JP3638428B2 - Image forming apparatus and fixing temperature control method for image forming apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
  The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus capable of forming a toner image on thick paper such as drawing paper and a fixing temperature control method for such an image forming apparatus.
[0002]
[Prior art]
  In an image forming apparatus such as a copying machine, a printer, or a facsimile machine, a toner image is transferred onto a sheet in an image forming unit, and the sheet is conveyed to a fixing unit to heat and fix the toner image formed on the surface. . The fixing unit usually includes a heating roller having a fixing heater therein and a pressure roller that presses against the heating roller, and heats and fixes the toner image while sandwiching and transporting the sheet between the two rollers.
[0003]
  The fixing unit is provided with a temperature sensor, and the fixing heater is turned on / off so that the temperature of the fixing unit becomes a predetermined standby temperature until an operation button for receiving an instruction for an image forming operation is operated. ing. When the operation button is operated to instruct an image forming operation, the fixing heater is turned on to raise the temperature of the fixing unit, and at the same time, the image forming operation is started. Thereafter, until the image forming operation is completed, the fixing heater is turned on / off so that the temperature of the fixing unit is maintained at the normal fixing temperature.
[0004]
  When image formation is performed on normal copy paper (plain paper), for example, the standby temperature is set to 175 ° C. and the fixing temperature is set to 189 ° C. The fixing unit is set at a standby temperature until an image forming operation is instructed. When the image forming operation is instructed, the fixing heater is turned on to start the operation of the image forming unit while raising the temperature of the fixing unit. Then, the fixing heater is turned on / off so that the temperature of the fixing portion becomes 189 ° C.
[0005]
  When image formation is performed on thick paper such as drawing paper, the toner component melted when heat-fixing the toner image formed on the paper surface is difficult to penetrate into the paper, and the heating roller and pressure roller Therefore, in order to convey the sheet at a certain peripheral speed, it is necessary to set the temperature of the fixing unit to be considerably higher than that of plain paper. In particular, when the heating roller and the pressure roller are configured with a pair of small diameter rollers for downsizing the apparatus, a sufficient nip width between the two rollers cannot be obtained, and the heat capacity of the heating roller is small. As a result, the heat of the heating roller is rapidly taken away by contact with the paper, and it becomes difficult to maintain the temperature necessary for fixing.
[0006]
  Furthermore, when performing image formation on thick paper, paper is often fed from a paper feed unit exposed to the outside, such as a bypass tray, rather than being set in a normal paper feed cassette. Since the temperature difference between the temperature and the sheet is greatly different, the amount of heat taken by the sheet in the fixing unit is further increased. In order to reduce the size of the image forming apparatus, when the paper conveyance path is configured to be short, the path from the paper feed unit such as a bypass tray to the fixing unit is shortened, and the temperature difference between the fixing unit and the paper is still not increased. It becomes even larger, and it is necessary to set the temperature of the fixing portion to be considerably high.
[0007]
  For this reason, a mode selection button for performing image formation on such thick paper is provided, and when this mode selection button is selected, the fixing unit temperature is set to a higher fixing temperature than that of plain paper. It is conceivable to form an image by controlling the fixing heater. When drawing paper or the like is used as the thick paper, for example, it is considered necessary to perform the image forming operation by controlling the fixing unit at 205 ° C. or higher.
[0008]
[Problems to be solved by the invention]
  On the other hand, when the temperature of the fixing unit becomes high, there is a risk that a so-called offset occurs in which the toner on the paper adheres to the heating roller. Usually, the minimum temperature at which this offset occurs is considered to be about 210 ° C. Therefore, when forming an image on cardboard, it is necessary to control the temperature of the fixing unit to be in the range of 205 ° C. or more and 210 ° C. or less, and the fixing unit is maintained at about 207 ° C. However, it is considered preferable to perform image formation.
[0009]
  In the image forming apparatus as described above, when image formation on thick paper is selected by the mode selection button, the fixing heater is turned on to raise the temperature of the fixing unit and start image formation. The fixing unit maintains the standby temperature of 175 ° C. until an image forming operation instruction is issued. When the image forming operation instruction is issued, the fixing heater is turned on to raise the temperature to the thick paper fixing temperature of 207 ° C. At this time, in order to speed up the image forming operation, it is necessary to rapidly heat the fixing unit until the temperature of the fixing unit reaches about 207 ° C.
