JP4806942B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to a fixing device for fixing an unfixed image formed on a recording material and an image forming apparatus including the fixing device.

  In image forming apparatuses such as copying machines, printers, and facsimiles, a toner image formed on a photosensitive drum or the like is directly or indirectly transferred onto a sheet, and an unfixed toner image transferred onto the sheet is heated and pressure-fixed. By doing so, a permanent image is formed on the paper. Here, as a fixing device for heating and pressure-fixing an unfixed toner image on a sheet, a heating member (for example, a heating roll) provided with a heating source such as a halogen lamp inside, and the heating member are arranged in pressure contact with each other. A pressure member having a pressure member (for example, a pressure roll or a pressure belt) is widely used. In this fixing device, the sheet is passed through the nip portion between the heating member and the pressure member, and the unfixed toner image is fixed on the sheet by the heat from the heating member and the pressure by the heating member and the pressure member.

  In such a fixing device, in order to fix the unfixed toner image on the paper, the surface temperature of the heating member is maintained at a predetermined temperature until at least the final paper in one job passes through the nip portion of the fixing device. It is required to be. Therefore, a sensor for measuring the surface temperature of the heating member is provided, and the heating operation of the heating source is controlled based on the temperature measurement result by this sensor (see Patent Document 1).

Japanese Patent Laid-Open No. 11-231702 (page 4, FIG. 2)

Incidentally, in the fixing device, as described above, heat is applied to the paper (and the unfixed toner image formed on the paper) from the heating member. In other words, this means that the heat of the heating member is taken away by the paper. For this reason, when image formation is continuously performed on a plurality of sheets, sheets passing through the fixing device take heat from the heating member one after another, and the surface temperature of the heating member gradually decreases. Invite the situation to go. At this time, the heating source heats the heating member, but it takes some time for the heat received from the heating source to reach the surface of the heating member. Take away. As a result, the surface temperature of the heating roll cannot be immediately increased and continues to decrease. Then, after outputting a certain number of sheets, when the heat supplied from the heating source and the heat taken by the sheets are balanced, the surface temperature of the heating roll becomes substantially constant.
For this reason, in the fixing device, the surface temperature of the heating member at the start of the image forming operation (the target temperature) is set so that the temperature at which the heat supplied from the heating source described above and the heat deprived by the paper are balanced is the final fixing temperature. Is set to a temperature higher than the fixing temperature.

  However, when such a setting is performed, the sheet that has passed through the fixing device during the period from the start of the image forming operation to the balance between the heat supplied from the heating source and the heat deprived by the sheet, and the heating source Since the temperature of the heating member differs between the paper that passes through the fixing device after the supplied heat and the heat taken away by the paper are balanced, there is a difference in the amount of heat received by the unfixed toner image on the paper and the paper. . As a result, the fixing degree of the toner image (the adhesion strength of the toner to the paper) and the gloss (gloss) are changed for each sheet, and the appearance of the image for each sheet is different.

  In particular, recently, an image forming apparatus called a high-speed machine capable of outputting, for example, 40 sheets or more per minute has been supplied for the light printing market. In this type of image forming apparatus, hundreds to thousands of prints may be output per job. In addition, in such an image forming apparatus, as a matter of course, the number of sheets supplied per unit time is large. Therefore, such a problem of temperature reduction is likely to occur in a fixing device used in a high-speed machine.

The present invention has been made to solve such a technical problem, and the object of the present invention is, for example, a case where a decrease in fixing temperature occurs due to heat being taken away by a recording material during a job. Another object is to suppress fluctuations in the obtained image quality.
Another object is to ensure a certain productivity even when the fixing speed is changed with a decrease in the fixing temperature during the job, for example.

  For this purpose, the fixing device to which the present invention is applied fixes the unfixed image on the recording material by the fixing means by passing the recording material carrying the unfixed image, and heats the fixing means by the heating means. The fixing unit is driven by the driving unit, and the fixing is set in consideration of the heat taken away from the fixing unit by the recording material passing through the fixing unit according to the number of recording materials passing through the fixing unit during one job. The drive speed information of the means is stored in the storage means, and the drive speed of the fixing means by the drive means is controlled by the control means based on the drive speed information read from the storage means.

  Here, the control unit can change the driving speed of the fixing unit by the driving unit when the recording material does not pass through the fixing unit. The drive speed information stored in the storage means is such that when the number of recording materials is less than or equal to a predetermined number, the driving speed of the fixing means is gradually decreased according to the number of recording materials and the number of recording materials is equal to the predetermined number. After exceeding, the driving speed of the fixing unit is made constant.

  From another point of view, an image forming apparatus to which the present invention is applied includes a transfer unit that transfers an image formed on an image carrier to a recording material at a predetermined transfer speed, and a transfer unit that transfers the image to the recording material. A fixing unit that heat-fixes the printed image at a predetermined fixing speed, and a conveyance unit that conveys the recording material from the transfer unit to the fixing unit at a predetermined conveyance speed. The predetermined transfer speed in the transfer unit is kept constant. The predetermined fixing speed in the fixing unit is reduced in accordance with the number of recording materials passing through the fixing unit during one job, and the predetermined conveying speed in the conveying unit is varied between the transfer speed and the fixing speed. It is characterized by that.

  Here, the predetermined fixing speed when the first recording material in one job passes through the fixing unit may be higher than the predetermined transfer speed. The predetermined fixing speed in the fixing unit may be constant after the number of recording materials passing through the fixing unit during one job exceeds a predetermined number. At this time, the predetermined fixing speed after the number of recording materials passing through the fixing unit during one job exceeds the predetermined number may be the same as the transfer speed. Furthermore, the recording material conveyance direction length of a conveyance part can be larger than the maximum length of the recording material which can be used. The predetermined transport speed in the transport unit is changed from the transfer speed to the fixing speed after the trailing end of the recording material in the transport direction passes through the transfer unit, and the fixing speed after the rear end in the transport direction of the recording material passes through the transport unit. To a transfer speed.

According to the present invention, since the driving speed of the fixing unit is controlled based on the driving speed information read from the storage unit, for example, the fixing temperature is lowered due to heat being taken away by the recording material during the job. Even in such a case, fluctuations in the obtained image quality can be suppressed.
In addition, according to the present invention, the transfer speed in the transfer unit is kept constant, while the transport speed in the transport unit is appropriately adjusted. Even in the case of changing, it is possible to ensure a certain productivity.

