Control for forcibly repeating ON / OFF of energization to the halogen heater 28 by setting the ON time ratio (DUTY ratio) in the ON / OFF 1 cycle to a predetermined value at which the roller temperature decreases (hereinafter referred to as second control). ).

  Specifically, for example, by setting the DUTY ratio to 60%, the roller temperature gradually decreases compared to the conventional control (broken line in the figure) in which the energization to the halogen heater 28 is turned off (DUTY ratio 0%). To do. Accordingly, since the temperature detection of the center thermistor 37 can follow the decrease in the roller temperature with a relatively small time lag, a significant decrease (undershoot) in the surface temperature of the sheet passing region due to the detection delay of the center thermistor 37 is suppressed. This makes it possible to prevent the occurrence of fixing failure during warm-up.

  The duration of the second control is not set to a predetermined time set in advance, but until the time when the detected temperature of the center thermistor 37 is lowered to the control temperature, and thereafter, the first control is shifted to. As a result, switching to the first control is always performed at the control temperature, and the problem of undershoot at the time of control switching does not occur. Further, when printing is finished before the detected temperature falls to the control temperature, the second control is also finished at that time.

  Here, when the DUTY ratio in the second control is too small, the roller temperature is drastically lowered as in the case where the first control is performed from the beginning and the current supply to the halogen heater 28 is turned off. To do. On the other hand, if the DUTY ratio in the second control is too large, the roller temperature rises conversely, and the temperature detected by the center thermistor 37 does not drop to the control temperature, so that the temperature control of the present invention is not established.

  Therefore, the DUTY ratio in the second control needs to be set in a range in which the temperature detected by the center thermistor 37 is surely lowered and the roller temperature is not suddenly lowered. The appropriate range of the DUTY ratio varies depending on the specifications of the image forming apparatus, particularly the specifications of the heating roller 17a and the halogen heater 28, and may be appropriately set according to the heating roller 17a and the halogen heater 28 to be used.

  Further, as the energization ON / OFF cycle in the second control is shortened, the roller temperature decreases more slowly, but the load on the control unit 33 also increases. Therefore, it is desirable to set the time per ON / OFF cycle to such an extent that the roller temperature does not decrease rapidly and does not cause an excessive load on the control unit 33.

  In the case where single printing is repeatedly performed, or even when continuous printing is performed at a predetermined number or less, the halogen heater is used until the detected temperature of the center thermistor 37 reaches the control temperature and then the detected temperature decreases again to the control temperature. Even with only the first control to turn off the power to 28, the sheet finishes passing through the fixing roller pair 17 before the undershoot of the roller temperature occurs, so that no fixing failure occurs.

  Further, when the roller temperature is high to some extent when the print start signal is received and the temperature difference from the target temperature is small, it is not necessary to rapidly increase the roller temperature, and therefore no undershoot occurs when the roller temperature decreases. Furthermore, in consideration of the safety of the device, it is safer to suppress the second control for forcible energization to the minimum necessary. For this reason, it is desirable to perform the second control under the condition that the roller temperature when the print start signal is received is equal to or lower than a predetermined value and the continuous printing is equal to or higher than the predetermined number.

  FIG. 5 is a flowchart showing the fixing temperature control procedure of the present invention. The roller temperature control during image formation will be described with reference to FIGS. First, when the control unit 33 receives a print output signal transmitted by the operation of the operation panel 35 (step S1) and image formation is started, the roller temperature (hereinafter referred to as the sheet temperature) of the sheet passing region by the center thermistor 37 and the edge thermistor 38 is started. Detection of the detection temperature T1) and the roller temperature in the non-sheet-passing area (hereinafter, detection temperature T2) is started (step S2), and simultaneously warm-up is started by energizing the halogen heater 28 (step S3).

  Next, it is determined whether or not the detected temperature T1 of the center thermistor 37 is equal to or higher than the control temperature 160 ° C. (step S4). When T1 is 160 ° C. or higher, the actual roller temperature also reaches the target temperature of 180 ° C. Then, it is determined whether or not the continuous printing is greater than or equal to a predetermined number (step S5), and in the case of single-shot printing or when the number of printed sheets is equal to or less than the predetermined number, all printing is completed before undershoot occurs. Therefore, the temperature control is not performed and the energization is turned off (step S6), printing is performed as it is, and the process is terminated.