[0010]
  However, when heating is performed rapidly as described above, it is difficult to maintain the cardboard fixing temperature of 207 ° C. by continuously increasing the temperature by so-called overshoot even if the fixing heater is turned off. In particular, since the thick paper fixing temperature 207 ° C. suitable for thick paper is close to the offset generation temperature 210 ° C., even if the fixing heater is turned on / off near the thick paper fixing temperature 207 ° C., the offset generation temperature exceeds 210 ° C. Is a problem. When the on / off control of the fixing heater is performed in consideration of such overshoot, there is a problem that it takes a long time to reach the target temperature and the image forming operation cannot be performed at high speed.
[0011]
  An object of the present invention is to provide an image forming apparatus capable of appropriately controlling the fixing temperature in an image forming operation on special paper such as thick paper with a simple configuration.
[0012]
  Another object of the present invention is to propose a fixing temperature control method for an image forming apparatus that appropriately controls the temperature of a fixing unit in an image forming operation on special paper such as thick paper.
[0013]
[Means for Solving the Problems]
  An image forming apparatus according to the present invention includes an image forming unit, a fixing unit, a fixing temperature detecting unit, an instruction receiving unit, a plain paper fixing control unit, a special paper selection instruction receiving unit, and a special paper fixing control unit. It has. The image forming unit reads a document image and forms a toner image on a sheet. The fixing unit includes a heating roller having a fixing heater therein, and a pressure roller pressed against the heating roller. The sheet is sandwiched between both rollers to heat and melt the toner image formed on the sheet. Fixes and conveys paper. The fixing temperature detecting means detects the temperature of the fixing unit. The instruction receiving unit receives an instruction for an image forming operation. The plain paper fixing control unit controls the fixing heater on / off so that the temperature of the fixing unit detected by the fixing temperature detecting unit becomes a predetermined standby temperature until the instruction receiving unit receives an instruction for the image forming operation. When the image forming operation instruction is received by the receiving unit, the operation of the image forming unit is started, and on / off control is performed so that the temperature of the fixing unit detected by the fixing temperature detecting unit becomes a predetermined plain paper fixing temperature. . The special paper selection instruction accepting means accepts a special paper selection mode setting instruction and sets the special paper selection mode. The special paper fixing control unit is set when the special paper selection mode is set by the special paper selection instruction receiving unit and the instruction receiving unit receives an image forming operation instruction.FixedTurn on the landing heater, and the temperature of the fixing part detected by the fixing temperature detection meansHigher than special paper fixing temperature set higher than plain paper fixing temperatureWhen the first predetermined temperature is reached, the fixing heater is turned off, and the temperature of the fixing unit detected by the fixing temperature detecting means is lower than the first predetermined temperature.Higher than special paper fixing temperatureWhen the second predetermined temperature is reachedPaintingStart the image forming unitDuring operation of the image forming unit, the fixing heater is turned on / off so that the fixing unit temperature becomes the special paper fixing temperature.The
[0014]
  In this case, the temperature of the fixing unit is controlled to a predetermined standby temperature until an instruction for an image forming operation is given. When an instruction for the image forming operation using plain paper is given, the fixing heater is turned on / off to raise the temperature from the standby temperature to the plain paper fixing temperature, and the plain paper fixing temperature is maintained until the image forming operation is completed. The fixing heater is turned on / off. When the special paper selection mode is set and an image forming operation is instructed, the fixing heater is turned on to raise the temperature of the fixing unit from the standby temperature to the first predetermined temperature. When the temperature of the fixing unit reaches the first predetermined temperature, the fixing heater is turned off, and when the temperature of the fixing heater reaches the second predetermined temperature, the image forming operation is started.
[0015]
  Here, the first predetermined temperature is an offset start temperature at which the toner on the paper starts to adhere to the heating roller, and the second predetermined temperature is slightly lower than the offset start temperature and lower than the special paper fixing temperature. A sufficiently high temperature can be set.