The best mode for carrying out the present invention (hereinafter referred to as an embodiment) will be described below in detail with reference to the accompanying drawings.
FIG. 1 shows an image forming apparatus to which the present embodiment is applied. This is a so-called tandem type or intermediate transfer type image forming apparatus, for example, a plurality of image forming units 10 (specifically, 10Y, 10M, 10C, 10K) on which each color component toner image is formed by electrophotography. And an intermediate transfer belt 20 as an image carrier for sequentially transferring (primary transfer) holding each color component toner image formed by each image forming unit 10, and a superimposed image transferred onto the intermediate transfer belt 20 as a recording material. Secondary transfer device 30 as a transfer portion for secondary transfer (collective transfer) to paper P as a fixing device, and a fixing device for fixing the secondary transferred image on paper P as a recording material or a fixing device as a fixing portion 70.

  In the present embodiment, each image forming unit 10 forms an electrostatic latent image on the photosensitive drum 11 and the charger 12 that charges the photosensitive drum 11 around the photosensitive drum 11 that rotates in the direction of arrow A. Laser exposure device 13 for writing (exposure beam is indicated by symbol Bm in the drawing), developing device 14 for accommodating each color component toner and making the electrostatic latent image on the photosensitive drum 11 visible, on the photosensitive drum 11 The electrophotographic devices such as a primary transfer roll 15 for transferring the respective color component toner images to the intermediate transfer belt 20 and a drum cleaner 16 for removing residual toner on the photosensitive drum 11 are sequentially arranged.

Further, the intermediate transfer belt 20 is stretched over a plurality of (in the present embodiment, five) support rolls and is rotated in the direction of the arrow B. These five support rolls are a drive roll 21, driven rolls 22 and 25 of the intermediate transfer belt 20, a tension roll 23 that adjusts the tension of the intermediate transfer belt 20, and a backup roll that functions as a backup roll of the secondary transfer device 30 described later. 24. The intermediate transfer belt 20 is formed using a resin such as polyimide or polyamide containing an appropriate amount of a conductive agent such as carbon black, and has a volume resistivity of 10 ⁸ to 10 ¹⁴ Ω · cm. The thickness is set to 0.1 mm, for example. The primary transfer roll 15 is applied with a primary transfer bias having a polarity opposite to the charging polarity of the toner, and the toner images on the photosensitive drum 11 are sequentially electrostatically attracted to the intermediate transfer belt 20. Then, a superimposed toner image is formed on the intermediate transfer belt 20.

Further, the secondary transfer device 30 includes a secondary transfer roll 31 disposed in pressure contact with the toner carrying surface side of the intermediate transfer belt 20 and a counter electrode of the secondary transfer roll 31 disposed on the back surface side of the intermediate transfer belt 20. And a backup roll 24 formed. A metal power supply roll 32 to which the secondary transfer bias is stably applied is disposed in contact with the backup roll 24. The secondary transfer roll 31 is made of urethane rubber tube with carbon dispersed on the surface, and the inside is made of foamed urethane rubber with carbon dispersed. Further, the roll surface is coated with fluorine, and its volume resistance is 10 ³ to 10 ¹⁰ Ω. The roll diameter is 28 mm, and the hardness is set to 30 ° (Asuka C), for example. Further, the backup roll 24 is a tube of EPDM and NBR blend rubber with carbon dispersed on the surface, and the inside is made of EPDM rubber, and the surface resistivity is 7 to 10 logΩ / □ and the roll diameter is 28 mm. The hardness is set to, for example, 70 ° (Asuka C).

  Further, a static eliminator 33 for neutralizing the surface of the intermediate transfer belt 20 after the secondary transfer is disposed downstream of the secondary transfer roll 31 in the moving direction of the intermediate transfer belt 20, and further, the surface of the intermediate transfer belt 20 is disposed downstream thereof. A belt cleaner 34 for cleaning is provided. On the other hand, an image density sensor 35 for adjusting image quality is disposed on the upstream side in the moving direction of the intermediate transfer belt 20 as viewed from the secondary transfer roll 31. Further, on the upstream side of the yellow image forming unit 10Y, a reference sensor (home position sensor) that generates a reference signal serving as a reference for taking image forming timing in each of the image forming units 10 (10Y, 10M, 10C, 10K). ) 36 is arranged. The reference sensor 36 recognizes a predetermined mark provided on the back side of the intermediate transfer belt 20 and generates a reference signal. Each image forming unit 10 (10Y, 10M, 10C, 10K) is configured to start image formation according to an instruction from the control unit 100 as a controller based on the recognition of the reference signal. Note that a UI (user interface) 110 is connected to the control unit 100 so that an instruction from the user can be received.

  Furthermore, the paper transport system feeds the paper P from the paper tray 40 at a predetermined timing by the paper feed roll 41, transports it by the transport roll 42, and temporarily stops the positioning by the registration roll (registration roll) 43. Then, it is sent to the secondary transfer position at a predetermined timing. The sheet P after the secondary transfer is guided to the belt conveyance unit 80 and conveyed to the fixing device 70 via the belt conveyance unit 80 as a conveyance unit. Here, the belt conveyance unit 80 includes a first belt conveyance device 50 provided on the secondary transfer device 30 side and a second belt conveyance device 60 provided on the fixing device 70 side.

  Next, an image forming process of the image forming apparatus according to the present embodiment will be described. For example, when a user turns on a start switch (not shown) of the UI 110, a predetermined image forming process is executed. Specifically, for example, when this image forming apparatus is configured as a digital color copying machine, a document set on a document table (not shown) is read by a color image reading device (not shown), and the read signal is read as an image. It is converted into a digital image signal by signal processing and temporarily stored in a memory, and toner images of each color are formed based on the stored digital image signals of four colors (Y, M, C, K).

  That is, the image forming units 10 (10Y, 10M, 10C, 10K) are driven based on the digital image signals of the respective colors obtained by the image signal processing. In each of the image forming units 10Y, 10M, 10C, and 10K, a laser exposure unit 13 applies an electrostatic latent image corresponding to a digital image signal to the photosensitive drum 11 as an image carrier uniformly charged by the charger 12. To write each. Then, each formed electrostatic latent image is developed by a developing device 14 containing toner of each color to form a toner image of each color. In the case where the image forming apparatus is configured as a color printer, it is only necessary to generate toner images of respective colors based on image signals input from the outside.