  If the number of continuously printed sheets is equal to or greater than the predetermined number in step S5, it is then determined whether or not the detected temperature T2 of the edge thermistor 38 is equal to or lower than a predetermined temperature (for example, 120 ° C.) in step S1, which is the start of warm-up. Step S7). When T2 is 120 ° C. or higher, undershoot does not occur even if the first control is performed from the beginning, so that the process proceeds to (1) and the energization to the halogen heater 28 is turned off (step S8). Then, it is determined whether or not printing has been completed (step S9). If the printing has been completed, the process proceeds to (3) and the process is terminated. When printing is continuing, it is determined whether or not the detected temperature T1 has decreased to 160 ° C. or lower (step S10). When the temperature has decreased to 160 ° C. or lower, energization to the halogen heater 28 is resumed (step S11).

  Then, it is determined whether or not printing is completed (step S12). If printing is completed while the halogen heater 28 is energized, the process proceeds to (4), the power is turned off (step S6), and the process is terminated. Further, when printing is in progress, it is determined whether or not the detected temperature T1 has risen to 160 ° C. or higher (step S13), and when it has risen to 160 ° C. or higher, the flow returns to (1) to energize the halogen heater 28 again. Is turned off (step S8), and thereafter, the first control is repeated until printing is completed (steps S9 to S13).

  On the other hand, when T2 is 120 ° C. or lower in step S7, it is necessary to prevent the occurrence of undershoot during continuous printing. Therefore, the DUTY ratio during energization is determined based on the detected temperature T2 during warm-up (step S14). Specifically, the DUTY ratio is set corresponding to the detected temperature T2 such that 60% when T2 is less than 80 ° C and 30% when T2 is less than 120 ° C and stored in a table. 34 is stored. Then, the DUTY ratio corresponding to the actual detected temperature T2 is read and transmitted to the control unit 33. The controller 33 repeats ON / OFF of the energization to the halogen heater 28 at the determined DUTY ratio (step S15).

  Thereafter, it is determined whether or not T1 has decreased to 160 ° C. or less (step S16). If T1 has not decreased to 160 ° C., it is determined whether or not printing has been completed (step S17), and the processing has ended. In this case, the energization is turned off (step S6) and the process is terminated. If printing is continuing, the process returns to step S15 to continue the second control (steps S15 to S17).

  When T1 becomes 160 ° C. or lower in step S16, the process proceeds to (2), and the energization is continued by switching from the second control to the first control (step S11). Thereafter, the first control is repeated until printing is completed (steps S12 to S11).

  In the control of FIG. 5, the detection temperature T2 of the edge thermistor 38 is used as the roller temperature at the start of warm-up. For example, the control is performed using only the detection temperature T1 of the center thermistor 37 without providing the edge thermistor 38. You can also However, since the center thermistor 37 is a non-contact type thermistor, the detection accuracy may vary depending on the operation state of the apparatus and the ambient environmental temperature. Therefore, it is preferable to provide a contact-type edge thermistor 38 capable of highly accurate temperature detection, and determine the necessity of the second control and the DUTY ratio using the detection temperature T2 of the edge thermistor 38. Thereby, stable temperature control becomes possible regardless of the use conditions and use environment of the apparatus.

  It should be noted that an unexpected rise in roller temperature may occur due to some cause during the forced energization by the second control, such as when the paper is not normally passed due to a jam or the like. Therefore, even during the second control, when the detected temperature of the center thermistor 37 reaches a predetermined temperature (for example, 180 ° C.), it is set so that the control is ended, and safety against an unexpected temperature rise is set. It is preferable to ensure.

  In addition, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the spirit of the present invention. For example, in the above embodiment, the DUTY ratio in the second control is set in two stages according to the initial value of the roller temperature, but may be set in one stage or may be set in three stages or more. In addition, instead of storing the relationship between the initial value of the roller temperature and the DUTY ratio in a table, the optimum DUTY ratio is calculated each time based on the temperature difference between the initial value of the roller temperature and the control temperature. Anyway.

  Further, the present invention is not limited to the color printer as shown in FIG. 1, but can be applied to other image forming apparatuses having a fixing device of a heat roller fixing system, such as a monochrome printer, a monochrome and color copying machine, and a facsimile. Of course.