[0016]
  thisLikeFixing part temperatureTheOvershootLet me onceFirst predetermined temperaturetaller thanOver offset start temperatureWest,Thereafter, the image forming operation is started when the temperature of the fixing unit decreases and reaches the second predetermined temperature. Therefore, the temperature is sufficiently lower than the offset start temperature and the image is formed on the special paper. Image formation can be performed at an appropriate fixing temperature.As a result, the toner image formed on the paper does not adhere to the heating roller side, and a good fixing state can be obtained in the fixing unit.In addition, the fixing unit can be rapidly heated to the target temperature, and the image forming operation can be performed quickly.
[0017]
  Also,The special paper fixing control means is arranged so that the temperature of the fixing section detected by the fixing temperature detecting means becomes a special paper fixing temperature lower than the second predetermined temperature while the image forming section performs the image forming operation on the special paper. On / off control of fixing heaterdoing.
[0018]
  Also, plain paper can be stored, a paper feed cassette that supplies the stored plain paper to the image forming unit, and special paper can be placed, and the loaded plain paper or special paper is imaged And a bypass tray that is supplied to the image forming unit, and is set on the bypass tray when the special paper selection instruction receiving unit has set the special paper selection mode and the instruction receiving unit receives an instruction for an image forming operation. It is also possible to configure so that the special paper is supplied to the image forming unit. In this case, when performing image formation on special paper placed on the bypass tray, it is possible to perform image formation quickly at an appropriate temperature by performing temperature control in the fixing unit as described above. It becomes.
[0019]
  Such a configuration can be applied when the special paper placed on the bypass tray is thick paper.
[0020]
  The fixing temperature control method for an image forming apparatus according to the present invention includes an image forming operation in an image forming apparatus that forms a toner image on a sheet supplied to an image forming unit and heat-fixes the formed toner image in the fixing unit. Until the temperature of the fixing unit is set to a predetermined standby temperature until the instruction is received, and when the image forming operation instruction is received when the paper supplied to the image forming unit is plain paper, The temperature is raised from a predetermined standby temperature to a predetermined plain paper fixing temperature, and the temperature of the fixing unit is set to the predetermined plain paper fixing temperature during the image forming operation, and the temperature is supplied to the image forming unit. When the paper is special paper, when the image forming operation instruction is accepted, the temperature of the fixing unit is raised and the temperature of the fixing unit isHigher than special paper fixing temperature set higher than plain paper fixing temperatureWhen the temperature reaches the first predetermined temperature, the temperature of the fixing unit is stopped, and the temperature of the fixing unit is lower than the first predetermined temperature.Higher than special paper fixing temperatureAnd starting the image forming operation by the image forming unit when the predetermined second temperature is reached, and setting the temperature of the fixing unit to a predetermined special paper fixing temperature during image formation.
[0021]
  In this way, even when image formation is performed on special paper such as thick paper, the temperature of the fixing unit can be quickly set to an appropriate temperature, and occurrence of offset can be suppressed. It becomes.
[0022]
DETAILED DESCRIPTION OF THE INVENTION
  〔overall structure〕
  FIG. 1 shows a cross-sectional view of a copying machine in which an embodiment of the present invention is employed. The copying machine 1 includes a main body 10 and a document presser 11 that is mounted on the upper portion of the main body 10 so as to be freely opened and closed.
[0023]
  The main body 10 is provided with a document table 15 on which an original document is placed. Inside the main body 10, an image forming unit 16 is provided at a substantially central position in the vertical direction at the left end when viewed from the front. In addition, a paper feed cassette 17 for storing paper is detachably provided at the lower portion, and a bypass paper feed unit 18 is provided on the left side thereof. An optical system 19 for reading image information of a document placed on the document table 15 is provided below the document table 15. A fixing device 20 for fixing the image on the sheet is provided obliquely above the left side of the image forming unit 16, and a discharge roller 21 is provided obliquely above the left side. A paper discharge tray (not shown) is provided outside the main body of the discharge roller 21, and a bypass tray (not shown) is provided outside the main body of the bypass paper feeding unit 18.
[0024]
  Between the paper feed cassette 17 and the bypass paper feed unit 18 and the image forming unit 16, there is provided a vertical conveyance path 25 that conveys the supplied paper in the vertical direction toward the image forming unit 16. Further, an oblique conveyance path 26 is provided between the image forming unit 16 and the discharge roller 21 to convey and discharge the sheet on which the image is formed obliquely upward.