  The toner image formed on each photosensitive drum 11 is transferred to the primary transfer roller 15 by a primary transfer bias at a primary transfer position where the photosensitive drum 11 and the intermediate transfer belt 20 are in contact with each other. To the surface of the intermediate transfer belt 20. The toner image primarily transferred to the intermediate transfer belt 20 in this way is superimposed on the intermediate transfer belt 20 and conveyed to the secondary transfer position as the intermediate transfer belt 20 rotates. On the other hand, the paper P is conveyed to the secondary transfer position of the secondary transfer device 30 by the registration roll 43 at a predetermined timing, and the secondary transfer roll 31 removes the paper P from the intermediate transfer belt 20 (backup roll 24). Nip. The superimposed toner image carried on the intermediate transfer belt 20 is secondarily transferred to the paper P by the action of a secondary transfer electric field formed between the secondary transfer roll 31 and the backup roll 24. Thereafter, the sheet P on which the toner image is transferred is conveyed to the fixing device 70 by the belt conveying unit 80, and the toner image is fixed. On the other hand, the intermediate transfer belt 20 after the secondary transfer is discharged from the residual charge by the charge eliminator 33, and the residual toner is removed by the belt cleaner 34.

  FIG. 2 is a diagram illustrating the secondary transfer device 30, the belt conveyance unit 80 (the first belt conveyance device 50 and the second belt conveyance device 60), and the fixing device 70 among the image forming apparatuses described above. Among these, the registration roll 43 is provided with a registration roll driving motor 43 a that drives the registration roll 43. The rotation speed (hereinafter referred to as transfer speed) VT of the intermediate transfer belt 20 rotated by the drive roll 21 (see FIG. 1) is always constant (266 mm / sec). A paper detection sensor 90 that detects the passage of the paper P is disposed between the registration roll 43 and the secondary transfer device 30.

  The first belt conveying device 50 includes an endless first conveying belt 51 provided with a large number of perforations (not shown), a drive roll 52 and a driven roll 53 that stretch the first conveying belt 51. It has. The first belt conveyance device 50 is provided inside the first conveyance belt 51 and the first conveyance belt drive motor 54 that rotates the first conveyance belt 51 via the drive roll 52. And a suction fan 55 for adsorbing the paper P (not shown) conveyed by the first conveying belt 51 through perforations (not shown). The first conveying belt drive motor 54 drives the first conveying belt 51 so that the rotation speed (hereinafter referred to as the first belt conveying speed) VB1 becomes the same speed (266 mm / sec) as the transfer speed VT. .

  On the other hand, the second belt conveying device 60 is similar to the first belt conveying device 50 described above, and includes an endless second conveying belt 61 provided with a large number of perforations (not shown), and the second conveying belt 61. A driving roll 62 and a driven roll 63 are provided. The second belt conveyance device 60 is provided inside the second conveyance belt 61 and a second conveyance belt drive motor 64 that rotates the second conveyance belt 61 via the drive roll 62. And a suction fan 65 for adsorbing the sheet P (not shown) conveyed by the sheet 61 to the second conveying belt 61 through perforations (not shown). The second conveyor belt drive motor 64 is formed of a stepping motor, and can change the rotation speed (hereinafter referred to as second belt conveyance speed) VB2 of the second conveyor belt 61.

  As described above, in the present embodiment, the first conveyance belt 51 and the second conveyance belt 61 of the belt conveyance unit 80 can be rotated at different and independent speeds. Further, the conveyance direction length L of the first conveyance belt 51 disposed on the upstream side in the conveyance direction of the paper P (not shown) and the second conveyance belt 61 disposed on the downstream side in the conveyance direction (recording material conveyance direction). The length is the same, and is set to be slightly larger than the maximum size paper P that can be used in this image forming apparatus (for example, 420 mm in the case of A3SEF).

  The fixing device 70 includes a heating roll 71 provided with a heating source (not shown, which will be described later) such as a halogen lamp (not shown), and an endless pressure belt 72 disposed in pressure contact with the heating roll 71. And a heating roll drive motor 73 as a driving means for rotationally driving the heating roll 71. The heating roll drive motor 73 is formed of, for example, a DC brushless motor, and can change the rotation speed (hereinafter referred to as a fixing speed) VF of the heating roll 71.

FIG. 3 is a diagram for explaining the details of the fixing device 70.
The heating roll 71 includes a cylindrical core bar 71a, an elastic layer 71b provided on the core bar 71a, and a release layer 71c formed on the surface of the elastic layer 71b. Here, the cored bar 71a can be made of a metal material having high thermal conductivity such as aluminum. The elastic layer 71b can be made of a material having elasticity and a function of storing heat, such as silicon rubber. Furthermore, the release layer 71c can be made of a material having excellent release properties and heat resistance, such as fluororubber. Furthermore, an internal halogen lamp 74 that functions as an internal heating source is disposed inside the cylinder of the core bar 71 a constituting the heating roll 71. Furthermore, a first external heating roll 75 and a second external heating roll 76 are provided on the outer peripheral surface of the heating roll 71 so as to be in contact with the release layer 71c. The first external heating roll 75 and the second external heating roll 76 have a first external halogen lamp 75a and a second external halogen lamp 76a, respectively, and are rotatably arranged. The first external heating roll 75 and the second external heating roll 76 are driven to rotate as the heating roll 71 is rotated by the heating roll drive motor 73 (see FIG. 3). Further, the heating roller 71 is further downstream in the rotation direction of the heating roller 71 than the second external heating roller 76 and more upstream in the rotation direction of the heating roller 71 than the fixing nip region N where the heating roller 71 and the pressure belt 72 are in contact. A temperature detection sensor 77 that measures the surface temperature of the roll 71 is disposed in contact with the heating roll 71. As such a contact-type temperature detection sensor 77, a soft touch sensor using a thermistor can be used. As the temperature detection sensor 77, a non-contact sensor disposed at a position separated from the heating roll 71 by a predetermined distance can also be used. In the present embodiment, the internal halogen lamp 74, the first external halogen lamp 75a, and the second external halogen lamp 76a have a function as heating means.