  The present invention has an unfixed toner on a sheet having a pair of fixing rollers each including a heating roller having a heating unit disposed therein and a pressure roller that is disposed opposite to the heating roller and forms a nip through which the sheet is inserted. A fixing device that fixes the toner, a first detection unit that is disposed so as to be opposed to a sheet passing area of the heating roller, and detects a surface temperature of the heating roller in a non-contact state, and a heating unit according to a detection result of the first detection unit An image forming apparatus that performs a first control for switching on / off the energization of the heating unit when the detection result of the first detection unit reaches a predetermined control temperature. The control means receives the first detection result when the surface temperature of the heating roller is not more than a predetermined value when continuous printing of a predetermined number or more is input at the time of warm-up and the print start signal is received. From the time when the temperature rises above the control temperature to the time when the temperature drops again to the control temperature or until the end of continuous printing, whichever comes first, the energization is turned on at a predetermined DUTY ratio at which the surface temperature of the heating roller decreases. The second control that repeats / OFF is executed.

  As a result, it is possible to provide an excellent image forming apparatus that can suppress the undershoot of the heating roller temperature in the sheet passing area due to energization OFF immediately after warm-up, and can effectively prevent the occurrence of fixing failure during continuous printing. . Further, since the second control for forcibly energizing ends at the latest until the detection result of the first detection means falls to the control temperature, the safety of the apparatus is also ensured.

  In addition, since the optimum DUTY ratio is set based on the temperature difference between the surface temperature of the heating roller and the control temperature when the print start signal is received, both the effect of preventing undershoot and the safety against temperature rise are combined. An image forming apparatus is obtained.

  Further, if the contact-type second detection means is provided and the surface temperature of the heating roller when the print start signal is received using the second detection means is detected with high accuracy, the operation state and use environment of the apparatus Regardless of this, the image forming apparatus is capable of stable fixing temperature control.

FIG. 1 is a schematic configuration diagram of an image forming apparatus of the present invention. FIG. 3 is a side sectional view showing a configuration example of a fixing device used in the image forming apparatus of the present invention. FIG. 3 is a block diagram showing a control path of the image forming apparatus of the present invention. These are graphs showing the surface temperature change of the heating roller and the detected temperature change of the center thermistor when the fixing temperature control of the present invention is executed. These are flowcharts showing a fixing temperature control procedure in the image forming apparatus of the present invention. FIG. 9 is a side sectional view showing a schematic configuration of a conventional fixing device. These are graphs showing the surface temperature change of the heating roller and the detected temperature change of the non-contact thermistor when the conventional fixing temperature control is executed.

Explanation of symbols

17 Fixing roller pair 17a Heating roller 17b Pressure roller 18 Fixing device 25 Housing 26 Inlet 27 Outlet 28 Halogen heater (heating means)
32 Image forming unit 33 Control unit (control unit)
34 storage unit 37 center thermistor (first detection means)
38 Edge thermistor (second detection means)
100 Image forming apparatus

Claims (3)

Fixing unit that fixes a non-fixed toner on a sheet having a pair of fixing rollers including a heating roller in which a heating unit is disposed and a pressure roller that is disposed opposite to the heating roller and forms a nip through which the sheet is inserted. Equipment,
A first detection unit that is disposed to face the paper passing area of the heating roller and detects a surface temperature of the heating roller in a non-contact state;
Control means for controlling energization to the heating means according to the detection result of the first detection means, and when the detection result of the first detection means reaches a predetermined control temperature, In the image forming apparatus performing the first control for switching ON / OFF of energization,
The control means detects a detection result of the first detection means when a predetermined number or more of continuous prints are input during warm-up and the surface temperature of the heating roller when a print start signal is received is equal to or less than a predetermined value. The surface temperature of the heating roller decreases for a predetermined period from when the temperature first rises above the control temperature to when the temperature decreases again to the control temperature or until the end of continuous printing, whichever comes first. An image forming apparatus that executes a second control that repeats ON / OFF of energization according to a DUTY ratio.   The DUTY ratio used for the second control is set based on a temperature difference between the surface temperature of the heating roller and the control temperature when a print start signal is received. Image forming apparatus.   In the non-sheet passing area of the heating roller, second detection means for detecting the surface temperature of the heating roller in a contact state is arranged, and when the print start signal is received using the second detection means, The image forming apparatus according to claim 1, wherein a surface temperature of the heating roller is detected.

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