[0025]
  (Image forming part)
  As shown in an enlarged view in FIG. 2, the image forming unit 16 includes a photosensitive drum 30 on which an electrostatic latent image is formed, and a main charging device 31 disposed around the photosensitive drum 30. A blank lamp 32, a developing device 33, a transfer device 34, and a cleaning device 35 are provided.
[0026]
  The main charging device 31 is a device for charging the surface of the photosensitive drum 30, and is disposed on the upper left side of the photosensitive drum 30. The blank lamp 32 is a lamp for discharging a desired area on the surface of the photosensitive drum 30, and is disposed adjacent to the right side of the main charging device 31. A developing device 33 that forms a toner image on the photosensitive drum 30 is disposed on the photosensitive drum 30 diagonally to the right of the photosensitive drum 30 with a predetermined gap from the blank lamp 32. The transfer device 34 is a device for transferring the toner image on the photoconductive drum 30 to a sheet, and is disposed on the lower left of the photoconductive drum 30. A downstream side of the vertical conveyance path 25 is located between the transfer device 34 and the developing device 33. The cleaning device 35 is a device for removing residual toner and the like on the surface of the photosensitive drum 30, and is disposed obliquely above and to the left of the photosensitive drum 30.
[0027]
  As shown in FIG. 1, a toner cartridge 36 that stores replenishment toner is provided above the developing device 33.
[0028]
  [Paper Feeder]
  The paper feed cassette 17 is provided so that the end portion on the paper exit side is located on the left side of the apparatus and on the right side of the vertical conveyance path 25, and the other end side extends to the right side of the apparatus. A feeding roller 37 for feeding the paper in the cassette to the vertical conveyance path 25 is provided above the end on the paper exit side. The bypass paper feeding unit 18 feeds paper manually from the lower left side of the apparatus or supplies paper placed on the bypass tray to the image forming unit 16, and is continuous with the vertical conveyance path 25.
[0029]
  〔Optical system〕
  The optical system 19 includes a lamp unit 40 having a light source and a reflecting plate, mirror units 41 and 42, a lens unit 43, and a reflecting mirror 44. Then, the light from the reflection mirror 44 passes between the blank lamp 32 and the developing device 33 and is irradiated onto the photosensitive drum 30. The incident angle θ1 (see FIG. 2) of light with respect to the photosensitive drum 30 is set to 26 ± 2 ° (24 to 28 °) with respect to the horizontal plane.
[0030]
  [Conveyance path]
  As shown in FIG. 2, the vertical conveyance path 25 includes a first conveyance roller pair 50, a second conveyance roller pair 51 disposed above the first conveyance roller pair 50, and each conveyance roller pair 50, 51. And a guide member 53 disposed in the front-rear direction. As shown schematically in FIG. 3, the vertical conveyance path 25 includes a first conveyance unit 25 a formed to extend obliquely upward to the left between the paper feed cassette 17 and the first conveyance roller pair 50. The second transport unit 25b formed to extend obliquely upward to the right between the first transport roller pair 50 and the second transport roller pair 51, and the second transport roller pair 51 and the image forming unit 16 (photosensitive drum). 30) and a third conveying portion 25c formed so as to extend obliquely upward to the left, and extends in the vertical direction as a whole.
[0031]
  As shown in FIG. 2, the oblique conveyance path 26 includes a guide member 55 a formed by a part of the opening / closing frame 55, a guide member 20 a formed by a part of the frame of the fixing unit 20, the fixing unit 20, and the discharge. And a guide member 56 provided between the rollers 21. The inclination angle θ2 (see FIG. 3) of the oblique conveyance path 26 is set to about 50 ° with respect to the horizontal plane. The angle θ2 of the oblique conveyance path 26 is preferably set to 30 to 60 °.
[0032]
  As shown in FIG. 2, the opening / closing frame 55 is supported so as to be rotatable about a fulcrum 54 with respect to the main body frame. The opening / closing frame 55 is mounted with the transfer device 34, the left roller 50a of the first transport roller pair 50, the left roller 51a of the second transport roller pair 51, and the left member of the guide member 53. ing. For this reason, by opening and closing the opening / closing frame 55, these members can be set on the copying machine body or separated.