  Further, the pressure belt 72 is stretched over a plurality of (three in this embodiment) support rolls 78 and is rotated in the arrow direction following the heating roll 71. These three support rolls 78 include a pressure roll 78a, an idle roll 78b, and a steering roll 78c. Here, the pressure roll 78 a is disposed in pressure contact with the heating roll 71 via the pressure belt 72 at a position downstream of the lowest point of the heating roll 71 in the rotation direction. As a result, the pressure roll 78a has a function of stretching and supporting the pressure belt 72 and a function of ensuring the self-stripping property of the paper P after fixing. The idle roll 78b is disposed on the upstream side in the horizontal direction from the lowest point of the heating roll 71, and carries a function of stretching and supporting the pressure belt 72 and an unfixed toner image T as an unfixed image. It has a function of guiding the paper P as the recorded material. Furthermore, the steering roll 78c is disposed such that one end of the roll shaft can be moved in the horizontal direction by a displacement mechanism (not shown). The steering roll 78c is feedback-controlled based on a detection signal from a sensor (not shown) that detects the meandering of the pressure belt 72, and suppresses the meandering of the pressure belt 72. Furthermore, a predetermined fixing nip region N is formed between the heating roll 71 and the pressure belt 72 by pressing the pressure belt 72 toward the heating roll 71 inside the pressure belt 72. A pressure pad 79 is attached.

  Next, a method for controlling the speed (fixing speed) in the fixing device 70, which is a characteristic point of the image forming apparatus according to the present embodiment, will be described in detail. In this image forming apparatus, the speed of the fixing speed VF is adjusted in accordance with the image forming operation, that is, the number of sheets P (number of printed sheets) from the start of the job. Corresponding to the speed adjustment of the fixing speed VF, the speed adjustment of the second belt conveyance speed VB2 (conveyance speed) is also performed.

FIG. 4 is a block diagram for explaining the control unit 100 as control means. The control unit 100 controls the entire image forming apparatus. FIG. 4 shows only functional blocks related to the control of the fixing operation.
The CPU 101 of the control unit 100 executes processing while appropriately exchanging data with the RAM 103 in accordance with a program stored in the ROM 102. The control unit 100 is provided with a counter 104 for counting the number of prints from the start of the job. In addition, the job start instruction information, the image formation mode information, and the print number information for one job received by the UI 110 are input to the control unit 100 via the input / output interface 105. Further, temperature information from the temperature detection sensor 77 provided on the heating roll 71 (see FIG. 3) and paper detection information from the paper detection sensor 90 are input to the control unit 100 via the input / output interface 105. . Further, the control unit 100 is connected to the registration roll drive motor 43a, the first transport belt drive motor 54, the second transport belt drive motor 64, the heating roll drive motor 73, the internal halogen lamp 74, the first via the input / output interface 105. The driving and lighting of the external halogen lamp 75a and the second external halogen lamp 76a are controlled. Note that the ROM 102 functions as a storage unit in the present embodiment, as will be described later.

  Next, processing related to the fixing operation will be described with reference to the flowchart shown in FIG. In the present embodiment, the internal halogen lamp 74, the first external halogen lamp 75a, and the second external halogen lamp 76a (described below) even during a period when the image forming operation (job) is not performed (standby period). Then, these are collectively referred to as a halogen lamp group). Specifically, power is supplied (lighted) to the halogen lamp group so that the temperature measured by the temperature detection sensor 77 (surface temperature of the heating roll 71) becomes a set temperature (175 ° C. as will be described later in the present embodiment). Is controlled.

First, an instruction from the user, specifically, an input of an image forming mode is received via the UI 110 (step 101) and input to the control unit 100. Here, examples of the image forming mode include the type of paper used for image formation (plain paper, thick paper, thin paper, etc.), the number of colors used for image formation (full color or monochrome), and the like. Further, the number of prints (number of sheets) output in one job is also input to the control unit 100 via the UI 110.
Next, the control unit 100 reads a fixing speed table corresponding to the input image forming mode from the ROM 102 (step 102). Then, a fixing speed schedule is set based on the read fixing speed table (step 103). Here, the fixing speed schedule includes not only the rotation speed of the heating roll 71 (fixing speed VF) in the fixing device 70 but also the rotation speed of the first transport belt 51 in the first belt transport device 50 of the belt transport unit 80. The first belt conveyance speed VB1 and the rotation speed of the second conveyance belt 61 in the second belt conveyance device 60 (second belt conveyance speed VB2) are also included. Details of the fixing speed table stored in the ROM 102 will be described later.

  Then, the control unit 100 determines whether or not a job start instruction is received via the UI 110 (step 104). If no job start instruction has been received, the process returns to step 104 to wait for an instruction. On the other hand, when an instruction to start a job is received, the control unit 100 next drives the fixing device 70 (heating roll 71) and the belt conveyance unit 80 (first conveyance belt 51 and second conveyance belt 61), and Power supply to the halogen lamp group (internal halogen lamp 74, first external halogen lamp 75a, and second external halogen lamp 76a) is started (step 105). Then, while the first belt conveyance speed VB1, the second belt conveyance speed VB2, and the fixing speed VF are set according to the set fixing speed schedule, the paper P on which the unfixed toner image T is formed is sequentially conveyed and fixed. (Step 106).

  Thereafter, it is determined whether or not the number of prints input in step 101 has been output, that is, whether or not the job has been completed (step 107). Here, if the job is still continued, the process returns to step 106 to continue the processing. On the other hand, when the job is completed, the control unit 100 drives the fixing device 70 (heating roll 71) and the belt conveyance unit 80 (first conveyance belt 51 and second conveyance belt 61) and supplies power to the halogen lamp group. Is stopped (step 108), and the series of processes is terminated. In the halogen lamp group, the power supply is controlled again so that the temperature measured by the temperature detection sensor 77 becomes the set temperature.

  Here, FIG. 6 shows an example of a fixing speed table as driving speed information stored in the ROM 102. Here, the fixing speed table corresponding to the paper type (plain paper, thick paper) as one of the image forming modes is illustrated. The fixing speed table includes the image forming mode (here, paper type), the set temperature of the heating roll 71 in each image forming mode (plain paper, thick paper), the first belt conveyance speed VB1, and the number of sheets and the fixing speed ratio ( The fixing speed VF / first belt conveying speed VB1) is associated. The second belt conveyance speed VB2 is set based on the first belt conveyance speed VB1 and the fixing speed VF, as will be described later.

In the example shown in FIG. 6, for example, in the plain paper mode, the set temperature of the heating roll 71 is 175 ° C., and the first belt conveyance speed VB1 is 266 mm / sec. The first belt conveyance speed VB1 is the same as the transfer speed VT. For example, the fixing speed VF is set to 1.500 times the first belt conveyance speed VB1 (399 mm / sec in this example) for the first sheet P, and then gradually decreases to the 200th sheet P. It is supposed to be. For the 201st sheet P and thereafter, the fixing speed VF is set to the same speed (266 mm / sec) as the first belt conveying speed VB1. On the other hand, in the thick paper mode, for example, the set temperature of the heating roll 71 is 190 ° C., and the first belt conveyance speed VB1 is 266 mm / sec. The set temperature of the heating roll 71 is higher than that of plain paper because thick paper has a larger heat capacity than plain paper, and the amount of heat required for fixing increases accordingly. The first belt conveyance speed VB1 is the same as the transfer speed VT, as in the case of plain paper. For example, the fixing speed VF is set to 1.500 times the first belt conveyance speed VB1 (399 mm / sec in this example) for the first sheet P, and then gradually decreases to the 100th sheet P. It is supposed to be. For the 101st and subsequent sheets P, the fixing speed VF is set to the same speed (266 mm / sec) as the first belt conveying speed VB1.
Here, the fixing speed table corresponding to the paper type as the image forming mode is exemplified, but actually, other than this, for example, color mode, monochrome mode, single-sided or double-sided, normal mode and photo mode, etc. Fixing speed tables corresponding to various image forming modes are stored in the ROM 102.

  Next, the operation in step 106 shown in FIG. 5 will be described in detail. FIG. 7 shows a timing chart when an image forming operation (job) for 400 sheets of paper P is executed in the plain paper mode shown in FIG. FIG. 8 shows a state where the paper P is conveyed from the registration roll 43 to the fixing device 70. 7A shows the power feeding operation (ON, OFF) for the halogen lamp group, FIG. 7B shows the transfer speed VT of the intermediate transfer belt 20, and FIG. 7C shows the driving of the registration roll 43 (ON, OFF). (D) shows the paper detection signal (ON, OFF) by the paper detection sensor 90, respectively. 7, (e) shows the first belt conveyance speed VB1 of the first conveyance belt 51, (f) shows the second belt conveyance speed VB2 of the second conveyance belt 61, and (g) shows the heating roll 71 ( And the fixing speed VF of the pressure belt 72) are shown. Further, in FIG. 7, (h) indicates the number of prints counted when the paper P passes through the paper detection sensor 90 and stored in the counter 104.

  Now, a specific description will be given with reference to FIGS. When an instruction to start a job is received at time t0 (Yes in step 105 shown in FIG. 5), power supply to the halogen lamp group is started. The intermediate transfer belt 20 has a transfer speed VT = 266 mm / sec, the first transport belt 51 has a first belt transport speed VB1 = 266 mm / sec, and the second transport belt 61 has a second belt transport speed VB2 = 266 mm / sec. Then, each drive is started. On the other hand, the heating roller 71 of the fixing device 70 is driven at a fixing speed VF = 399 mm / sec, which is 1.500 times the first belt conveying speed VB1 (transfer speed VT). Then, the supply of the first sheet P from the sheet tray 40 is started. At this time, the registration roll 43 stops rotating, and the leading edge of the first sheet P that has been conveyed contacts the registration roll 43 and stops.

  Next, the registration roll 43 starts rotating at the timing when the toner image formed on the intermediate transfer belt 20 reaches the secondary transfer device 30, and the first sheet P is transferred to the secondary transfer device 30. Nipped. The sheet detection signal is turned on when the leading edge of the first sheet P reaches the sheet detection sensor 90, and then the timing when the trailing edge of the first sheet P passes the sheet detection sensor 90. The paper detection signal is turned off. Then, the toner image on the intermediate transfer belt 20 is secondarily transferred to the first sheet P. The first sheet P passing through the secondary transfer device 30 and carrying the toner image (unfixed toner image T) is conveyed by the first conveying belt 51. At this time, the first belt conveyance speed VB1 of the first conveyance belt 51 is set to the same speed (266 mm / sec) as the transfer speed VT. Therefore, for example, as shown in FIG. 8A, the leading edge side of the first sheet P is on the first conveying belt 51 side, while the trailing edge side is still nipped by the secondary transfer device 30. In this case, the first sheet P is smoothly conveyed. Note that at the timing when the trailing edge of the first sheet P passes the sheet detection sensor 90, the driving of the registration roll 43 is stopped, and the arrival of the next sheet P (second sheet P) is awaited. Become.

  The first sheet P carrying the unfixed toner image T is further transported from the first transport belt 51 to the second transport belt 61. At this time, the second belt conveyance speed VB2 of the second conveyance belt 61 is set to the same speed (266 mm / sec) as the first belt conveyance speed VB1. Therefore, for example, as shown in FIG. 8B, even if the leading edge side of the first sheet P is on the second conveying belt 61 side and the trailing edge side is still on the first conveying belt 51, one sheet The eye paper P is smoothly conveyed.

  Then, the rear end of the first sheet P carrying the unfixed toner image T passes through the first conveyance belt 51. Here, in the present embodiment, since the length L in the conveyance direction of the second conveyance belt 61 is set to be longer than the maximum usable paper length, the first sheet P is shown in FIG. As shown in (), the second carrier belt 61 is carried and conveyed only. At this timing, that is, when the first sheet P is entirely carried on the second conveying belt 61, the second belt conveying speed VB2 of the second conveying belt 61 is the same as the first belt conveying speed VB1. The speed is increased from 266 mm / sec to 399 mm / sec, which is the same speed as the fixing speed VF. This speed increase is completed until the leading edge of the first sheet P reaches the fixing nip region N of the fixing device 70.

  Thereafter, the first sheet P carrying the unfixed toner image T is further transported from the second transport belt 61 to the fixing device 70. At this time, the second belt conveyance speed VB2 of the second conveyance belt 61 has already been increased to the same speed (399 mm / sec) as the fixing speed VF, as described above. Therefore, for example, as shown in FIG. 8D, the leading end side of the first sheet P is nipped by the fixing device 70, while the rear end of the first sheet P is still on the second conveying belt 61. Even so, the first sheet P is smoothly conveyed.