[0033]
  As shown in FIG. 1, a cover 57 is provided on the outside of the discharge roller 21 so as to be openable and closable. By opening the cover 57, the fixing unit 20 and the discharge roller 21 can be exposed. .
[0034]
  [Fixing part]
  The fixing unit 20 includes a heating roller 61 and a pressure roller 62 that can be pressed against the heating roller 61. The heating roller 61 includes a fixing heater 63 therein, and is surrounded by a fixing upper housing 64. On the sheet discharge side of the fixing upper housing 64, a peeling claw 65 that is rotatably supported is disposed. The peeling claw 65 is urged by a torsion spring (not shown) so that the tip is pressed against the peripheral surface of the heating roller 61. With this peeling claw 65, the paper to be wound around the heating roller 61 can be peeled off and conveyed to the paper discharge side.
[0035]
  A shaft 67 of the pressure roller 62 is supported by the lower fixing housing 66 through a bearing 68 and a spring 69. The bearing 68 can move up and down with respect to the fixing lower housing 66, and is always urged toward the heating roller 61 by a spring 69. The bearing 68 and the spring 69 are disposed at both ends of the roller in the axial direction.
[0036]
  Although not shown here, a temperature sensor for detecting the temperature of the fixing unit 20 is provided on the inner wall of the fixing upper housing 64 or the fixed housing 66.
[0037]
  (Control part)
  As shown in FIG. 4, a control unit 71 is provided inside the copying machine 1. The control unit 71 is composed of a microcomputer system including a CPU, RAM, ROM, various drivers, and the like. An operation panel 72 for performing various operation instructions is connected to the control unit 71. The operation panel 72 includes an input key unit including a numeric keypad and other command keys, a display unit including a liquid crystal display element, an LED, and the like. A bypass H mode designation key for instructing is also included.
[0038]
  An image reading unit 73 is further connected to the control unit 71. The image reading unit 73 includes the above-described optical system 19 and the like. The image reading unit 73 irradiates a document placed on the document table 15 with light, and reads the light reflected from the document with a CCD image sensor or the like as two-dimensional image data. To process. The control unit 71 is connected to the above-described image forming unit 16 and a paper feeding unit 74 that transports paper to the image forming unit 16.
[0039]
  The controller 71 is connected to a fixing heater 63 provided in the fixing unit 20 and a temperature sensor 75 that detects the temperature of the fixing unit 20. The control unit 71 constantly detects the temperature of the fixing unit 20 with the temperature sensor 75, and turns on / off the fixing heater 63 based on an operation instruction by the operation of the operation panel 72 and the temperature of the fixing unit 20 by the temperature sensor 75. To control the temperature of the fixing unit 20. In addition to this, various input / output units 76 are connected to the control unit 71.
[0040]
  [General operation]
  A general operation of the copying machine 1 configured as described above will be described with reference to the flowchart of FIG. When the power of the copying machine 1 is turned on, reception of an instruction input from the operation panel 72 is prohibited in step S1 shown in FIG. 5, and various parameters are initialized in step S2. In step S3, the fixing heater 63 is turned on to raise the temperature of the fixing unit 20. In step S4, it is determined whether or not the temperature of the fixing unit 20 detected by the temperature sensor 75 has reached a predetermined standby temperature. The predetermined standby temperature can be set to 175 ° C., for example, and if the temperature of the fixing unit 20 detected by the temperature sensor 75 is 175 ° C. or less, this loop is repeated and the temperature of the fixing unit 20 detected by the temperature sensor 75 is repeated. When the temperature reaches 175 ° C., it is determined that the fixing unit 20 has reached a predetermined standby temperature, and the process proceeds to step S5.
[0041]
  In step S5, the fixing heater 63 is turned on / off so that the fixing unit 20 maintains a predetermined standby temperature. For example, when the fixing heater 63 is on and the temperature of the fixing unit 20 detected by the temperature sensor 75 reaches 175 ° C., the fixing heater 63 is turned off, and the temperature sensor 75 detects that the fixing heater 63 is off. When the temperature of the fixing unit 20 reaches 175 ° C., the temperature of the fixing unit 20 can be maintained at about 175 ° C. by turning on the fixing heater 63.