  Then, at the timing when the trailing edge of the first sheet P passes through the second conveyance belt 61, the second belt conveyance speed VB2 of the second conveyance belt 61 is 399 mm / sec, which is the same speed as the fixing speed VF, The belt speed is reduced to 266 mm / sec, which is the same speed as the first belt conveyance speed VB1 of the belt speed 51. Then, it waits for the next paper P (second paper P) to be carried. In addition, after the trailing edge of the first sheet P passes through the fixing device 70, the fixing speed VF is 1.399 times the first belt conveyance speed VB1 according to the fixing speed schedule, from 399 mm / sec to 1. The speed is changed to 498 mm / sec, which is 495 times, and the arrival of the next paper P (second paper P) is awaited.

  Whether or not the rear end of the paper P has passed through the first transport belt 51, the second transport belt 61, or the fixing device 70 is determined after the rear end of the paper P has passed through the paper detection sensor 90 (turns OFF. And based on the elapsed time. Therefore, it is not necessary to provide a sheet detection sensor between the first conveyance belt 51 and the second conveyance belt 61, between the second conveyance belt 61 and the fixing device 70, or downstream of the fixing device 70.

  In addition, the supply of the second sheet P from the sheet tray 40 is started while the first sheet P is being conveyed and fixed. At this time, the registration roll 43 stops rotating, and the leading edge of the second sheet P that has been conveyed contacts the registration roll 43 and stops.

  Next, the registration roll 43 starts to rotate in accordance with the timing at which the toner image formed on the intermediate transfer belt 20 reaches the secondary transfer device 30, and the second sheet P is transferred to the secondary transfer device 30. Nipped. The sheet detection signal is turned on when the leading edge of the second sheet P reaches the sheet detection sensor 90, and then the timing when the trailing edge of the second sheet P passes the sheet detection sensor 90. The paper detection signal is turned off. Then, the toner image on the intermediate transfer belt 20 is secondarily transferred to the second sheet P. Then, the second sheet P passing through the secondary transfer device 30 and carrying a toner image (unfixed toner image T: not shown) is conveyed by the first conveying belt 51. At this time, the first belt conveyance speed VB1 of the first conveyance belt 51 is set to the same speed (266 mm / sec) as the transfer speed VT. Therefore, for example, as shown in FIG. 8A, the leading edge side of the second sheet P is on the first conveying belt 51 side, while the trailing edge side is still nipped by the secondary transfer device 30. Even so, the second sheet P is smoothly conveyed. Note that at the timing when the trailing edge of the second sheet P passes through the sheet detection sensor 90, the driving of the registration roll 43 is stopped and the next sheet P (third sheet P) is awaited. Become.

  The second sheet P carrying the unfixed toner image T is further transported from the first transport belt 51 to the second transport belt 61. At this time, the second belt conveyance speed VB2 of the second conveyance belt 61 is set to the same speed (266 mm / sec) as the first belt conveyance speed VB1. Therefore, for example, as shown in FIG. 8B, even if the leading edge side of the second sheet P is on the second conveying belt 61 side, while the trailing edge side is still on the first conveying belt 61, The eye paper P is smoothly conveyed.

  Then, the trailing edge of the second sheet P carrying the unfixed toner image T passes through the first conveyance belt 51. Here, in the present embodiment, since the length L in the transport direction of the second transport belt 61 is set to be longer than the maximum usable paper length, the second paper P is shown in FIG. As shown in (), the second carrier belt 61 is carried and conveyed only. At this timing, that is, when the second sheet P is entirely carried on the second conveyance belt 61, the second belt conveyance speed VB2 of the second conveyance belt 61 is the same speed as the first belt conveyance speed VB1. The speed is increased from 266 mm / sec to 398 mm / sec, which is the same speed as the changed fixing speed VF. This speed increase is completed until the leading edge of the second sheet P reaches the fixing nip region N of the fixing device 70. That is, for the second sheet P, the speed of the second transport belt VB2 is different from that for the first sheet P.

  Thereafter, the second sheet P carrying the unfixed toner image T is further transported from the second transport belt 61 to the fixing device 70. At this time, the second belt conveyance speed VB2 of the second conveyance belt 61 has already been increased to the same speed (398 mm / sec) as the fixing speed VF, as described above. Therefore, for example, as shown in FIG. 8D, the leading end side of the second sheet P is nipped by the fixing device 70, while the rear end of the first sheet P is still on the second conveying belt 61. Even so, the second sheet P is smoothly transported.

Then, at the timing when the trailing edge of the second sheet P passes through the second conveying belt 61, the second belt conveying speed VB2 of the second conveying belt 61 is from 398 mm / sec, which is the same speed as the fixing speed VF. The belt speed is reduced to 266 mm / sec, which is the same speed as the first belt conveyance speed VB1 of the belt speed 51. Then, it waits for the next paper P (third paper P) to be carried. In addition, after the trailing edge of the second sheet P passes through the fixing device 70, the fixing speed VF is 1.498 times the first belt conveyance speed VB1 according to the fixing speed schedule, and from 398 mm / sec to 1. The speed is changed to 397 mm / sec, which is 492 times, and the next paper P (third paper P) is awaited.
Thereafter, up to the 200th sheet P, the sheet P is conveyed and fixed while the fixing speed VF is gradually decelerated so as to approach the first belt conveyance speed VB1. In the meantime, the second belt conveyance speed VB2 is sequentially switched between the first belt conveyance speed VB1 which is a constant speed and the fixing speed VF which is gradually decelerated.

  For the 201st sheet P, the second belt conveying speed VB2 of the second conveying belt 61 and the fixing speed VF of the heating roll 71 (pressure belt 72) are conveyed according to the fixing speed schedule. The speed is set to 1.000 times the speed VB1, that is, the same speed (266 mm / sec) as the first belt conveyance speed VB1. Thereafter, the second belt conveyance speed VB2 and the fixing speed VF are continuously set to the same speed as the first belt conveyance speed VB1 up to the 400th sheet P as the final sheet of the job.

  Thereafter, at time t 1 after the 400th sheet P is discharged from the fixing device 70, the power supply to the halogen lamp group, the intermediate transfer belt 20, the first transport belt 51, the second transport belt 61, and the heating roll 71 are performed. The driving is stopped and the series of jobs is completed.

  As described above, in the image forming apparatus according to the present embodiment, the transfer speed VT during a job is constant regardless of the number of printed sheets, while the fixing speed VF during a job is the number of printed sheets at the initial stage of the job. Is gradually decelerated so as to approach the transfer speed VT (first belt conveyance speed VB1) in accordance with the increase of the print speed, and after a constant number of prints is output during the job, it is maintained constant, that is, the same speed as the transfer speed VT. The Hereinafter, the reason why such setting is performed will be described.

  FIG. 9 shows the fixing temperature of the fixing device 70 (heating roll 71) when 1000 sheets of paper P are printed out using the image forming apparatus according to the present embodiment, that is, the temperature measurement result by the temperature detection sensor 77. Is shown. In FIG. 9, plain paper is used as the paper P. Therefore, the fixing temperature in the initial state (at the start of printout) is set to 175 ° C. In the example shown in FIG. 9, the heating roll 71 is heated by the halogen lamp group from the start of printout.

  As shown in the figure, when plain paper is used as the paper P, the fixing temperature starts to drop from 175 ° C. from the start of printout, and after approximately 200 sheets of paper P are printed out, it is substantially constant at 155 ° C. It is understood that This is due to the following reason. When the printout is started, the paper P is nipped and fixed to the heating roll 71 and the pressure belt 72. At this time, the paper P takes heat from the heating roll 71. At this time, the heating roll 71 is heated by the halogen lamp group, but in the initial state, the heat taken away from the surface of the heating roll 71 is larger than the heat supplied to the surface of the heating roll 71. . More specifically, the surface of the heating roll 71 is directly supplied with heat from the first external heating roll 75 and the second external heating roll 76. Insufficient to prevent Moreover, since heat is supplied to the surface of the heating roll 71 from the internal halogen lamp 74 provided in the heating roll 71 via the elastic layer 71b having heat storage properties and the release layer 71c, the heating roll 71 actually There is a time lag before heat is supplied to the surface. For this reason, a decrease in fixing temperature occurs in the initial stage of the job.

FIG. 10 shows the relationship between the fixing temperature and the fixing degree of the toner image on the fixed paper P when the fixing speed VF is used as a parameter. FIG. 10 illustrates a case where the fixing speed VF is 1.50 times, 1.34 times, and 1.00 times the transfer speed VT (the same speed as the first belt conveyance speed VB1). Yes. From the figure, it is understood that, as the fixing temperature is higher, an image having a higher fixing degree and a higher gloss is obtained. Accordingly, assuming that the fixing speed VF is constant, for example, the fixing degree obtained is different between the case where the fixing temperature is 175 ° C. and the case where the fixing temperature is 155 ° C. Specifically, the fixing degree is lower as the fixing temperature is lower. It is understood that Further, from the figure, assuming that the fixing temperature is constant, for example, the fixing speed VF is set to the same speed (1.00 times) as the transfer speed VT when the fixing speed VF is set to 1.50 times the transfer speed VT. It is understood that the degree of fixing obtained differs depending on the case, specifically, the fixing degree increases as the fixing speed VF decreases.
In fixing, the degree of fixing is determined by the amount of heat received by the paper P (and the unfixed toner image T on the paper P) per unit time. That is, the fixing degree is determined based on the correlation between the fixing speed VF and the fixing temperature. Therefore, as is apparent from FIG. 10, if the ratio of fixing temperature / fixing speed VF is made constant, the degree of fixing obtained becomes substantially constant.

  Therefore, in the present embodiment, as shown in FIG. 11, a fixing speed ratio (fixing speed VF / first belt conveyance speed VB1) corresponding to a temperature change depending on the number of printed sheets is set, and the number of sheets set in this way is set. The fixing speed ratio for each image is stored in the ROM 102 as a fixing speed table for each image forming mode. For example, when plain paper is used as the paper P, the fixing speed VF is set to the first belt conveyance speed VB1 (for the first to 200th paper P whose fixing temperature is higher than 155 ° C. In this period, the fixing speed can be kept constant by gradually decreasing the fixing speed VF as the number of printed sheets increases, that is, the fixing temperature decreases. For the 201st and subsequent sheets of paper P, the fixing temperature of which is substantially constant at 155 ° C., the fixing speed is set to the same speed as the first belt conveyance speed VB1 (transfer speed VT). Can be kept constant.

  FIG. 12 shows the relationship between the number of printed sheets and the obtained fixing degree. The solid line indicates the case where the image forming apparatus according to this embodiment is used, and the alternate long and short dash line indicates the fixing speed regardless of the number of printed sheets. The conventional cases where VF is set constant (266 mm / sec) are shown. From this figure, it is understood that the fixing degree is maintained constant regardless of the increase in the number of printed sheets in the present embodiment. On the other hand, in the conventional case, it is understood that the fixing degree decreases at the initial stage of the job and becomes constant after exceeding the 200th sheet. In this example, the fixing degree 77 is constant, but the set fixing degree varies depending on the required image quality and the like.

As described above, in the present embodiment, since the fixing speed VF is changed according to the number of printed sheets P, the amount of heat applied to each sheet P passing through the fixing device 70 is made substantially constant. be able to. For this reason, the fixing degree of the toner image formed on each paper P after fixing can be made substantially constant. In particular, in the image forming apparatus according to the present embodiment, since the initial value of the fixing temperature is set in anticipation of a temperature drop from the start of paper passing, the fixing temperature for about 100 sheets of paper gradually increases from the start of the job. descend. On the other hand, by using the method described in this embodiment, it is possible to prevent a change in the fixing degree (gloss) of the toner image after fixing in the initial stage from the start of the job.
In particular, in the present embodiment, the fixing speed VF is changed during a period in which the sheet P does not exist in the fixing nip region N, that is, between the sheets P. For this reason, a situation in which the fixing speed VF fluctuates during the fixing of one sheet P does not occur, and fluctuations in fixing degree and gloss due to speed fluctuations in one sheet P can be suppressed.

  In the present embodiment, the belt conveyance unit 80 includes the first belt conveyance device 50 and the second belt conveyance device 60. The first belt conveying speed VB1 of the first conveying belt 51 in the first belt conveying device 50 is the same as the transfer speed VT of the intermediate transfer belt 20, while the second conveying belt 61 in the second belt conveying device 60 is the same. The second belt conveyance speed VB2 is sequentially switched between the first belt conveyance speed VB1 (transfer speed VT) and the fixing speed VF which is decelerated or set at a constant speed according to the number of printed sheets. Therefore, the paper P can be smoothly transported from the secondary transfer device 30 to the fixing device 70. In this embodiment, since the conveyance speed of the paper P is switched in the belt conveyance unit 80, the transfer speed VT can be kept constant irrespective of the set value of the fixing speed VF, and the productivity is constant. Can be maintained.