[0042]
  In step S6, the prohibition of accepting instruction input from the operation panel 72 is canceled, and various operation instructions can be input.
[0043]
  In step S7, it is determined whether or not there has been a mode selection input from the operation panel 72. Here, it is mainly determined whether or not a bypass H mode designation key for instructing image formation on thick paper has been operated. If the key is operated, the process proceeds to step S14.
[0044]
  In step S7, it is determined whether an instruction to start an image forming operation has been issued. Here, when the start key on the operation panel 72 is operated, it is determined that there is an instruction to start an image forming operation, and the process proceeds to step S9.
[0045]
  In step S9, the fixing heater 63 is turned on / off, and the plain paper fixing temperature control is performed so that the temperature of the fixing unit 20 becomes a temperature setting when image formation is performed on plain paper. In this plain paper fixing temperature control, the fixing heater 63 is turned on and the fixing unit 20 is heated from the standby temperature of 175 ° C. so that the plain paper fixing temperature is 189 ° C. After the temperature of the fixing unit 20 detected by the temperature sensor 75 has reached the normal paper fixing temperature 189 ° C., every time the temperature of the fixing unit 20 detected by the temperature sensor 75 reaches the normal paper fixing temperature 189 ° C., the fixing heater If 63 is on, it is turned off, and if it is off, the temperature of the fixing unit 20 is maintained at a normal paper fixing temperature of 189 ° C.
[0046]
  Simultaneously with the plain paper fixing temperature control in step S9, the copying operation in step S10 is started. Here, normal copy paper stored in the paper feed cassette 17 is conveyed to the photosensitive drum 30 side via the vertical conveyance path 25, and the toner image on the surface of the photosensitive drum 30 is transferred onto the paper. The sheet is conveyed to the fixing unit 20 to melt and fix the toner image. During this copying operation, the plain paper fixing temperature control is continuously performed, and the temperature of the fixing unit 20 is maintained at the ordinary paper fixing temperature of 189 ° C. In step S11, it is determined whether or not the instructed copying operation has been completed. Here, when a plurality of copying operations are instructed, it is determined whether or not all of these operations have been completed. If it is determined that the operation has been completed, the process proceeds to step S12.
[0047]
  In step S12, the temperature control of the fixing unit 20 is the standby temperature control similar to that in step S5. In step S13, other processing is performed.
[0048]
  In step S7, if the bypass H mode designation key is operated, the process proceeds to step S14. In step S14, it is determined whether an instruction to start an image forming operation has been issued. Here, when the start key on the operation panel 72 is operated, it is determined that there is an instruction to start an image forming operation, and the process proceeds to step S15.
[0049]
  In step S15, special paper fixing temperature control is performed so that the fixing heater 63 is controlled to set the temperature of the fixing unit 20 to a temperature setting for image formation on thick paper. In this special paper fixing temperature control, the fixing heater 63 is turned on so that the temperature of the fixing unit 20 is approximately the special paper fixing temperature 207 ° C. In step S <b> 16, the image forming unit 16 is fed via the bypass sheet feeding unit 18, the toner image formed on the photosensitive drum 30 is transferred onto the sheet, and the toner image is melted and fixed by the fixing unit 20. .
[0050]
  In step S17, it is determined whether or not the instructed copying operation has been completed. Here, when a plurality of copying operations are instructed, it is determined whether or not all of these operations have been completed. If it is determined that the operation has been completed, the process proceeds to step S12.
[0051]
  [Bypass H mode]
  When the bypass H mode is selected, the temperature of the fixing unit 20 is controlled by the special paper fixing temperature control in step S15 in FIG. This special paper fixing temperature control can be controlled as shown in the time chart of FIG.
[0052]
  When the copying machine 1 is on standby, the fixing heater 63 is turned on / off so that the temperature of the fixing unit 20 is 175 ° C. When the start button is operated in the bypass H mode, the registration switch is turned on, and at the same time, the fixing heater 63 is turned on (time t1). As a result, the fixing heater 63 raises the temperature of the fixing unit 20 with the first predetermined temperature (K1) as a target value. Here, the first predetermined temperature (K1) is set to 210 ° C., which is the offset start temperature. At the same time, the main motor remote is turned on.