  In the present embodiment, for example, the fixing speed VF is changed for each sheet P for the first to 200th sheets P. However, the present invention is not limited to this. For example, for each sheet P10. Alternatively, the fixing speed VF may be changed every 20 sheets. This is effective when, for example, a motor whose rotation speed cannot be changed steplessly is used as the heating roll drive motor 73.

  In this embodiment, the tandem type and intermediate transfer type image forming apparatus has been described as an example. However, the present invention is not limited to this, and can be applied to various image forming apparatuses including the fixing device 70. it can. Further, in the present embodiment, the fixing device 70 including the heating roll 71 and the pressure belt 72 has been described as an example. However, the present invention is not limited to this, and a combination of a heating roll and a pressure roll, Any combination of a heating belt and a pressure belt, such as a type that nips and fixes the paper P, can be applied as appropriate.

1 is a diagram for explaining an overall configuration of an image forming apparatus according to an embodiment. It is an enlarged view of a belt conveyance unit. It is a figure for demonstrating a fixing device. It is a block diagram for demonstrating the function of a control part. 6 is a flowchart for explaining processing in a fixing operation. It is a figure which shows an example of the fixing speed table stored in ROM. 6 is a timing chart when an image forming operation (job) is executed. (a)-(d) is a figure for demonstrating conveyance of a sheet | seat. It is a graph showing the relationship between the number of prints and the fixing temperature. FIG. 6 is a graph showing a relationship between a fixing temperature and a fixing degree of a toner image on a paper after fixing when the fixing speed is a parameter. It is a graph which shows the relationship between the number of printed sheets and a speed setting ratio. It is a graph which shows the relationship between the number of printed sheets, and the relationship between the fixing degree obtained.

Explanation of symbols

10 (10Y, 10M, 10C, 10K) ... image forming unit, 20 ... intermediate transfer belt, 24 ... backup roll, 30 ... secondary transfer device, 31 ... secondary transfer roll, 40 ... paper tray, 43 ... registration roll (Registration roll), 43a ... Registration roll drive motor, 50 ... First belt conveyance device, 51 ... First conveyance belt, 54 ... First conveyance belt drive motor, 60 ... Second belt conveyance device, 61 ... Second conveyance belt 64 ... second conveying belt drive motor, 70 ... fixing device, 71 ... heating roll, 71a ... core metal, 71b ... elastic layer, 71c ... release layer, 72 ... pressure belt, 73 ... heating roll drive motor, 74 ... Internal halogen lamp, 75 ... first external heating roll, 76 ... second external heating roll, 77 ... temperature detection sensor, 79 ... pressure pad, 80 ... belt conveyance unit 90, paper detection sensor, 100, control unit, 110, UI, VT, transfer speed, VF, fixing speed, VB1, first belt conveyance speed, VB2, second belt conveyance speed.

Claims (2)

A transfer section for transferring an image formed on the image carrier to a recording material at a predetermined transfer speed;
A fixing unit that heat-fixes the image transferred to the recording material at the transfer unit at a predetermined fixing speed;
A conveyance unit configured to convey the recording material from the transfer unit to the fixing unit at a predetermined conveyance speed, and to set the conveyance direction length of the recording material to be larger than the maximum usable recording material. Have
In one job in which image transfer by the transfer unit, conveyance by the conveyance unit, and fixing by the fixing unit are sequentially performed on a plurality of recording materials sequentially supplied.
The fixing temperature of the fixing unit before the start of the one job is set to a magnitude that takes into account the amount of heat taken by the recording material passing through the fixing unit when the one job starts,
The transfer speed in the transfer section is kept constant over the one job ,
The fixing speed in the fixing unit is set to be higher than the transfer speed when the first recording material in the one job passes through the fixing unit, and is previously set from the second sheet in the one job. While the determined predetermined number of recording materials pass through the fixing portion, the recording material is slower than the first recording material and faster than the transfer speed, and the fixing portion is is set to so that is sequentially reduced in accordance with the number of recording materials which pass, when the recording material after exceeding the predetermined number-th in the one job is passing through the fixing unit is the same as the corresponding transfer rate Is set to
The conveyance speed in the conveyance unit is determined from the transfer speed to the fixing speed after the recording material conveyance direction rear end passes through the transfer unit and before the recording material conveyance direction front end reaches the fixing unit. After the rear end of the recording material in the transport direction has passed through the transport section, and before the front end of the next recording material in the transport direction reaches the transport section, the fixing speed is changed to the transfer speed. image forming apparatus characterized by that. A transfer section for transferring an image formed on the image carrier to a recording material at a predetermined transfer speed;
A first transport unit that transports the recording material on which the image has been transferred by the transfer unit at a first transport speed;
The recording material transported by the first transport unit is transported at a second transport speed, and the length of the recording material in the transport direction is set to be larger than the maximum usable recording material . A transport section;
A fixing unit that fixes the recording material conveyed by the second conveying unit at a predetermined fixing speed;
In one job in which image transfer by the transfer unit, conveyance by the first conveyance unit, conveyance by the second conveyance unit, and fixing by the fixing unit are sequentially performed on a plurality of recording materials sequentially supplied.
The fixing temperature of the fixing unit before the start of the one job is set to a magnitude that takes into account the amount of heat taken by the recording material passing through the fixing unit when the one job starts,
The transfer speed in the transfer section is kept constant over the one job ,
The first transport speed in the first transport unit is set to be the same as the transfer speed over the one job ,
The fixing speed in the fixing unit is set to be higher than the transfer speed when the first recording material in the one job passes through the fixing unit, and is previously set from the second sheet in the one job. While the determined predetermined number of recording materials pass through the fixing portion, the recording material is slower than the first recording material and faster than the transfer speed, and the fixing portion is is set to so that is sequentially reduced in accordance with the number of recording materials which pass, when the recording material after exceeding the predetermined number-th in the one job is passing through the fixing unit is the same as the corresponding transfer rate Is set to
The second transport speed in the second transport unit is determined after the recording material transport direction rear end passes through the first transport unit and before the recording material transport direction leading edge reaches the fixing unit. The transfer speed is changed to the fixing speed, and after the trailing end of the recording material in the transport direction passes through the second transport section, before the next recording material reaches the second transport section, the fixing speed is changed. An image forming apparatus that is changed to the transfer speed .

Images