[0053]
  When the temperature of the fixing unit 20 detected by the temperature sensor 75 reaches the first predetermined temperature (K1) (time t2), the fixing heater 63 is turned off. Even after the fixing heater 63 is turned off, the temperature of the fixing unit 20 detected by the temperature sensor 75 continues to increase due to overshoot, and starts to decrease gradually when the first predetermined temperature (K1) exceeds several degrees Celsius.
[0054]
  The image forming operation is started when the temperature of the fixing unit 20 detected by the temperature sensor 75 continues to decrease and reaches the second predetermined temperature (K2) (t3). The second predetermined temperature (K2) is set to a temperature slightly lower than the first predetermined temperature (K1), and can be set to 209 ° C., for example.
[0055]
  When the image forming operation is started, an original image on the original table 15 is read to form an electrostatic latent image on the surface of the photosensitive drum 30, and the electrostatic latent image is visualized as a toner image and supplied by bypass. The paper is conveyed from the paper unit 18 to the image forming unit 16 to transfer the toner image. The sheet with the toner image transferred on the surface is conveyed to the fixing unit 20 and the toner image on the surface is melted and fixed.
[0056]
  During this image forming operation, the fixing heater 63 is turned on / off so that the temperature of the fixing unit 20 detected by the temperature sensor 75 becomes the thick paper fixing temperature (K3). The thick paper fixing temperature (K3) can be set to a temperature suitable for thick paper fixing as described above, and can be set to 207 ° C., for example. Accordingly, during the image forming operation, when the temperature of the fixing unit 20 detected by the temperature sensor 75 reaches 207 ° C., the fixing heater 63 is turned off if it is on, and if it is off, the temperature sensor 75 is turned on. The temperature of the fixing unit 20 detected by 75 can be maintained at 207 ° C., which is an appropriate temperature for fixing the toner image on the thick paper.
[0057]
  As described above, when the thick paper having the toner image formed on the surface thereof is conveyed to the fixing unit 20, the temperature of the fixing unit 20 is substantially equal to the thick paper fixing temperature (K3), and the toner image is surely melted and fixed. This is done and no offset occurs.
[0058]
【The invention's effect】
  In the image forming apparatus according to the present invention, when an instruction for an image forming operation is received in the special paper selection mode, the temperature of the fixing unit is temporarily set.Higher than special paper fixing temperatureRaised to a first predetermined temperature,after thatThe fixing heater is turned off and the temperature is lower than the first predetermined temperature.Higher than special paper fixing temperaturePredetermined second temperatureToStart image forming operation when it reachesIn addition, during operation of the image forming unit, the fixing heater is turned on / off so that the fixing unit temperature becomes the special paper fixing temperature.Therefore, even when the temperature suitable for fixing is close to the temperature at which offset occurs, the temperature of the fixing unit can be set to an appropriate temperature setting, and image formation is possible. It is possible to shorten the time from when the operation instruction is given until the fixing unit reaches an appropriate temperature, thereby enabling a rapid image forming operation.
[0059]
  If the diameter of the heating roller and pressure roller is reduced in order to reduce the size of the image forming apparatus, the nip width between the two rollers is reduced, the heat capacity of the heating roller is reduced, and the temperature of the fixing unit must be set high. However, the toner image cannot be reliably fixed to the thick paper. However, according to the present invention, it is possible to prevent the occurrence of offset and to obtain a good fixing temperature.
[0060]
  In the fixing temperature control method of the image forming apparatus according to the present invention, when the paper supplied to the image forming unit is special paper, the temperature of the fixing unit isHigher than special paper fixing temperature set higher than plain paper fixing temperatureThe temperature is raised until the first predetermined temperature is reached, and when the temperature reaches the first predetermined temperature, the temperature rise of the fixing unit is stopped, and the temperature of the fixing unit isLower than the first predetermined temperature and higher than the special paper fixing temperatureThe image forming operation is started when the second predetermined temperature is reached, and an appropriate fixing temperature can be set for special paper such as thick paper.
[Brief description of the drawings]
FIG. 1 is a schematic vertical sectional view of a copying machine employing a first embodiment of the present invention.
FIG. 2 is an enlarged view of a main part thereof.
FIG. 3 is a schematic diagram of the paper transport path.
FIG. 4 is a control block diagram showing a schematic configuration thereof.
FIG. 5 is a control flowchart thereof.
FIG. 6 is a timing chart of the bypass H mode.
[Explanation of symbols]
1 Copying machine
10 Device body
11 Document holding
15 Document platen
16 Image forming unit
17 Paper cassette
18 Bypass paper feeder
20 Fixing part
30 Photosensitive drum
61 Heating roller
62 Pressure roller
63 Fixing heater
71 Control unit
72 Operation panel
75 Temperature sensor

Claims (4)

An image forming unit that reads a document image and forms a toner image on paper;
A heating roller having a fixing heater inside and a pressure roller pressed against the heating roller are sandwiched between the rollers, and the toner image formed on the paper is heated and fused. And a fixing unit for conveying the paper;
Fixing temperature detecting means for detecting the temperature of the fixing unit;
An instruction receiving means for receiving an instruction of an image forming operation;
Until the instruction receiving unit receives an instruction for an image forming operation, the fixing heater is turned on / off so that the temperature of the fixing unit detected by the fixing temperature detecting unit becomes a predetermined standby temperature. When an instruction for an image forming operation is received, the operation of the image forming unit is started, and on / off control is performed so that the temperature of the fixing unit detected by the fixing temperature detecting unit becomes a predetermined plain paper fixing temperature. Paper fixing control means;
Special paper selection instruction accepting means for accepting a special paper selection mode setting instruction and setting a special paper selection mode;
When the special paper selection instruction receiving means has set the special paper selection mode and the instruction receiving means receives an image forming operation instruction, the fixing heater is turned on and the fixing temperature detecting means detects the fixing. The fixing heater is turned off when the temperature of the section reaches a first predetermined temperature higher than the fixing temperature of the special paper set higher than the fixing temperature of the plain paper, and the temperature of the fixing section detected by the fixing temperature detecting means is the first to initiate the operation of the image forming section when reaching higher than the low KuKatsu the special paper fixing temperature than the predetermined temperature a second predetermined temperature during operation of said image forming unit temperature of the fixing unit Special paper fixing control means for turning on and off the fixing heater so that the temperature becomes the special paper fixing temperature ;
An image forming apparatus comprising: A paper feed cassette capable of storing plain paper and supplying the stored plain paper to the image forming unit;
A bypass tray on which special paper can be placed, and the plain paper or special paper placed thereon is supplied to the image forming unit;
And the special paper selection mode accepting means has been set by the special paper selection instruction accepting means, and the special paper placed on the bypass tray when the instruction accepting means accepts an instruction for an image forming operation. The image forming apparatus according to claim 1, wherein the image forming unit is supplied to the image forming unit.   The image forming apparatus according to claim 2, wherein the special paper placed on the bypass tray is cardboard. In an image forming apparatus that forms a toner image on a sheet supplied to an image forming unit and heat-fixes the formed toner image in a fixing unit.
Setting the temperature of the fixing unit to a predetermined standby temperature until receiving an instruction for an image forming operation;
When the paper supplied to the image forming unit is plain paper, when receiving an instruction for an image forming operation, the temperature of the fixing unit is increased from the predetermined standby temperature to a predetermined plain paper fixing temperature, A step of setting the temperature of the fixing unit to be the predetermined plain paper fixing temperature during an image forming operation;
When the paper supplied to the image forming unit is special paper, the temperature of the fixing unit is raised when an instruction for an image forming operation is received, and the temperature of the fixing unit is set higher than the fixing temperature of the plain paper When the first temperature higher than the special paper fixing temperature is reached, the temperature of the fixing unit is stopped, and the temperature of the fixing unit reaches the second temperature lower than the first temperature and higher than the special paper fixing temperature . Sometimes starting the operation of the image forming unit, and setting the temperature of the fixing unit to be the special paper fixing temperature during the operation of the image forming unit ;
A fixing temperature control method for an image forming apparatus.

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